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PC/104 FAQ
Last updated 1/11/2006

This PC104 FAQ was written by Roger Arrick starting in the 1990's. It was hosted at for many years and is now here for preservation purposes.

Many of the links here to external resources and email addresses have suffered bit rot and are now invalid.

-Roger Arrick,

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PC/104 - PC/104plus - EBX Embedded Systems

Copyright © Arrick Publishing & Roger Arrick. All rights reserved.

General PC/104 Information

* What is PC/104?
* What is PC/104-plus?
* What is PCI-104?
* What is EBX?
* About the PC/104 Embedded Systems FAQ
* Contribute to the FAQ
* PC/104 related information on the Internet
* The PC/104 Consortium
* The PC/104 Specifications
* Shows, conferences, seminars
* Trade Journals
* Technical Books
* Technical Overview

PC/104 Technical Questions and Answers

* How many PC/104 boards can be stacked?
* How can you separate PC/104 cards without bending pins?
* Are standoffs required when mounting PC/104 modules?
* Should a custom designed card be considered?
* How do I add serial ports to my PC/104 system?
* Configuring Windows 95/NT to recognize PC/104 serial module
* What serial communications interfaces are available for PC/104?
* How do I perform synchronous serial communications on my PC/104 system?
* How can several serial ports share the same IRQ?
* What is signal Conditioning?
* Can you extend the PC/104 bus height to accommodate tall components?
* Can you have multiple CPUs on a single PC/104 stack?
* What are the PC/104 pinouts?
* Why are some PC/104 CPU boards bigger than PC/104 modules?
* Is PC/104 supported by a large enough vendor base?
* What types of boards are offered for PC/104?
* How can PC/104 systems address thermal management?
* What type of power does PC/104 require?
* Driving large loads
* What is the most common media used in Solid State Disks (SSDs)?
* What about SSD and a real-time operating systems?
* Year 2000 issues in embedded systems
* Hand soldering PC/104 connector issues
* The Five Steps to Data Acquisition
* Pressfit Style PC/104 Connectors
* What support is available for using Linux in PC/104 based systems?
* Collecting data from an IP address (Ethernet or Internet) directly into Access, Excel, etc.
* Making data from my RS232 device available at an IP address on a TCP/IP network
* Are the four pads for the PC/104 board standoffs connected to ground or are they isolated?
* How many PC/104-Plus (PCI) modules can be placed on a stack?
* Mixing 3.3V and 5V signal levels
* How can I interface ISA and PC/104 products?
* What tools are available to debug/troubleshoot PC/104 boards?
* What alternative is there to stacking PC/104 cards?
* How can I connect more than two Ethernet products?
* How can I eliminate constant polling of my I/O and still get the data?


What is PC/104?

PC/104 is a standard for PC-compatible modules (circuit boards) that can be stacked together to create an embedded computer system. These types of systems are often found in factories, laboratories, and machinery to provide programmable control of a complex system.

PC/104 systems are very similar to standard desktop PCs but with a different form factor. The name "PC/104" is derived from this likeness and the special stackable bus connector having 104 pins. These systems can be programmed with the same development tools used with full-size PCs which reduces the need and cost of custom development efforts. Although only about 4" x 4", PC/104 boards are very powerful for their size. PC/104 products are designed for minimal power consumption, small foot print, modularity, expandability, and ruggedness - basic needs of the embedded systems designer.

Almost any type of module you can think of is now available for the PC/104 bus including CPUs, video controllers, network interfaces, sound I/O, data acquisition boards, and specialized interfaces. More than 200 vendors world-wide supply hardware, software, and engineering services to support the growing PC/104 standard.

Would you like to see a typical PC/104 module.

Return to the table of contents.


What is PC/104-plus?

PC/104-plus is basically a PCI (Peripherial Component Interface) bus addition to the PC/104 standard. PCI gives peripherial devices more direct access to the CPU which can greatly improve system performance. PC/104-plus has arrived just in time to serve video controllers, processors, and other high-throughput devices while maintaining backward compatability with PC/104.

The PC/104-plus specification Defines the PCI addition to PC/104 including the connector details. The new connector has 120 pins with 2mm spacing, an overall width of about 2.4 inches, and occupies the space at the opposite side of the board from the existing bus connectors. The spec can be seen at

Although vendors are finding it hard to place additional circuitry onto PC/104 boards that now have the plus PCI connector consuming space, they're being helped by the availability of video controllers, CPUs, gate arrays and other components with smaller footprints.

The obvious result of PC/104-plus is increased performance for the ever-increasing demands of the embedded systems designer.

Answer provided by Roger Arrick.

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What is PCI-104?

PCI-104 is the terminology used for the PCI-only (32-bit PCI bus) specification within the PC/104 product family. PC/104 supports the ISA bus with a 104-pin connector, and PC/104-Plus supports ISA and PCI through separate connectors. PCI-104 eliminates the on-board support for the ISA bus, retaining only the 120-pin connector for PCI. ISA support may still be obtained through use of a PCI to ISA bridge.

Because PCI is gradually replacing ISA as the bus of choice for device designs, the need for the ISA connector on the PC/104-Plus architecture is declining. The PCI-104 specification was adopted in 2003 by the PC/104 Consortium. The specification provides an industry standard for PC/104 footprint compatible devices that offer PCI, but not ISA bus connections. By not including the ISA connector, a small PC/104 format board regains space that can be used to provide additional functionality.

Answer submitted by Michele Lukowski at VersaLogic Corp.

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What is EBX?

EBX (Embedded Board eXpandable) is a standard that defines a non-backplane embedded controller that can contain PC/104-plus modules and other devices. The standard was designed by Ampro and Motorola and is based on the "Little Board" form factor from Ampro. EBX boards are 8" x 5.75" which is large enough to contain a highly integrated system and yet maintain expandablilty using additional PC/104-plus modules.

Download the EBX specification here (225k PDF)

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About the PC/104 Embedded Systems FAQ

What is a FAQ?
Many special interest groups in cyberspace have a FAQ file (frequently asked questions) associated with them. This reduces repetitive questions from new users (newbes), reduces the overall clutter of messages on newsgroups, and generally educates the masses. In addition to answering common questions, this FAQ will also serve as a tutorial for those learning about PC/104 systems.

