The "Bus" LED on the CTD shows when there is bus activity. A short heartbeat is shown every 5 seconds. When a module 'wakes up' the bus, the LED will be red. But it must turn off after a few seconds. However, if this "Bus" LED is continuously lit, this is probably due to a contradictory configuration or caused by a (broken) module on the bus.
In order to eliminate this last option, simply disconnect all bus wires and check the led...
A possible contradictory configuration can be: An input that is set as "Normally open" or "Normally closed" can not be forced. The same applies to an output that is connected to a detector. Or conflicting logic was used... This continuous bus communication must certainly be remedied.