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NetBSD/mvmeppc Frequently Asked Questions

NetBSD/mvmeppc Frequently Asked Questions

General Questions

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General Questions

What are the jumpers on the backplane (top)

The jumpers on the backplane are for Bus Grant and Interrupt Acknowledge. Some VMEbus boards, like disk controllers, do their own VMEbus I/O instead of using DMA from the CPU. The I/O board requests control of the VMEbus, the arbiter on the CPU board releases control of the VMEbus, and then the arbiter on the CPU sends a Bus Grant signal down the backplane.

There are only 4 Bus Grant levels, and you can have several boards, on the same Bus Grant level. The Bus Grant signal daisy-chains down the backplane through each I/O board. If you have an empty slot you break the daisy-chain.

Backplane manufacturers put jumper pins next to the connectors so you can jumper across empty slots. Unfortunately the jumpers may be to the right or left of a slot depending on the manufacturer.

You may also find that some VMEbus boards don't pass on daisy-chain signals that they don't use. This means that you may have to leave jumpers on even when a board is in a slot.

If the boards are side-by-side without an empty slot between the CPU and the VMEbus boards you may not need any jumpers. That might be worth a try.


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