Our signals are controlled by SIGM20 LocoNet signal controllers. These units work out the correct signal aspect from the block occupancy sensors, the positions of points and other signal aspects. For a manually operated train this would generally be fine. Sometimes, though, Traincontroller isn't ready to let a train go but the track ahead is clear. This could be if the exit from a block is locked, for example. Under those circumstances, it would be good to make the signal stay red until Traincontroller was ready for it to go. In Traincontroller this is indicated by a block's "block signal" being red.
A SIGM20 signal can be forced to red by settings its DCC accessory address to THROWN. When set to CLOSED, it chooses its aspect according to the track ahead. Traincontroller can cause that "make signal go red" to happen.
It's easy to do. Add a 2 aspect signal to your switchboard, and set the DCC address to the signal's DCC address. For block "green 3" the corresponding SIGM20 signal is DCC 221. Make sure the signal is configured to be green if closed, and red if thrown.
To set its state from a block signal: Set the Traincontroller aspect to be red if the accessory is thrown, and green if closed. Choose the "Trigger" tab; choose the "red" aspect. In the left window select block "green 3". In checked elements on the right, selected "checked" at the bottom, expand "main signal" and select "calculated block signal to the bottom is stop/red".
Then choose the "green" aspect, and choose the block signal you want to make it green (in this case green 3, at the downward end).
Now, that signal will be forced to red if the Traincontroller block signal is red. That's all you need to do!