I've got a PLC that handles alarm notifications. This PLC runs only 2
things - a 120DB horn, and the brightest strobe light you've ever seen
in your life (OK, some of you guys have probably seen something
brighter, just not me ;-).
When a NEW alarm comes in (which is a output BIT that writes to a PLC
control register), the PLC clears that bit, resets a couple of latching
relays, and then sets off the horn, and/or the strobe. This happens
every time this bit is written as a '1'. As long as the alarm bit in
EPICS is set, I don't want to output the bit to the PLC again. Only
when the bit is initially set to a non-zero do I want to write to the
PLC.
