Re: VME bus confusion

[mooney@aps.anl.gov (Tim Mooney)]

re...

> 1) There MUST ALWAYS be a board of some kind in EVERY slot of the VME
>    crate starting on the left and working right.  If you MUST have
>    a missing board, you may simulate it by installing the BG and IACK
>    jumpers often found on the VME backplane.

Part of the responsibility for this confusion is mine.  Most, but not
all, of the full-size crates we've purchased for beamline controls, and
loaned out to developers, are equipped with "bus-grant" P1 connectors,
which contain teeny-weeny little switches that pass daisy-chained
arbitration signals down the bus when no card is installed in their
slot.

For these crates (Mupac model 5099KDL20FK-100), you CAN leave empty
slots between modules, and this has given many beamline-controls
developers the impression that *all* VME crates operate in this sane
manner.

Tim Mooney