SL503 Comm with Honeywell Experion DCS on Modbus using Prolinx

My colleague is commissioning a system (at remote place where he doesn't have access to internet) involving SLC 5/03 to Honeywell Experion DCS communication on Modbus RTU. The SLC DH485 port is connected to DH485 to Modbus Gateway (Prolinx unit from Prosoft). The communication parameters are as follows -
Baud rate: 9600
Data Bits: 8
Stop bits: 1
Parity: Even
SlaveID: 1

When the RS485 wires meant for conection to DCS are connected to DCS end (RCS5000) the DCS doesn't communicate with SLC even though all the parameters are set properly to match those on Prolinx side. But when the same wires are connected to a laptop using RS232/RS485 converter (ADAM4520) the communication takes place properly with KEPServerEX and Modtest softwares. He tried Hyperterminal to check what frames DCS is sending but since the protocol is Modbus RTU and Hyperterminal display is ASCII he is not able to decode the data. I have also told him to try RS232 port on DCS instead of RS485. I am trying to send him Modbus Parser utility also so that he can decode the modbus frames from DCS

Has anybody tried a similar communication between SLC and Honeywell Experion DCS using Prolinx Gateway (not the rack-mounted MVI card)? What could be the problem in the above case?
What more information is needed from him?

Thanks in advance

SVN

Modsim by Wintech (software) can emulate a Modbus slave device using a standard laptop and a RS232/RS485 converter. I think I would benchtest the Honeywell in that respect, as well as recheck the polarity of the cabling. I have run into a few companies that have interchanged their RS485 +/- markings--

Do you have a screenshot of the RCS5008 configuration?

Do you have a screenshot of the Quickbuilder configuration?

Do you have a pinout of the cable b/w Prolinx and RCS5008?