On 11/22/2011 5:59 AM, Brett Frankenberger wrote:
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. -- Brett
Indeed. All solid-state controllers, microprocessor or not, are required to have a completely independent conflict monitor that watches the actual HV outputs to the lamps and, in the event of a fault, uses electromechanical relays to disconnect the controller and connect the reds to a separate flasher circuit. The people building these things and writing the requirements do understand the consequences of failure. Matthew Kaufman