Modbus Scada Communication

[rajsiyer 7]

Diagram_485Link.doc Hi geeks. Will someone please take a look at the diagram below. At the 485 side of the convertor, the D+ & D- wires are paralleled. Problem is, the PLCs communicate with scada, when single. When both are connected we lose contact with the right hand PLC which is 100m. away. The PLC on the left communicates continuously, whether the 2nd PLC is connected or not. I've noticed that when the D+ wire is momentarily disconnected from the convertor and re-connected, the @ PLCs could be visible in scada at the same time. I've noticed this behaviour despite or not connecting a termination resistor at both ends (near both PLCs). Thanks a million to anybody who can figure it out and tell me how to do it right Raj S. Iyer

[PdL 71]

Is the converter suitable for star net configurations ? Not all converters are suitable for this configuration; most need a bus structure for multidrop networks.Is it possible to put the converter (for testing) at either one of the slave ends ?

[rajsiyer 7]

Thanx pDL; I get a strong feeling that you could be right! I'll change the wiring right away and report success if any. Once again I get the hunch that MrPlc will come to my rescue and save the day! Best wishes for your happy day. Raj S. Iyer