About Roger Arrick
Hello. My name is Roger Arrick. I'm founder and Engineer at Arrick Robotics where I design and sell motion control systems. When PC/104 products first became available I took an interest and began to collect information, articles, and advertisments. Over the years this grew into a pretty large stack. When the Internet started becoming popular I went publishing crazy and started an on-line publication known as The E-Zine of Computer Controlled Systems in which I distribute control-related product and vendor information for several popular buses including PC/104. The E-Zine has become very popular to both users and vendors of PC/104 products. The PC/104 FAQ is a result of this effort. To eliminate any conflict of interest, I'm not a user or provider or PC/104 related products and am not associated with any vendor or with the PC/104 Consortium.

Why do I do it?
Why do I go to all this effort and give it away for free? Well, to enhance PC/104 awareness, promote the general welfare of the embedded-computing community, stamp out technical hog-wash, elevate certain writers to even higher ivory towers, and finally, to be a part of the techno-cool PC/104 movement. Oh, and did I mention it also gives us a warm, fuzzy feeling?

Legal and Copyright information
The official name of this document is:
"PC/104 Embedded Systems Frequently Asked Questions (FAQ) copyright (c) 1998-2000+ all rights reserved".

This FAQ is compiled and written by Roger Arrick of Arrick Publishing with contributions by vendors and users of PC/104 systems, among others.

This document is Copyright (c) Roger Arrick - Arrick Publishing all rights reserved. Do not distribute this document without express permission from the author. Do not remove this notice.

Contact the author via email at

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Contribute to the PC/104 FAQ

We welcome contributions and comments from PC/104 users, vendors, engineers, programmers, and well, you get the idea - everyone. We're specifically looking for common questions asked about PC/104 systems. The Author will be allowed to place their name, company information, etc at the end of the contributed text. This gives companies a little P.R. to justify their effort. Naturally, self-promotion and marketing hype will not be allowed. After we receive your contribution, we'll do some minor editing, check for technical accuracy, add it to the FAQ, and notify you of your success. Sorry, we can't use information specific to a certain piece of equipment or software unless is offers insight on more universal issues.

We hope you'll consider being a part of this noble effort. It'll look great on your resume!

Send comments, articles, and information to the editor via email at Electronic images can be sent as email attachments. Hardcopy can be sent to:

E-Zine of PC/104 Controlled Systems
PC/104 FAQ Contribution
Arrick Publishing
10768 CR 2335
Tyler, TX 75707 USA
Fax: (903) 566-4709

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PC/104 related information on the Internet

As you can imagine, the Internet is full of information about PC/104 and embedded systems. Here are a few links:

Web Sites

Frequently asked Questions available for downloading via FTP. You'll need an FTP (File Transfer Protocol) client or a browser that will let you download the file and save it. Newsgroups
You'll need a newsreader or a web browser with one to access newsgroups also known as USENET. This is where many questions get answered that are not covered in a FAQ. Most newsgroups have a FAQ associated with them. Return to the table of contents.


The PC/104 Consortium

Founded in 1991, the PC/104 Consortium is a member-supported, non-profit organization designed to support and promote PC/104 as an industry standard architecture for embedded PC applications.

The Consortium maintains and publishes the PC/104 Specification, maintains an on-line resource guide, attends trade shows and promotes the PC/104 Embedded Systems Industry.

Several categories of membership exist to meet a wide variety of participation levels. For more information on how you can become a member of the PC/104 Consortium please go to the On-line Consortium Information at

On-line Consortium Information at

Contact the PC/104 Consortium at:

PO Box 78008
San Francisco, CA 94121
Phone: 415-243-2104
Fax: 415-836-9094

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The PC/104 Specifications

Initially released in 1992, the PC/104 standard document describes the technical details needed by Engineers and Programmers. Below you can download the current specification for PC-104, PC-104-Plus, and PCI-104.

Download the PC-104 V2.5 specification (200k PDF)
Download the PC-104-Plus V2.0 specification (270k PDF)
Download the PCI-104 V1.0 specification (250k PDF)

The IEEE-P996 document describes the mechanical and electrical specifications for standard PC-style systems. Since PC/104 is based on this specification, we suggest you obtain a copy from IEEE at:

IEEE Standards Office
445 Hoes Lane
Piscataway, NJ 08854

A listing of the PC/104 draft standard and others can be found on the IEEE on-line catalog at

PC/104 differs from standard PC systems in the following ways:

  • Small form-factor.
    3.6 by 3.8 inches.

  • Stacking bus with pin/socket connectors.
    Eliminates the need for motherboards and card cages, and enhances reliability.

  • Reduced bus signal drive (4ma).
    Reduces power consumption to 1-2 Watts per module and minimizes component count.

Would you like to see a mechanical drawing of a PC/104 Module?

Ways to use PC/104 Modules

  • A single PC/104 module can be used as a complete stand alone system.
  • PC/104 modules can be added as components to larger systems.
  • Multiple PC/104 modules can be stacked together to create a system.

Electrical Specifications
PC/104 bus signals are identical to the standard PC bus in definition and function. Additional grounds enhance bus integrity and connector keys insure proper mating. AC signal timing and DC levels are identical to the P996 specification.

Most signals have a reduced bus drive of 4ma which reduces power consumption and heat dissipation. Open collector signals must drive 330 ohm pull-ups for compatibility. Components driving MEMCS16, IOCS16, MASTER, and ENDXFR signals must be able to sink 20ma.

Termination of the control signals is recommended to increase reliability. A network consisting of a 40 ohm resistor and a 30pf capacitor connected between each signal and ground is recommended by the P996 specifications. See the product database for a listing of companies that offer bus termination products.

Interrupt sharing can be accomplished by following the recommendations mentioned in the P996 specification. Certain precautions must be taken to insure that signal specifications are not violated. See the P996 specification for additional information.

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Shows, Conferences, Seminars

Attending a show or exhibition can be an educational and fun experience. Most major cities are visited by a shows, conference, exhibit or seminar on a regular basis. The following list describes several shows that would be of interest to the embedded systems user:

The Real-Time Computer Show
Contact: Cindy Chikahisa at The RTC Group
Ph: (714) 489-7575
Fax: (714) 489-8502

Embedded Systems Conference
Exhibits, tutorials, classes, speakers, product demos.
When: March 31-April 2, 1998
Where: Chicago, IL
Ph: (617) 821-9210
Fax: (617) 828-9992

Embedded Computing Shows
Contact: Steve Grimaldi at The RTC Group
Ph: (714) 489-7575
Fax: (714) 489-8502

IEEE Conference Listings

A more complete list of shows can be found at the E-Zine of PC/104 Controlled Systems.

