Troubleshoot Modbus communication problem between DCS / SCADA and third party devices.?
For hardware problems ,You need RS-232 break-out boxes, RS-232 to RS-485 converters, continuity testers, voltmeters to trace the signal between master(DCS/SCADA) and slaves(third party devices)
If it was not a hardware problem you should use a Protocol Analyzer to check for software problems.