Reson8 strobe wiring?

I have just changed an old bell box on my Santronic 9200 alarm system. The new sounder is a Reson8 bell box. All is working ok except the strobe. I have a problem with the wiring.

The old strobe (An addition to a bell box) was wired with two wires direct from the 9200 circuit board.
The reason8 has one connection for a strobe (-ST) but what do I do with the second (Return?) wire.