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Trade Journals

Several industry trade journals cover issues related to embedded computing, here are a few:

RTC Magazine (Real-Time Computing)
The RTC Group
27312 Calle Arroy
San Juan Capistrano, CA 92675 USA
Phone: 714-443-4400
Fax: 714-489-8502

PC/104 Embedded Solutions Magazine
Fax: 810-774-8182

Embedded Systems Programming Magazine
Miller Freeman, Inc
525 Market St., #500
San Francisco, CA 94105 USA
Phone: (415) 905-2200

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Technical Books

Several book stores can be found on line that carry technical books including:

On-Line Book Stores

We think the following books may be helpful to programmers, engineers and users of PC/104 embedded systems:

Indispensable PC Hardware Book
Covers the desktop IBM-style PC. Complete PC hardware reference including interrupt, bus, I/O, EISA/Microchannel/PCI.
Author: Peter Messer
Publisher: Addison-Wesley
ISBN: 0201876973

The Art of Programming Embedded Systems
Author: Jack G. Gansale
Publisher: Academic Press
ISBN: 0122748808

Embedded Systems Programming In C and Assembly Language
Author: John Forrest Brown
Publisher: Van Nostrand Reinhold
ISBN: 0-442-01817-7

Practical Guide to Real-Time Systems Development
Author: Sylvia Goldsmith
Publisher: Prentice Hall
ISBN: 0137185030

Real-time Systems: Specification, Verification, and Analysis
Covers program structures, timing analysis and scheduling theory and specification and verification using an assertional method, using Timed CSP and using the duration calculus.
Author: Mathai Joseph
Publisher: Prentice Hall Intl.
ISBN: 0134552970

A more complete list of reading material can be found at the E-Zine of PC/104 Controlled Systems.

Return to the table of contents.



PC/104 Technical Questions and Answers



How many PC/104 boards can be stacked?

As with so many questions, the answer is "It Depends". Some PC/104 CPU manufacturers will only guarantee 5 or 6 cards in a stack, while others have tested their CPU modules with up to 10 boards or more.

The answer depends on the amount of bus drive capability on the CPU as well as the bus loading of the add-on modules. A well-designed module presents a maximum of one load on any bus signal that it uses. A poorly designed one may leave out buffer ICs and present 2 or more loads on some lines, which limits the number of add-on modules that you can put on the stack without overloading the CPU.

You will have to try your proposed configuration to know for sure. Ask the CPU manufacturer about their experience with their various CPU modules, and also ask the add-on module vendors about the bus loading of their modules.

Sometimes, adding AC bus termination, as indicated in the PC/104 specification, will help ensure signal reliability on systems with large numbers of modules.

Answer by Johnathan Miller at Diamond Systems

Sometimes there can also be physical limitiations to the number of PC/104 modules in a stack. Each module consumes about .6" of height. Vibration, space constraints and other factors of your specific application may be an issue.

Addition offered by Roger Arrick.

How can you separate PC/104 cards without bending pins?

The pin-and-socket bus of PC/104 offers one of the most reliable and rugged board connections available, yet PC/104 users often bend or damage these header pins when disassembling board stacks and separating modules.

To prevent breaking pins, PC/104 board extraction tools can be used to easily and quickly disconnect system components. These kind of tools are commonly made of polypropylene plastic to prevent damage to static sensitive device. Users vertically position them next to a PC/104 or EBX circuit board and carefully lift all PC/104 pins out of the adjacent PC/104 socket.

Answer provided by parvus Corporation.

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Are standoffs required when mounting PC/104 modules?

PC/104 modules have traditionally been stacked together or onto baseboards using 0.60-inch nylon or aluminum standoff spacers between board mounting holes. While standoffs provide an economical mounting method, many PC/104 users have found them to be tedious and time-consuming to assemble or disassemble.

As an alternative, PC/104 modules can be secured in railed card cages, which use four slotted aluminum rails and two steel end caps bolted together to support the PC/104 card stack. Holding up to 14 boards, card cages securely hold the four corners of each PC/104 circuit board and enables quick removal and replacement of defective modules or reorganization of the stack as it's being configured for thermal management, connector orientation, and cable routing. With cards installed, an entire card cage assembly can measure as small as four inches on a side. This method also enables shock/vibration isolators to be attached to the card cage for portable or otherwise rugged applications.

Answer provided by parvus Corporation.

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Should a custom designed card be considered?

Custom designs can provide precise system requirements by:

  • Providing functions that are not available with standard products; and/or,
  • Reducing card count by combining functions of two or more standard products into a single card.

Most designs can be accommodated by 2-layer and 4-layer PCB's. Typical cost and delivery for a custom designed card in which a function specification is provided are as follows:

# Layers Production Prototype Additional Units (1-10) Delivery
2 $1,500-$2,000 $175-$225 4-5 Wks
4 $2,500-$3,000 $250-$300 4-5 Wks

Concerning the number of cards usually required to justify the cost of the production prototypes:

As an example, suppose two standard products are available that provide a system requirement using partial functions of each card. Suppose the combined costs of the cards is $375. A 2-layer design that combines these functions for n units might typically be:

(1) Custom design for n cards = $1750 + (n-1) * $200

where $1750 is the proto and $200 is the additional cost/card.

The two standard units would cost:

(2) Standard products for n cards = n * $375

Equating (1) and (2) we find the break even cost occurs for n = 9. Thereafter, each card saves $175. In general, typical designs offer savings at 9 or 10 units.

Answer submitted by John Hilburn at Microcomputer Systems.

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How do I add serial ports to my PC/104 system?

PC/104 systems treat serial ports as do ISA bus systems. All you need is a serial communication module that is compliant with the PC/104 bus architecture. There are usually no special steps involved in configuring the port, other than the standard port address/IRQ jumper settings often found on the module. Once the module is plugged in the stack, any communications software will be able to access the module in the same way it would be accessed if it were an ISA bus serial port.

Answer provided by Sealevel Systems.

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Configuring Windows 95/NT to recognize PC/104 serial module?

You need to inform Windows of the new serial ports. The steps involved in doing this vary slightly depending on whether you are using 95 or NT.

If you are using Win95:

  • choose the "Add New Hardware" Icon under "Control Panel" (found on the start menu).
  • Do NOT let Windows detect your hardware, but instead, tell it that you are installing a new Port (choose "Standard Port Types" | "Communications Port").
  • Windows 95 will assign values for "Address" and "IRQ" to the new port (which are probably going to be incorrect.)
  • You now need to double click on the "System" icon in the "Control Panel", choose the "Device Manager" tab, and bring up the properties for the newly created com port.
  • Lastly, under the "Resources" tab, change the Address and IRQ properties to match your com port (Select "Basic Configuration 8" in order to change the Address and IRQ)
  • Repeat this process for every serial port you are installing.

