TECH TALK

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Crash Course: CompactPCI Voltage (I/O)

Have you ever turned on your CompactPCI chassis for the first time to find some boards didn't power up? It is not uncommon to discover that the cause is a missing Voltage (I/O) jumper. Not only is this very irritating, it's compounded by a lack of documentation. Many times, the CompactPCI enclosure is shipped with the V(I/O) jumper and keys uninstalled leaving you responsible for properly configuring the V(I/O). The following provides background on V(I/O) you should know to solve your dilemma.

Backplane Construction & V(I/O) Jumper

The P1 and P2 connectors have power pins for +3.3V, +5V, +12V, and -12V plus eleven additional pins called Voltage I/O, labeled V(I/O). The V(I/O) pins are configured to 5V or 3.3V to power boards with buffers that drive the backplane signals. However, CompactPCI specification requires that boards with PCI I/O buffers draw power from V(I/O) pins - not from the 5V or 3.3V power pins.

Within the CompactPCI backplane, there is a V(I/O) power plane. This plane is not directly connected to power but is powered by physically connecting a jumper from the V(I/O) plane to either the 3.3V or 5V power planes. With no jumper in place, the V(I/O) pins are 0 volts leading to power up problems for boards looking for power on these pins. Although some backplanes provide multiple jumper positions, only one V(I/O) jumper should be used.

V(I/O) Keying

CompactPCI implements a keying mechanism to differentiate between 5V and 3.3V signaling. This is designed to prevent a board requiring 3.3V or 5V from being inserted into a system jumpered to the other voltage. Keys are inserted into the P1 connector. Blue keys indicate 5V and yellow keys, 3.3V. Every slot needs a key of the same color. Do not attempt to plug a 5V board into a system jumpered to 3.3V or vice versa. You cannot mix the signaling!

Fortunately, the market is providing universal boards that may operate in either 5V or 3.3V systems. These boards are not keyed. However, they still require a V(I/O) voltage. To avoid a headache, ask the backplane manufacturer to jumper and key your backplane before shipping. Take time to understand the voltages required by your CompactPCI boards.

Contact me if you would like further information. I have complete instructions on configuring your CompactPCI backplane's V(I/O).

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