PC/104 technology questions and answers

Since the PC/104 Embedded Consortium gets a lot of inquiries about PC/104 technology, I decided to publish a few of the questions and answers to help others who may have the same questions.

Let me start with a couple of general questions we often receive: Can I use PC/104 for an avionics application? Does anyone make a [insert your favorite device] in PC/104 architecture? What types of enclosures are available for PC/104? To answer these types of questions, we direct people to the product section of the PC/104 Consortium website, www.pc104.org.

Here are a few more specific frequently asked questions.

How many cards can I put in a PC/104, PC/104-Plus, or PCI/104-Express stack?

Like the ISA specification, the PC/104 specification does not have a limit on the number of cards in the stack. Although I have heard a typical number of six, I have seen systems with as many as 12 cards operate successfully. It really depends on the drive strength of the CPU and the loads on the peripheral cards. System problems can generally be traced to boards that don’t follow good engineering practices.

Unlike the ISA specification, the PCI specification has very specific limits for the number of cards on the bus. The PCI implementation on PC/104-Plus and PCI-104 has a limit of four cards in addition to the host CPU. This is due to the PCI load specification and enforced by the fact that the bus only has four DEVSEL signals. Cards have switches to select which of the four slots they occupy.

PCI/104-Express supports x1 PCI Express (PCIe) boards, USB 2.0 boards, and PCIe x16, which can be used as x8 or x4 with host support. The bus can have one x16 PCIe, four x1 PCIe, and two USB. While these can be in any sequence, it is good practice to keep the x16 PCIe close to the host CPU. PCI/104-Express has one additional feature: The card can use a PCIe packet switch to replace the link designers are using, thereby increasing the number of cards in the stack.

Where can we find timing specs for the buses?

The PC/104 Consortium recommends the following books and specifications:

  • ISA and EISA Theory and Operation by Edward Solari
  • ISA System Architecture by MindShare, Inc.
  • PCI and PCI-X Hardware and Software (Architecture and Design Library) by Edward Solari
  • PCI System Architecture by MindShare, Inc.
  • PCI Local Bus Specification Revision 2.2 by the PCI-SIG. Contact the PCI-SIG office for the latest revision of the PCI specification (e-mail: administration@pcisig.com, website: www.pcisig.com).

Should a CPU provide power for all cards on the bus?

Obviously, system power must come from somewhere. The PC/104 specifications define +3.3 V, +5 V, -5 V, +5 V standby, +12 V, and -12 V power on the various buses. It is rare for a PC/104-compliant CPU to supply any power due to its limited size; however, an EPIC or EBX board can pass through the power from an external ATX power supply to the other boards in the stack. A compliant CPU is typically coupled with a PC/104 power supply board that supplies system power.

In practice, the power required in the system is determined by the cards in the system. Many cards operate on +5 V only, while others require all of the supplies or a subset of the supplies. Designing a system with less than all of the supplies requires that designers select the appropriate supply and know all of the boards in the system as well as their power requirements.

Why does the connector show a variance of the depth it can be plugged in, but the standoffs get very little flexibility?

The board stack-up drawing should have tolerances in it. The standoffs are a fixed size. Variance in connector dimensions allows connectors from many companies to be used with the standoff. PC/104 connectors are specifically designed for 0.600" standoffs. All of the PC/104 Consortium’s specifications include the standoff as 0.600" ± 0.005".

Why is the thread of the standoffs on the PC-104 multiboard stacks 4-40 and not metric?

The specification does not define a thread for the standoffs. The standoffs are 4-40 because the specification was originated in the U.S. and the thread, like all other measurements, was in U.S. dimensions.

Should the mounting holes be grounded?

The PC/104 specification does not require that the mounting holes be connected to digital, analog, earth, chassis, or any other ground. It also does not prevent the mounting holes from being connected to ground. This is left to the system integrator/end user.

The bottom line is that some PC/104 manufacturers ground them, others leave them unconnected, and others provide an option to ground them. Because the issue of grounding the pads was never addressed or specified, then it needs to remain undefined.

PC/104 Consortium 916-270-2016 info@pc104.org • www.pc104.org