If you are using NT, the process is even easier:

  • Choose the "Ports" icon under "Control Panel".
  • From this dialog box, add and configure the ports to match those available on your serial communication module.
  • Hardware manufacturers often provide configuration utilities that extend installation options. Choosing a module that provides a configuration utility can make installation even simpler.
Answer provided by Sealevel Systems.

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What serial communications interfaces are available for PC/104?

Any interface that is available in ISA format. Product availability depends mainly on where you obtain your PC/104 modules. Some manufacturers provide products supporting multiple electrical interface i.e.(RS-232, RS-422, RS-485,... ). Following are descriptions of some of the common interfaces:

  • RS 232
    Quite possibly the most widely used communication standard is RS 232. This implementation has been defined and revised several times and is often referred to as RS 232 or EIA/TIA 232. The IBM PC computer defined the RS 232 port on a 9 pin D sub connector and subsequently the EIA/TIA approved this implementation as the EIA/TIA 574 standard. This standard is defined as the 9 Position Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange. Both implementations are in wide spread use and will be referred to as RS 232 in this document. RS 232 is capable of operating at data rates up to 20 Kbps at distances less than 50 ft. The absolute maximum data rate may vary due to line conditions and cable lengths. RS 232 often operates at 38.4 Kbps over very short distances. The voltage levels defined by RS 232 range from 12 to +12 volts. RS 232 is a single ended or unbalanced interface, meaning that a single electrical signal is compared to a common signal (ground) to determine binary logic states. A voltage of +12 volts (usually +3 to +10 volts) represents a binary 0 (space) and 12 volts ( 3 to 10 volts) denotes a binary 1 (mark). The RS 232 and the EIA/TIA-574 specification defines two type of interface circuits, Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The Sealevel Systems adapter is a DTE interface.

  • RS 422
    The RS 422 specification defines the electrical characteristics of balanced voltage digital interface circuits. RS 422 is a differential interface that defines voltage levels and driver/receiver electrical specifications. On a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or inputs. In contrast, a single ended interface, for example RS 232, defines the logic levels as the difference in voltage between a single signal and a common ground connection. Differential interfaces are typically more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces also have greater drive capabilities that allow for longer cable lengths. RS 422 is rated up to 10 Megabits per second and can have cabling 4000 feet long. RS 422 also defines driver and receiver electrical characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS 422 signal levels range from 0 to +5 volts. RS 422 does not define a physical connector.

  • RS 485
    RS 485 is backwardly compatible with RS 422; however, it is optimized for partyline or multi drop applications. The output of the RS 422/485 driver is capable of being Active (enabled) or Tri State (disabled). This capability allows multiple ports to be connected in a multi drop bus and selectively polled. RS 485 allows cable lengths up to 4000 feet and data rates up to 10 Megabits per second. The signal levels for RS 485 are the same as those defined by RS 422. RS 485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one line. This interface is ideal for multi drop or network environments. RS 485 tri state driver (not dual state) will allow the electrical presence of the driver to be removed from the line. Only one driver may be active at a time and the other driver(s) must be tri stated. The output modem control signal RTS controls the state of the driver. Some communication software packages refer to RS 485 as RTS enable or RTS block mode transfer. RS 485 can be cabled in two ways, two wire and four wire mode. Two wire mode does not allow for full duplex communication, and requires that data be transferred in only one direction at a time. For half duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx to Rx ). Four wire mode allows full duplex data transfers. RS 485 does not define a connector pin out or a set of modem control signals. RS 485 does not define a physical connector.

  • MIDI
    The MIDI (Musical Instrument Digital Interface) specification grew out of the need for electronic musicians to link together synthesizers, drum machines, and all manner of other electronic instruments. Until the implementation of MIDI, each manufacturer of electronic musical equipment had its own proprietary method of passing data. With MIDI, a universal language for synthesizer control could be adopted by all manufacturers. MIDI was introduced in 1983 and very quickly was considered the standard for passing data between musical instruments. The MIDI specification has been continually updated and in 1987 was edited to include MIDI Time Code and in 1992 MIDI Machine Control. The MIDI specification is administered by the MIDI Manufactures Association. From a hardware stand point, MIDI is a simple current loop data signal traveling serially at 31.25K bits per second. MIDI defines the mechanical connector as a 5 pin DIN connector. There are only two ways to connect the instruments with cables: MIDI IN on one instrument to MIDI OUT on another or connect the MIDI THRU to the MIDI IN. The MIDI THRU cable "echoes" or re transmits the data from the MIDI IN port, thus providing a means of "daisy chaining" MIDI instruments.

Answer provided by Sealevel Systems.

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How do I perform synchronous serial communications on my PC/104 system?

In order to perform synchronous communications, you will need a non-UART based communication module. A popular approach has been implemented by various manufacturers using the Zilog 85x30 family of serial communication controllers (SCC). These SCC based modules may or may not support Direct Memory Access (DMA) depending on the design. If the module does support DMA, then much higher data rates can be attained. Keep in mind that maximum data rates depend on factors including but not limited to CPU, electrical interface, line distance / quality, software efficiency, and whether the card supports DMA or not.

Answer provided by Sealevel Systems.

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How can several serial ports share the same IRQ?

There are two basic ways to do this. A simple but inefficient method is to set each port to the same IRQ and poll each port every time an interrupt is generated. Analogy - Imagine paging each of 8 employees offices to find out who paged you on the intercom. This is an example of polling. Each port/office gets checked but it takes time.

The more efficient method is to have an Interrupt Status Port onboard the module. The interrupt status port tells you which port(s) generated the interrupt. An Interrupt Status Port (ISP) will provide greater efficiency/throughput when servicing multiple ports on a single interrupt line. The ISP is a read only 8-bit register that sets a corresponding bit when an interrupt is pending. Port 1 interrupt line corresponds with Bit D0 of the status port, Port 2 with D1 etc. The ISP is addressed at Base+7 on each port (Example: Base = 280 Hex, Status Port = 287, 28F etc.). This allows any one of the multiple locations to be read to obtain the value in the status register. All status ports on a single adapter are identical, so any one of the eight can be read.

Analogy - Imagine the same office environment. This time, when you receive a page, your phone illuminates with the location of the office issuing the page. Instead of paging each office individually, you know that Bob in office #7 issued the page and simply page Bob back. These two analogies illustrate the advantages associated with shared interrupts and the Interrupt Status Port.

Interrupt Status Port Example: The following indicates that Channel 2 has an interrupt pending.

Bit Position: 7 6 5 4 3 2 1
Value Read: 0 0 0 0 0 1 0

Answer provided by Sealevel Systems.

