Richard,
It sounds like making a Logix that will track those blocks around the
crossing and activate the Light-It when needed. Depending on the detector
wiring, like how long the blocks, does the crossing have an 'island' around
the gates, is it single track or multiple, etc... All of those come into
play.
Classic three block setup would also keep internal sensors for the state of
which direction. If the west block tripped first, you would set the west
direction and trigger the signal. Clearing the signal would be when all
blocks are clear OR west direction set and the east and island detectors
clear OR east direction set and the island and west detectors clear. There
are a couple of more statements needed, but this is the idea.
-Ken Cameron, Member JMRI Dev Team
www.jmri.org
www.fingerlakeslivesteamers.org
www.cnymod.org
www.syracusemodelrr.org
Ken Cameron