TECH TALK

A journal of bus architecture tips & techniques

VMEbus & CompactPCI Connectors: Keep 'Em Straight

If you work with both VMEbus and CompactPCI, you know these architectures specify a large number of pins and numerous pin labels. The two buses specify reversed socket and plug positions. Keeping all this straight is enough to induce an aneurism. I compiled the essentials into the simple matrix printed below. Hopefully, you'll find it handy, but first some basics...

Stick your head inside the card cages of a VMEbus and a CompactPCI system. You'll see sockets mounted on a VMEbus backplane. On a CompactPCI backplane you'll see an array of pins. A socket, also known as a jack, and a bank of pins, likewise called a plug, are designated "J" and "P" respectively.

The two jacks on the VMEbus backplane are denoted J1 and J2. A VMEbus board has mates named P1 and P2. VME64X adds an optional third connector between J1 and J2 called J0. J0 is mostly non-bused and used to pass I/O from the board to the rear of the enclosure. Emerging standards like VITA 41 propose a switched fabric over J0.

A CompactPCI backplane designates five plugs, P1 through P5, which mate to five jacks on a CompactPCI board, J1 through J5. P1 and P2 host the PCI bus and P3, P4, and P5 are optional and mostly used for user I/O. Note that the CompactPCI bus is located at the bottom half of a 6U space whereas the VMEbus is at the top half of a 6U space.

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