Thank you very much for the help. Now it is very clear to me, all the configurations,communication methods and the modbus commands.

For your info, we are using our own RS232 to RS485 converter. In this we are using SN75176 from TI for the RS485 convertion. In this chip, pin number 6 is used for " A " and pin number 7 is used for "B". That is the same we have connected to multirangers A and B terminals. We have made two of this dongle and tested sending and receiving data across, it worked fine.

But as you said if the naming convetions are not standardisd then there could be communication problem because of this. 

-> So one of my check is, swaping the terminals and test.

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Dear Chook

Thank you very much for suggesting the ModScan32 tool. It is very useful for our testing.

Regards

Rathinavel

Here is the summery of todays experiement in the field.

->As prepared I have started  my test one by one; First was to remove the extra '4' in the address field and check. but the response was same.. no reply from Multiranger; then I Swaped the A and B line of RS485. This cleared the commnunication. We got the response back. 1st success .

-> Still the response was not the expected value... thought of trying with different Modbus driver, so used ModScan32 tool... It started working. We did little analysis to understand the scenario. Then we found that ModScan automatically minus 1 from the register numer to get the addreess (means to access Register-1010, the address used will be 1009). 

->  Next step, we went ahead to read the parameter which we were interested (P925). We directly send a read command (03) to read register (4)4925; which did not give the correct result. We modified the primary index value in (4)3999 and secondary index in (4)3998 then we got the valid result. 

-> Mean while we faced a small problem in write register which did not put much effort to analysis it; the problem was the command 06 (write single register) did not work for us; but command 16 (write mulitple regiter ) worked well.

Regards

Rathinavel