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What is signal conditioning?

More generally, signal conditioning is converting a signal from some transducer to whatever your receiving system needs. Frequently it's a matter of amplifying the microvolt or millivolt signals you get from (for instance) a strain gauge to the 10V span you need to get good resolution for your analog to digital converter.

However, many transducers give you strange things that you have to convert. A tougher example is a Linear Variable Differential Transducer, which gives you a varying AC voltage (after you've given it a stable AC voltage). The voltage it gives you back varies in level with the position of the core, so you have to first convert that AC voltage to DC, then amplify scale it to fit into your ADC's span.

Some transducers give you a current, which you must convert to a scaled voltage.

These are all examples of signal conditioning, and you'll notice I haven't yet mentioned "cleaning up."

You frequently want to clean the signal up, too, but that's usually incidental to the process. Most transducers give a far cleaner signal than you can use, so the best approach is to protect the signal you get from the transducer and convert and scale it directly, rather than trying to clean up a signal that's been contaminated by careless wiring.

Answer submitted by John Hilburn at Microcomputer Systems.

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Can you extend the PC/104 bus height to accommodate tall components?

Although the PC/104 specification calls for 0.60-inches (15mm) between stacked boards, some processor, hard drive and power supply modules exceed the standard height (i.e. due to heatsinks, fans or special ICs) and take up more than one card slot. These boards can in some cases be placed at the end of a board stack to avoid component interference, but this arrangement may not be optimal for prototype development or working with user cabling/packaging requirements.

Another, more flexible method is to extend the height of the PC/104 bus by using double-height stack adapters. This type of low-cost PC/104 interconnection method provides a PC/104 system with the needed component clearance through an additional 16-bit pass-through header for all PC/104 bus signals. Such adapters have standard PC/104 board mounting holes and can be placed on top or bottom of any PC/104 module with a 104-pin and socket connector.

Answer provided by parvus Corporation.

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Can you have multiple CPUs on a single PC/104 stack?

It's important to remember that PC/104 is basically an ISA bus. This bus is intended to operate with a single CPU. There are many additional signals needed on a bus to handle the traffic on multi-CPU systems which PC/104 and ISA buses do not have.

If you need multiple CPUs in a single system, consider having seperate PC/104 stacks that communicat using the serial port or a network card. It's often not necessary for multiple CPUs to have constant access to shared memory and I/O ports. Satelite processors can act as intelligent I/O controllers in order to off-load tasks from a main processor that's handling user I/O.

Answer provided by Roger Arrick.

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What are the PC/104 connector pinouts?

The name "PC/104" is derived from the fact that there are 104 pins on the two bus connectors which carry signals between modules. P1 has 64 pins and P2 has 40.

Each connector is a dual-row header having .1" pin spacing. These connectors are offered by:

PIN    J1/P1 (ROW A)    J1/P1 (ROW B)    J2/P2 (ROW C)    J2/P2 (ROW D)
 0                                        0V               0V
 1      -IOCHCHK         0V               -SBHE            -MEMCS16
 2      SD7              RESET            LA23             -IOCS16
 3      SD6              +5V              LA22             IRQ10
 4      SD5              IRQ9             LA21             IRQ11
 5      SD4              -5V              LA20             IRQ12
 6      SD3              DRQ2             LA19             IRQ15
 7      SD2              -12V             LA18             IRQ14
 8      SD1              -ENDXFR          LA17             -DACK0
 9      SD0              +12V             -MEMR            DRQ0
 10     IOCHRDY          KEY              -MEMW            -DACK5
 11     AEN              -SMEMW           SD8              DRQ5
 12     SA19             -SMEMR           SD9              -DACK6
 13     SA18             -IOW             SD10             DRQ6
 14     SA17             -IOR             SD11             -DACK7
 15     SA16             -DACK3           SD12             DRQ7
 16     SA15             DRQ3             SD13             +5V
 17     SA14             -DACK1           SD14             -MASTER
 18     SA13             DRQ1             SD15             0V
 19     SA12             -REFRESH         KEY              0V
 20     SA11             CLK
 21     SA10             IRQ7
 22     SA9              IRQ6
 23     SA8              IRQ5
 24     SA7              IRQ4
 25     SA6              IRQ3
 26     SA5              -DACK2
 27     SA4              TC
 28     SA3              BALE
 29     SA2              +5V
 30     SA1              OSC
 31     SA0              0V
 32     0V               0V
Answer provided by Roger Arrick.

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Why are some PC/104 CPU boards bigger than PC/104 modules?

Many CPU boards are larger than the standard PC/104 form factor in order to provide additional functions that would not fit on a smaller board. Doing so reduces the number of modules needed for a system and makes production and maintenance easier. Large boards like this often act as a carrier board for additional modules and sometimes act as complete systems by themselves. These CPUs still retain a PC/104 stacking connector for future expansion or for the addition of functions not found on the main board.

Seek additional information on the new standard called "EBX".

Answer provided by Roger Arrick.

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Is PC/104 supported by a large enough vendor base?

PC/104 is definitely a standard with staying power. There are currently (Oct 2000) about 160 vendors who are members of the PC/104 consortium and many more that are not. Systems integrators and design firms world-wide are now designing, programming, and using PC/104 systems for real-world tasks every day. Selecting PC/104 as a platform for future product development is as safe a bet as can be made.

Answer provided by Roger Arrick.

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What types of boards are offered for PC/104?

Virtually every type of interface board imaginable is available in a PC/104 package. I've discovered the following types:

  • CPUs including everything from 8088 - Pentium
  • Most varieties of serial communications
  • Modems
  • Network interfaces
  • AC and battery power supplies
  • LCD and traditional CRT video drivers
  • Solidstate disk
  • Traditional rotating disk storage interfaces (SCSI, IDE, floppy, etc.)
  • Adapters to other types of busses
  • PCMCIA interfaces
  • Real time clocks
  • Prototype boards
  • Digitial I/O
  • Analog I/O
  • Sound and speech I/O
  • Motion control (servo and stepper)
  • Optical encoder interfaces
  • Touch screen interfaces
  • Video frame grabbers and processors
  • Relay and solenoid drivers
  • DSP (Digital Signal Processors)
  • GPS (Global Positioning Systems) receivers
  • Load cell and other specialized sensor interfaces
  • Field bus interfaces
  • Counter/timers
  • Keyboard and keypad interfaces
  • Avionics control systems interfaces
  • Coprocessors

Answer provided by Roger Arrick.

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How can PC/104 systems address thermal management?

