Hi Stu,
I got a personal message from you but I thought this is a better way to answer your question.
Here I give you my setp and connection of ERC-M and RRC.
RRC Setup
COM1 (ERC) Mode-3, 9600, 8, 1, 0, cts/rts(no), terminator(00)
(You can change buad rate of ERC with setup software of ERC, Service Tool)
Computer - Control RRC
     pin2 - pin2
     pin3 - pin3
     pin5 - pin5
ERC-M  - Radio RRC
     Tip - pin2
     Ring - pin3
     GND - pin5
Good luck.
Joe, HL5KY.