PC-104 Bus

[PC/104 Description] [PC/104 Pin-Out] [PC/104 Timing]
[PC/104 Interface ICs] [PC104 Connectors]
[PC/104 OnLine Standards] [PC-104 Cards]
[PCI Page] [ISA Page]

PC-104 Bus Description

The embedded PC/104 bus uses the PC/XT, and PCAT card specification (IEEE P996), but changes the form factor [board size]. The specification defines the new mechanical foot print and card power requirements.
This is a stacked bus with no backplane or interconnecting cable. The cards plug into each other. A PC-104 card may use an 8 bit interface (PC-XT) or 16 bit interface (PC-AT). Each interface uses its own connector. A PC/104 board must use have a PC/XT interface to support 8-bit operation, or it must have both a PC/XT interface and PC/AT interface to support 16-bit operations. The PC104 bus is AC terminated to reduce power. The pin signals remain the same as found in a desktop personal computer ISA board, except that the PC/104 board uses connectors instead of edge fingers, and the shape of the board is completely different. Because the aging PC/XT and PC/AT buses were no longer able to support the required buses speeds needed by higher speed devices, the PCI bus was added to the PCI/104 standard in 1996.

The PC/104-Plus enhancement allows the addition of the PCI bus onto the card. The PC/104-Plus adds another connector to accommodate the PCI signals. In other words a PC104 card is just like the [old] Personal Computer PC-XT interface card, or PC-AT interface card, and the PC104-Plus board is just like an older mother board with ISA slots and PCI slots. This standard is widely used for embedded computing application even though the PC ISA bus has been obsolete for years now. This PCI bus operates at 33MHz, while 66MHz PCI operations are not supported. When PC/104-Plus boards [PCI] are combined with the PC/104 boards the PC/104-Plus board should be located at the bottom of the stack. One of the PCI boards in a stack will be the PCI Host. The PCI Host board will determine the signaling level; 3.3 volt or 5 volt. However a board may be designed to accept either voltage interface.

PCI-104 is the newest variation of the standard. PCI-104 removes the old PC XT / AT bus from the specification leaving only the PCI bus. The PCI bus connector is a 4x30, 2mm stack-through connector. PCI-104 cards may still reside in a stack with either PC/104 cards or PC/104-Plus cards. However when PCI-104 boards are combined in a stack with other board types they must reside on the top of the stack [because they do not have the XT or AT connectors]. If PC/104-Plus cards are used they must reside underneath the PCI-104 cards. Component height above and below the board has changed. This PCI bus operates at 33MHz, 66MHz PCI operations are not supported.

The card size for each of the PC/104 standards is shown below.

PC104 Embedded Bus Card dimensions

Card size [dimensions] / Board Form Factor

Boards may be stacked 6 high when using the PC-AT [PC/104] bus, but only four of the boards may use the PCI bus, plus the Host board. In many cases the PC/104 stack is encased in a metal chassis. An embedded mother board format called EPIC contains a PC/104 stack area, allowing PC/104 boards to interface with the motherboard.

{PC104 Bus Index}

PC-104 Bus Pin-Out

PC/104 Pin Out ISA [PC-AT/PC-XT] pinouts. [ISA bus Interface]

PC/104-Plus Pin Out PCI pinout [PCI bus Interface]

{PC104 Bus Index}

Data Bus Transfer Timing

The PC104 Bus timing for is derived from the PC/PCAT specification, IEEE P996.
The PC/104-Plus [which adds the PCI bus], and PCI-104 timing is derived from the PCI specification

{PC104 Bus Index}

PC-104 Bus Interface IC Vendors

PC/104 and ISA use standard TTL logic levels for the AT and XT PC bus interfaces. An input Low is 0 to 0.4v, High is 2.4v to Vcc. An output Low is 0 to 0.8v, High is 2.0v to Vcc. The bus drive was reduced from 24mA used on the ISA bus to 4mA. The PC/104-Plus addition to PC104, and PCI-104 buses use normal CMOS devices for the PCI bus interface. Operational voltages for the PCI interface will either be 5 volts or 3.3 volts as determined by the PCI Host. The PC/104-Plus standard defines a maximum of 39 watts per board [total from all supply voltages].

Integrated Circuit manufacturers listed on this site:
PCI Chip Manufacturers
Memory Chip Manufacturers
Micro-Processor Manufacturers
PC Chip Set Manufacturers
Glue Logic IC Manufacturers

PC-104 Online Standards and Specifications

PC/104 and PC/104-Plus Specifications {VersaLogic Corp.}

PC/104 and PC/104-Plus Specifications {PC104.org}

PC/104 Spec ver 2.5, and PC/104-Plus ver 1.0 {PC/104 Consortium}

PC/104 Specification Version 1.0; March 1992
PC/104 Specification Version 2.1 / 2.2; July 1994 / September 1994
PC/104 Specification Version 2.3; June 1996
PC/104 Specification Version 2.4; August 2001
PC/104 Specification Version 2.5; November 2003
PC/104-Plus Specification Version 1.0; February 1997
PC/104-Plus Specification Version 2.0; November 2003

{PC104 Bus Index}

PC-104 Bus Connector Manufacturers

The Pin-and-Socket connectors used with the PC104 bus have .100" spacing, and may be 'stack-through' (pins extend out the other side of the PWB), or 'non-stack-through (normal PWB mounting).
The 8 bit XT card uses a 62 pin connector, The 16 bit AT version adds another 40 connector.
The PCI side of the card for PC/104-Plus or PCI-104 uses a 4x30 (120-pin) 2mm pitch stack-through connector.

Power supply in a PC104 format
PC/104 Card Stack

ept {PC104 Connectors-PC104 Plus Connector Manufacturer}

Teka Interconnection Systems {Solder Flux Bearing Lead Connectors}

Tyco Electronics {PC/104 Connector Manufacturer}

{Back to PC-104 Bus Index}

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Modified 6/13/15
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