PC/104 cards commonly support operational temperatures of 0C to +70C in standard modules and -40C to +85C in extended temperature-rated versions. As most temperature issues stem largely from a system's own CPU or power supply, heat sinks and/or fans can be used to maximize airflow and conductive/convective cooling.

For PC/104 boards in a sealed enclosure, the use of PC/104 fan cards can effectively create a push-pull circular cooling pattern to eliminate failure-causing hot spots, as well as providing heat transfer to the outer enclosure. Typically available with dual ball-bearing fans mounted on a PC/104 form factor, fan cards can also incorporate onboard thermostats to monitor temperature in an intelligent way, based on multiple digital inputs, outputs, and connections to external temperature sensors.

When heat transfer by airflow is not practical or sufficient, power supplies can be given a direct thermal connection to the body of the outer enclosure through low-profile conductive aluminum heatsinks. Externally mounted heatsinks, the length of the extrusion sides or enclosure end caps, can further dissipate heat away from the system.

Answer provided by parvus Corporation.

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What type of power does PC/104 require?

Like ISA the bus, PC/104 has 4 voltages available: +5v, -5v, +12v, -12v. The +5v supply is the most important and many PC/104 systems can run using it alone. Current levels range from 100ma up to many amps depending on the perpherials in the system. Vendors detail the power requirements for each module on their data bus.

The other voltages: -5v, -12v, and +12v are often only needed at very small current - less than 50ma. Sound cards, video cards, and serial communication cards are the most common users of these voltages.

Communications boards
Serial communications boards often require +12v and -12v for the drive circuitry. Drivers don't require very much current, usually less than 50ma. Some boards generate their own +12v and -12v from the +5v supply to eliminate this need. Check with your harware vendor and ask about the power supply requirements.

Disk drives
If your PC/104 system contains traditional rotating disk storage, you may need +12v for the spindle power. Sometimes this may be more than 1 amp. Today's hard and floppy disks require much less current than just a few years ago. Many hard disks are now available that need only +5v at very low current. Most 3.5" floppy drives now require only +5v also.

Linear and switching supplies
Two common types of power supplies are switchers and linear. Switchers are now the most popular due to their size and cost. Linears normally need a larger transformer but supply cleaner power and offer simplicity of design. Many switchers have a minimum current draw requirement for them to regulate properly - make sure to check the specs of your system and supply. Regulation should be better than 5% in most cases.

Battery operated
It's becoming more common to see PC/104 systems in environments that do not have access to 110v or 220v mains. This has given rise to battery operated supplies that create the necessary voltages from a single +12v source. Higher input voltages may also be available for aircraft and other applications.

The power supply on most systems is contained in a seperate package that mounts near the PC/104 stack and associated ciruitry. A short cable delivers power to the stack and other devices. Some supplies actually stack on the PC/104 bus along with other modules.

Read those data sheets
It's important to spend a little time and read the vendor's data sheets carefully for power requirements. It's smart to have excess current capacity to maintain your future expansion options. With a little effort, you'll be able to select a power supply that's right for the job.

Answer provided by Roger Arrick.

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Driving Large Loads

Many embedded systems must control large devices such as motors, lights, valves, etc. Driving such a load is normally done by attaching a digital output signal to a relay. The relay controls the large load from the small digital system in the same way that your car key switch controls your starter through a soleniod. There are both mechanical and solid-state relays on the market. Traditional mechanical relays rely on a coil which creates a magnetic field to cause the contacts to close or open. These coils normally require even more current than an digital output signal can provide requiring a buffer circuit in the form of a transistor. Mechanical relays can obviously wear out due to the moving parts involved. Contacts in the relay often arc creating a carbon deposit and electrical noise that can distrub near-by electrical equipment like the control computer.

Solid state relays
Solid state relays provide the same function as their mechanical counterpart but without many of the disadvantages. Since there are no moving parts, the need for contact cleaning is eliminated. Contact bounce and electrical noise are also non-existent in solid state models. Virtually all solid state relays also provide optical isolation which eliminates the direct electrical connection between the control computer and the load being switched. This goes a long way to protect sensitive digital computers and eliminate load noise from feeding back to the computer. Solid state relays also don't require as much current as mechanical models and can usually be driven directly from a digital output pin.

Digital I/O boards
Many digital I/O boards are available for PC/104 systems. Some will provide dozens of I/O pins to control devices and read digital sensors, switches, etc. Multi-function boards are also available that provide digitial I/O plus analog inputs, timers, counters, and other useful functions.

Using the parallel printer port
In applications where only a few digital I/O lines are needed, you may want to check into using a standard parallel printer port. Printer ports are simply digital I/O board designed to drive printers. The bits on the port accessed through the control program just like any other port. Since many single-board-computers already have such a port, using it for this function could eliminate the need for additional board on the stack.

Electrical spikes
When large loads are switched ON or OFF, electrical noise and voltage spikes can be created. If these effects make their way back to the control computer, it could stop the program, or worse, distroy circuits. Several devices exist to dampen spikes including MOVs (Metal Oxide Varistors), and Transzorbs. Most solid state relays already contain a protection device.

Cable and connectors
Remember to calculate the current needed for your load and size the cables and wires accordingly. Undersized wires can melt under heavy loads and cause fires. Connectors must also be capable of carrying the load.

It's normally safer to connect the frame of a machine having power loads to earth ground. Doing so, prevents an electrical short from creating an electrical shock condition for operators.

Answer provided by Roger Arrick.

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What is the most common media used in Solid State Disks (SSDs)?

The most common media in use today for creating solid state disks is Flash ROMs with some sort of BIOS extension (for bootable media) or installable device driver (for non-bootable data storage media).

Most manufacturers provide a BIOS extension that will allow the user to use the standard DOS/Windows commands and file structure. Assuming the designer is using DOS, for example, the steps would be as follows:

To make a bootable MS-DOS/Windows device:

Type Format /s driveletter : and this will create an MS-DOS file structure as well as transfer the operating system to the SSD.

To make a non-bootable device:

Type Format driveletter : Notice that the /s command line switch is not used in this case so the Operating System is not copied onto the SSD.

Answer contributed by Michael Peat at INSIDE Technology USA, Inc. .

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What about Solid State Disks (SSD) and a real-time operating systems?

Many single board computer manufacturers provide additional software/firmware support for a variety of operating systems that are specific to their hardware.

Another very simple way to implement an SSD for these types of applications is to use an IDE interface Flash Disk device. These are available from many of the single board computer manufacturers as well as third parties. These SSDs greatly simplify system development by using an IDE port on the single board computer. Thus, the user would follow the same steps as would be used in preparing any IDE hard drive for the operating system that is chosen.

