Hi Forum Members,
To day I started checking the problems about RS485.

The AC Drives are way far off from the Motors. The Cables from the Mains Supply is armoured cable till the Control Panel Mains Switch; But from there on it is unshielded till the AC Drive. The panel is about 7 feet high, and has only 4 AC Drives spaced apart by about 2 feet between them. The Terminal Strip is at the Top Side of the Panel, All the Incoming and Outgoing cables (Motor, AC Supply, RS485 etc.) are entering the panel from the Top Side and connected into the Terminal Strip. Till here the incoming and outgoing cables of the Motor and the AC Supply are armoured and connected to Earth. But from the Terminal Strip to the AC Drives they are going as per their own free will to the AC Drives, i.e. there is NO particular way. They are just randomly placed inside the channels to find the shortest route to the Source/Destination.

I placed the CAT5 cable, used the twisted pair from this CAT5 cable, and taken one more wire from RS232-RS485 Convertor Ground and goes till the other end of the cable, but it is left hanging over there because there is NO GND Terminal on those devices.

All other instruments DO NOT have a GND Terminal for RS485. But the AC Drive is having a SIGNAL GROUND. I have verified that the drive does NOT have any optos.

I tried by Connecting the GND of Convertor to Signal GND of AC Drive, but the communication fails.

I have found one peculiar thing that might help us. Till the AC Drive is in the STOP Mode, the RS 485 network is quite. But as soon as I put the AC Drive in RUN Mode, I start getting some random charachters on the RS 485 network. I checked this by starting Hyper Terminal and my screen gets filled with junk charachters in no time.i.e. I start recieving random charachters as soon as the AC Drive goes into RUN Mode. And as soon as I put the AC Drive in Stop Mode, these random charachters stop coming into Hyperterminal.
My feeling is that this RS485 network is not terminated with 3 resistors as mentioned by Erik (and in 10 ways). Because at this time(when testing with Hyperterminal) there is NO Master on the RS485 Bus.
I tried to implement this 3 resistor termination in the convertor, but the damned convertor, stopped working. I have asked for the circuit diagram of this convertor, but I think building one, out the spares I have in hand, will be a better idea than modifying the damned convertor.
Anyways I have 12 hours to build my own convertor.
If my todays work clicks some idea, then please inform on this forum.

Thanking you all,
With best regards from,
Mr. Kiran V Sutar.