Hi Forum Members,
Here is yet another situation for RS485. Please give your suggestions about it.

Background:
I have developed a software (Windows based using VB), that communicates with the Instruments connected over a RS485 cable using Modbus Protocol. The Software works fine in our office, No problems at all.
This Software is installed on a PC at the customer site (about 600 Km from here). I had visited this site a few months ago, and now I am visiting this place again on this Friday.

During Previous Visit:
There are 10 instruments in a Control Panel, Mounted close to one another. These are connected by using 4 core shielded cable for RS485 (2-wire mode). The cable length between 2 instruments is more than 6 inches (done that purposely...I read it somewhere that it is required to be more that 6 inches). These Instruments do NOT have a Signal Ground or Earth Terminal. So the sheild is shorted between the incoming and outgoing cable, but NOT Grounded. And yes it is terminated using a 100E resistor at the end of the cable. The cable runs from one instrument to another and it is ensured that there are maximum 2 wires at each terminal of Instrument (As per "10 Ways of Bulletproofing RS485").
The Cable comes out of this panel and enters another Panel that has 4 numbers of AC Drives having RS485 Port each. Similarly the cable runs into this panel and connects to each of the AC Drive.
The cable comes out of this panel and comes to the PC about 5-6 meters away and gets connected to the RS232-RS485 Convertor and then connects to the PC Serial port. The Convertor has 100E terminating resistor and Also a Convertor Ground Terminal (But this is NOT Earth Terminal) The Shield that is Continuous (electrically, but with physical break/shorting at each instrument to maintain continuous shield) from one end to the other and Connected to the GND Terminal of Convertor.

With this system The communication with Instruments and AC Drive works fine. But when the AC Drive starts, the communication fails and starts giving random characters instead of data. If I connect the cable to the Control Panel containing the Instruments only then it works fine even if the AC Drive is in the RUN Mode. But if the cable is connected to the Control Panel containing the AC Drive, then the problem is seen again when AC Drive is in RUN Mode.

Please give your suggestions regarding this situation.

Thanking you,
With Best regards from,
Mr. Kiran V Sutar.
Mumbai, INDIA.