Dotcomrecycling.com has received many inquiries regarding Compact PCI, ranging from "What is it?" to "Why the hell would I want it?", to "Why me?". After forwarding the third question to "Dear Abby", we worked on the others. We searched the Web for information that would make us appear knowledgeable, or, with luck, even brilliant. Unfortunately, we failed in our endeavor. There seems to be a dearth of introductory cPCI information, unless maybe if one wants to read the standards and draw their own conclusions. No thanks! So we changed our strategy and gathered the information we needed from our customers…success!

We’ll pass on as much information as we can to you in our little FAQ. This FAQ is intended to be a tutorial for people considering purchasing a Compact PCI system. However, the information below comes with no guarantees – use at your own risk!

Compact PCI (cPCI) is basically a re-engineered PCI bus specification designed for high-reliability and/or high-density applications such as telecom or network switches.

For reliability, most cPCI chassis are built with redundant hot-swappable fans and power supplies, and all cards are usually hot-swappable. CPUs have excellent airflow, so CPU fans are not required. Failure alarms are commonly built into the chassis. Each card is held firmly in place over Euro-style shrouded multi-pin connectors with many ground connections. NEBS compliant chassis are common.

For density, chassis can normally fit up to 16 cards side by side on a 19" rack, with space to spare for power supplies and hard drives. Some cPCI cards can fit up to two "PMC" mezzanine cards for telecom I/O, SCSI control, Ethernet, etc. Each card also has pass-through connectors for a rear transition card, which allows for a large number of connections coming out the back and/or front of the chassis for each card.

The latest cPCI bus specification is for a 66 Mhz, 64 bit bus. The cPCI specification also defines an optional independent set of bus connectors, which is often used for computer telephony (H.110 standard). H.110 includes such goodies as ring voltage, battery voltage and a time domain multiplexed bus for 4096 voice grade (64 Kbit/s) signals.

CPCI doesn’t seem to be intended for high speed single processor systems. For one thing, the cards are small and processor cards almost always have components filling both sides. High speed CPU cards alone can bust a budget! However, cPCI seems well suited for parallel processing of a multitude of data streams.

One slot of each bus is a system slot, usually identified by color coding differences somewhere. A system controller CPU card goes into this slot and controls the cPCI bus much like a PCI motherboard.

Other slots are peripheral slots and can accept peripheral cards, which can be CPU, DSP, I/O or other. The peripheral cards can be bus masters but do not supply the cPCI clock signals. The peripheral cards have access to the H.110 bus on compliant systems.

It is important to note that some CPU cards can be system controllers or peripheral controllers, jumper selectable, but most are one or the other. Also, not all cards are compatible with all chassis… please read on in this FAQ.

For the most part, yes. Sun cards run Solaris, and Pentium cards usually run Linux and Windows with no problem. PowerPC cards are commonly used too.

Disk drives are usually slightly harder to install than in PC systems. Some chassis have built-in slots for SCSI drives and cabling. These usually use SCSI SCA drives for hot-swap capability, sometimes in a RAID configuration. Pentium boards often have IDE interfaces only, and the cable is generally snaked from the CPU rear transition card through the chassis to the drive. Alternatively, one could plug a SCSI controller into a PMC slot, or some of the Pentium cards have a 2.5" HD in place of a PMC slot.

Many peripheral processor boards and some system processors boards (i.e. Sun), do not have a VGA controller built in. Solaris and Linux of course don’t have a problem with this. VGA cards are available for peripheral slots, or in PMC configuration (PMC is not cheap!).

The full (6U) cPCI system uses 5 connectors for each slot position labeled P1-P5. Not all connectors are installed in each position. Connector P1 is bussed and carries 32 bit PCI. For 64 bits, P2 is also bussed. The rear transition cards generally do not have access to these busses.

P3 and P5 are usually used to carry signals from each cPCI card to its matching rear transition I/O card. On the system slot, P4 is usually also used for this purpose. On H.110 compliant systems, P4 is bussed between the peripheral slots for the H.110 computer telephony signals.

For your protection, each slot has two locations for color coded keys, one in the P1 connector, and one in the P4 connector. CPCI cards have matching color keys so they ideally can’t be put into an incompatible slot. However, some early boards and non compliant boards and backplanes do not have any keys installed, so beware. It can be very bad to put a system controller into a peripheral slot on an H.110 backplane!

The voltage which cPCI cards will use to communicate in a system is set by the I/O voltage on the backplane. The backplane "V I/O" bus is connected to the power supply at either 3.3V or 5V. The color coded key in the P1 connector should indicate the I/O voltage. Brilliant Blue is for 5V, Cadmium Yellow is for 3.3V.

The existence of an H.110 bus is indicated by a Strawberry Red key in the P4 connector of the peripheral slots. A Net Brown key in the P4 connector indicates that the connector is usable for rear I/O. Unfortunately, a missing key in the P4 position generally tells you nothing.

Although some backplanes with 14 peripheral slots exist, it is much some common to see split backplanes when there are more than 7 peripheral slots. The most common split backplane has two sets of 8 slots, with the two system controller slots in the center. A special system controller with a PCI-PCI bridge fits into both the center slots. Of course, this type of backplane may be used to host two independent systems, with shared power supplies and often shared chassis-based alarms.

If you’re going to be using your cPCI chassis for anything besides general purpose or computer telephony, we highly recommend that you buy at least for your backplane card new from the manufacturer. There are many specialized backplanes available for things such as Packet Switching and Serial Mesh that don’t show up in the secondhand market much.

If you’re buying used, please consider:

Input power: AC (Convenient) or DC (Good for data centers with battery backup). If you need both for development/deployment, you can also buy a DC chassis and connect it to an AC to 48VDC power supply (with or without battery charging capability).

Form factor: If you only need 4-5 slots total, you caqn choose a tower style computer with handle on top, or a horizontal board rackmount. For 16 slots, or 8 slots with lots of space for floppy/HD/CD, units are always rackmountable. Steel chassis with redundant everything can weigh more than 100 pounds. Aluminum chassis are much lighter, but are less common and don’t usually have the beefy power supplies.

Disk drives: Some chassis have slots built in for RAID system drives. Some chassis have slots for 2-4 hot swap SCSI drives. Others have space for an internal hard drive (usually IDE), making them impossible to hot swap.

Backplane: The I/O voltage choice of 3.3V or 5V can usually be modified user at a later date. However it can be a little complicated. It’s best to choose your chassis I/O voltage based on the requirements of any specialty cPCI cards you plan to use. Choosing the H.110 bus will make the P4 connectors on all the peripheral slots unusable for rear I/O, but this is rarely a problem as most peripheral cards do not need more than P3 and P5 for rear I/O. In general, the P4 connector on the system slot is coded Nut Brown for rear I/O. But it is important to double-check both processor card and backplane before power-up!

Number of slots: Well, this shhould be pretty obvious. Remember that many I/O functions are available on PMC add-in cards and may not require a slot.