Answer contributed by Michael Peat at INSIDE Technology USA, Inc. .

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Year 2000 issues in embedded systems

The so-called "year 2000" problem is caused when software uses only 2-digits to represent date codes in order to save space in memory and/or disk. The obvious problem occurs at the transistion between 1999 and the 2000. The year "1999" is stored as "99" and the year 2000 is stored as "00". Any calculations based on these 2, 2-digit values must take into account the roll over from 99 to 00 as being only 1 year. If the program does not contain special code to deal with this condition, then a calculation could return bad results.

An example
A good example of this would be an embedded system that is used as a data-logger to track traffic. A sensor would detect the number of cars traveling a road and software would count the cars and create reports about the flow over time. The 1998 new year calculation would be 1998 - 1997 = 1 year using 4 digit values and 98 - 97 = 1 using 2 digit values. The result of these calculations are the same. The 2000 new year calculations would be 2000 - 1999 = 1 year, and 00 - 99 = -99 (woops!). The program could easily detect this condition and fix the result with the following statement: if result less than 0 then result=result+100. But often programmers underestimate the longevity of their creations, lack room in memory, or are simply lazy.

The solution
In order to solve the problem, a programmer must first decide if the error is taking place in the operating system or in the application code. The source code would then be modified to add additional calculations to detect the error and correct for it. Under some circumstances, it may be necessary to convert 2-digit dates into 4-digit dates. This would consume more memory and may be difficult. Another potential problem could be caused by the fact that the software development tools (compiliers, linkers, assembliers, etc) may have been upgraded and new program construction could cause other problems. Those that commented their code and documented their system will be greatly rewarded with reduced modification time.

The world won't end
In spite of all the panic surrounding the year 2000 problem, I predict that many people will return to work from new years eve in the year 2000 and notice either no problem, or that some report now displays silly results. You may even receive a notice that your electric bill is -99 years past due. Phones will ring, Programmers will be summoned, boards will meet, and like normal, problems will be solved.

For additional information you may want to visit Atlanta Year 2000 Users Group, and the Microsoft Year 2000 Resource center at

Answer provided by Roger Arrick.

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Hand soldering PC/104 connector issues

Is there any way to solder the stack through PC/104 bus connectors to a module PCB other than by hand soldering?

The unique self-stacking capability of the PC/104 bus is made possible by the bus connectors. This arrangement is unique as the bus connectors are hermaphroditic interconnects i.e., a single bus connector functions as both plug and receptacle. The long .025 square tails of a bus connector on the solder side of an upper module plug into the receptacle side of the bus connector on a lower module. The basics of a reliable separable interconnection for signal level contacts require a noble metal finish (typically gold) on both plug and receptacle contacts. Therefore, any solder deposits on the mating portion of the connector tails would result in an unreliable connection. This rules out wave or localized solder fountain soldering.

Solder preforms are an option to hand soldering. There are several drawbacks to using solder preform donuts; additional raw material cost, additional labor to install the loose donuts on each connector contact tail, retaining the donuts on the contact tails so that they dont fall off in normal handling, and additional labor to manually apply flux. At this point soldering can be done by directing hot air to the contact tails or sending the connector through reflow. Reflow soldering is only possible if the connector housings is of a high temp SMT compatible material.

PC/104 bus connectors have now been developed with the solder and flux pre-deposited on each contact at the factory. This is accomplished with solder and flux bearing clips that are designed to be economically applied with high speed automation and are also less expensive to manufacture that preform donuts. The connector housing is SMT compatible and oven reflow processing will yield top and bottom side filleted solder joints.

Answer provided by Jim Zanolli of Teka Interconnection Systems

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The Five Steps to Data Acquisition

When selecting the components for your data acquisition system, you must first identify the types of sensors and I/O signal types you are using. Typical I/O types for PC or VXI-based data acquisition systems include: ANALOG INPUT, ANALOG OUTPUT, DIGITAL INPUT/OUTPUT, TIMING INPUT/OUTPUT.

Many types of sensors and signals must be conditioned before connecting them to a data acquisition device. Use signal conditioning hardware for: APLIFICATION, ISOLATION, FILTERING, EXCITATION, MULTIPLEXING FOR HIGHER CHANNEL COUNT.

Criteria, such as accuracy, acquisition rates, number of channels, flexibility, reliability, expandability, ruggedness, and computer platform are used to determine the best data acquisition device for your application.

Cables connect your data acquisition board to your signal conditioning accessory or I/O connector block: SHIELDED OR RIBBON CABLE.

Two of the three data acquisition solution components are the driver and application software: DRIVER SOFTWARE, DEVELOPMENT ENVIRONMENTS AND LANGUAGES.

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Pressfit Style PC/104 Connectors

Until recently, all long tail PC/104 connectors required a through hole solder application. While the few through hole components may be hand or wave soldered, this is extremely difficult and time consuming for the 104 gold plated connector pins. Solder preform donuts or solder and flux bearing clips allow for improvements in the soldering process by eliminating the hand soldering operation. They do however still require a high temperature solder process and close visual inspection. The only other option for applying a long tail stack through connector is with compliant pin technology.

Adopted by the telecomm industry many years ago, pressfit technology has proven itself to be a cost effective alternative to soldering connectors to a board. Modern compliant sections deform to the plated through hole during the insertion process thus creating a reliable gas tight fit. Extensive laboratory testing and years of field operation have proven pressfit connector reliability in demanding environments.

PC/104 connectors are now available with compliant pin technology. This allows for fast and simple insertion of the connectors into the finished boards, without any risk of uneven solder flow or contamination to the gold tails of the connectors. Fast and simple insertion of the connector into the through hole is accomplished by means of a flat rock tool and a simple bottom support fixture mounted on a hand press. The entire press-in process takes less then 5 seconds, with all positions maintaining equal resistance across the hole. Another added benefit of pressfit connectors, is the ability to rework the board by pressing the connector in a reverse process, allowing a replacement connector to be inserted.

Answers provided by David Goodman at ept inc.

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What support is available for using Linux in PC/104 based systems?

Most PC/104 boards that are made with standard desktop and laptop chipsets are going to work fine under Linux. Just watch out for the more obscure interfaces like PCMCIA and SCSI. In the case of "Real world" I/O interfaces, like analog, digital, motion control, etc., support will vary according to the manufacturer of the interface card.

A whitepaper entitled "Using Linux in Embedded and Real Time Applications" is available at:

For more information on using Linux in PC/104 based embedded applications, visit "the Embedded Linux Portal," at:

Answers provided by Rick Lehrbaum at LinuxDevices.Com

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Collecting data from an IP address (Ethernet or Internet) directly into Access, Excel, etc.

