What pinout do I use with a Horner Electric RTU interface Module to talk RS-485, 2-wire?

I have a Series 90-30 with a Horner Electric RTU interface module. I am trying to get it to work with a simple PC-based master software package, communicating on the COM1 port and then through a 232/485 converter. The arrangement works with a GE Multilin 469 but not with the GE 90-30. The RTU module RXB light blinks in response to a data request, but returns no data. Can anyone help?

 

Maybe the Com port config on the Horner module doesn't match the master, so while it sees the request electrically, it cannot make any sense of it and so does not respond. I think if this were the case, it might respond to complain about a malformed request, but who knows? Also, is there any harware handshaking on the 485 port? You wouldn't think there would be on a multidrop network, but I have seen controllers that did look at it.