I am communicating with Ge-fanuc Versamax PLC.(point to point)
MMI is giving keyswitch(88 NO.) inputs to PLC and PLC switches required outputs such as contactotrs on (for various motors),solenoid coils. Also MMI gets input from PLC corresponding to status of various outputs and proximity switches which is displayed by lighting LED's(128no.) on MMI.
I am doing this by downloading/uploading 16 8-byte registers to/from PLC.(using modbus protocol on RS485)

I am using MAX488 for this purpose.
Ihave connected res. of 120 ohm between my receiver end.
Also used bias resisters of 1k from ground and vcc.
I am using cat-5 cable having 4 pairs of twisted wires , 2 pairs of which are connected to trasmitter and receiver outputs. 1 pair is used for connecting grounds of MMI and Plc. and one pair wires are connected to shield output of PLC while other side of that pair is unconnected.
Eric,
2) are you terminated in both ends

What termination is required at driver side?

I am using 5v SMPS as a power supply, inwhich Gnd is connected to Earth.
Also i am using 4-16 decoder to scan Keys and displaying LEDs.