----- Original Message -----
From: "Brett Frankenberger" <rbf+nanog@panix.com>
The typical implementation in a modern controller is to have a separate conflict monitor unit that will detect when conflicting greens (for example) are displayed, and trigger a (also separate) flasher unit that will cause the signal to display a flashing red in all directions (sometimes flashing yellow for one higher volume route).
So the controller would output conflicting greens if it failed or was misprogrammed, but the conflict monitor would detect that and restore the signal to a safe (albeit flashing, rather than normal operation) state.
"... assuming the *conflict monitor* hasn't itself failed." There, FTFY. Moron designers. Cheers, -- jra -- Jay R. Ashworth Baylink jra@baylink.com Designer The Things I Think RFC 2100 Ashworth & Associates http://baylink.pitas.com 2000 Land Rover DII St Petersburg FL USA http://photo.imageinc.us +1 727 647 1274