PC/104 Technical Information

This information is Copyrighted. Please do not copy it.

This information should help you understand some of the technical aspects of PC/104 controller boards. For additional information about the PC/104 specification, we suggest that you contact the PC/104 Consortium.

Pinouts for the PC/104 connectors are now on-line.

Also, see this overview of the ISA and PC/104 bus by Mark Sokos
and this Embedded Systems Tutorial by Joe Reeder

What is PC/104?

PC/104 gets its name from the popular desktop personal computers initially designed by IBM called the PC, and from the number of pins used to connect the cards together (104). PC/104 cards are much smaller than ISA-bus cards found in PC's and stack together which eliminates the need for a motherboard, backplane, and/or card cage. Power requirements and signal drive are reduced to meet the needs of an embedded system. Because PC/104 is essentially a PC with different form factor, most of the program development tools used for PC's can be used for a PC/104 system. This reduces the cost of purchasing new tools and also greatly reduces the learning curve for programmers and hardware designers.

The PC/104 form factor was developed by Ampro Computers in California in the late 1980's. The specification was published in 1992 in order to enhance popularity. Now over 150 vendors manufacture PC/104 compatible products including controller cards, software, and accessories.

The PC/104 Consortium was established to maintain the specifications, publish a resource guide, participate in standards activities, and to promote PC/104 at trade shows and through news releases. We encourage you to contact them for additional information.

The Embedded IEEE-P996 Specification

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 http://stdsbbs.ieee.org/products/catalog/drafts.html

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.

Mechanical Dimensions

technical drawing

Using PC/104 Modules

 Single-Module PC/104 Applications

Some applications can be developed using a single PC/104 card that contains a CPU, program memory, and I/O ports. The finished product includes the PC/104 module, the user's program, power supply, I/O devices, and cables.

technical drawing

 Stack Modules Together

PC/104 modules are designed to be stacked instead of using a backplane. There is about .6" between each module. Many designers use off-the-shelf modules for CPU and other common functions, then create their own module to perform a specific task. When stacked together, a custom controller is created with minimal effort.

technical drawing

 PC/104 Modules as Components

PC/104 modules can also be plugged on to a larger circuit board as a high-integration component. This allows the designer to use off-the-shelf modules instead of incorporating the chips and other components necessary for the desired function. Overall effort and time-to-market is greatly reduced.

technical drawing

Electrical Specifications

PC/104 bus signals are identical to the standard PC bus in definition and function. Pinouts for the PC/104 connectors can be seen on-line. 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.