To collect data from an IP address directly into Windows applications (such as Excel or Access) you can write a custom application to communicate with the IP address. Because of the popularity of the Internet there are several inexpensive "programmer toolkits" that make it easy to add support for TCP/IP to any Windows application. If you are a hard core Windows programmer, you do not even need any tools; you can simply make API calls to the WinSock DLLs that come with Windows. You just need the specs for the API which are readily available on Microsoft's web site ( If you are a Visual Basic programmer, the Pro Edition of VB5 comes with an ActiveX control called the "MSWinSck.OCX" that makes it easy to add TCP/IP support to VB programs.

If you are not a programmer TAL Tech makes a product called TCPWedge that allows you to collect data from practically any device on a TCP/IP network directly into any 32 bit Windows program including Excel, Access, etc. TCPWedge takes data from IP addresses, on any TCP/IP network, parses filters and formats the data and makes it available via DDE to other applications. It can also fully control devices over TCP/IP networks (Ethernet or Internet). It can support communications with up to 10,000 IP addresses from within Excel, Access and any DDE supporting Windows applications. For more information please visit

Answers provided by Susan Rogers at TAL Technologies

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Making data from my RS232 device available at an IP address on a TCP/IP network

There are "network protocol converters" available that will convert RS232 or RS485 serial data to TCP/IP network data. These protocol converters - also called terminal servers - make it possible to connect practically any device directly to any network. For example, you could attach RS232 device to an RS232 to TCP/IP converter and then connect the converter directly to a TCP/IP network and make it possible for any computer on the entire network to receive data from the RS232 device. Companies with RS232 to Ethernet converters include Lantronix (, model MSS1) and Z-World (, EM1000).

The same functionality can be acheived with TCP/com software, from TALtech Inc, and any Windows PC. TCP/com makes data from any RS232 port (up to 16 ports) available at IP addresses. A free down-loadable version of TCP/com is available on the Free Software page on TALtech's web site at

Answers provided by Susan Rogers at TAL Technologies

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Are the four pads for the PC/104 board standoffs connected to ground or are they isolated?

This issue is not defined in the specification. Therefore, it can (and has been) done either way by manufacturers. Even though there are valid reasons for either grounding or isolation, one will find a mix of both in boards available in the market today.

Answers provided by Bob Burckle at WinSystems

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How many PC/104-Plus (PCI) modules can be placed on a stack?

The maximum number of PC/104-Plus modules that can be placed on a stack is four. This is defined in section 3.2.2 of the PC/104-Plus Specification (version 1.2). However, the number of the boards on the stack may be less due to the number of PCI devices on the base CPU board. For example, a board based upon the Intel 430TX can directly support a maximum of 4 devices. If the base CPU board has the video and ethernet controllers included, then the maximum number of PC/104-Plus modules on a stack is two.

Answers provided by Bob Burckle at WinSystems

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Mixing 3.3V and 5V signal levels

The PCI bus and PCI on-board devices on many CPUs typically use 3.3V signaling (they drive the bus to 3.3V levels which are valid for either 3.3V or 5.0V receivers) and are 5.0V signal tolerant (they can receive either 3.3V or 5.0V signals). Some of the PCI signals are often pulled-up to 5.0V with a resistor. This means your module should be designed with components that are 5V tolerant.

The ISA bus and ISA bus peripherals on a CPU card are different. The ISA bus is driven with 5.0V drivers, thus any board attached to the ISA bus must be 5.0V tolerant. However, the ISA bus receivers will accept either 5.0V or 3.3V signals.

So the short answer to the question is "yes" an add on module which produces only 3.3V signals will work fine if it is also "tolerant" of 5V signals.

Answers provided by Kristin Allen at VersaLogic Corp.

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How can I interface ISA and PC/104 products?

While PC/104 and ISA PC cards are compatible electrically, they have different physical dimensions and different connectors - a pin/socket header versus a card edge. Using a PC/104 to ISA bus adapter card, which integrates both kinds of connectors, the two form factors can have a direct bus-to-bus interface and operates at full PC/AT system speeds without software modification or wait states to communicate. Developers can use these kinds of bus adapters to inexpensively develop and test PC/104 modules on an ISA-based computer or add an inexpensive ISA card to a PC/104 card stack when testing design concepts.

Answer provided by parvus Corporation.

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What tools are available to debug/troubleshoot PC/104 boards?

PC/104 bus line diagnostics boards and POST error code boards can be used to identify system failure points and provide a dynamic view of real-time bus activity. An alternative to traditional ohm-meters, diagnostics boards report bus line status through a series of LED indicators, each assigned to a specific bus signal. Similarly, POST boards display a two-digit hexadecimal code corresponding to the failure point for a PC-compatible BIOS' Power-On-Self-Test (POST) tasks.

Answer provided by parvus Corporation.

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What alternative is there to stacking PC/104 cards?

While PC/104 modules are typically stacked on top of each other, they can also be placed on off-the-shelf or custom baseboards. Such boards may include multiple PC/104 headers to enable PC/104 cards to be stacked on adjacent PC/104 buses rather than on top of each other. For height restrictive applications, this mounting method is especially useful. Four PC/104 boards mounted on a "quad motherboard" with four PC/104 headers measures less than 1.30" in height, as compared to 2.8" for four vertically stacked PC/104 modules.

Answer provided by parvus Corporation.

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How can I connect more than two Ethernet products?

PC/104 systems with multiple processors or peripherals that have Ethernet interfaces can use a PC/104 Ethernet switch or hub board to be networked together. Fast Ethernet and 10BaseT switches or hubs are available with auto-MDI-MDIX network installation and up to five auto-configurable 10/100 transceiver ports. Ethernet connections are made through either onboard RJ-45 jacks or via right-angle Molex connectors that can be used to mount RJ-45 jacks in a faceplate, endcap or enclosure.

Answer provided by parvus Corporation.

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How can I eliminate constant polling of my I/O and still get the data?

Polling of Digital Inputs from contact closures or other events can be a processor intensive operation. Some PC/104 boards include a useful feature called event detection, or change of state detection. This eliminates the need for constant polling and reduces processor overhead by generating an interrupt whenever any of the inputs change. Then the ISR (interrupt service routine) polls the inputs one time to collect the snapshot of the data. This feature typically adds to the cost of the board, but overall system performance can be significantly improved. In systems where the data is changing rapidly, this feature isn't as useful.

Answer submitted by Marty Wingett ( at ACCES I/O Products, Inc.

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