Garbled communication response from Modbus secondary(9):Incorrect Secondary Response

[dfabsher]

While performing single register writes to a SCADAPack PLC using Modbus RTU protocol, I  consistently recieve

"Garbled communication response from Modbus secondary(9):Incorrect Secondary Response"  and the communications fails.

The serial port diagnostic file is as follows:

18:39:11.5 - Station5_RTU ->
[05][06][00][C8][00][00][09][B0]
Station5_RTU - RTS low
18:39:11.7 - Station5_RTU <-
[05][03][16][01][0B][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][02][C9][5E][C7]

I have tried changes to the Modbus.ini file with no successful results.

Any hints would be greatly appreciated.

David

[Ryan.S]

Do you have any other software that can communicate with this PLC by Modbus RTU protocol?

If yes, you can compare the sent/received codes and find something difference.

[tkirk]

It looks like your diagnostic file is only logging the responses and
not the requests, since 05,06,... is either a request or response to a
function 6 (single register write, of register 40201 or 200 depending
on your numbering system), and 05,03,16,... is definitely a response
to a function 3 (multiple register read of 11 registers).

Try using Modbus Test Pro to talk to the ScadaPack to ensure the
ScadaPack is configured properly, and not acting as a Modbus Master
rather than a Modbus Slave. There is also a possibility that the
requested function 6 register is not a valid register on the
ScadaPack.

The ScadaPack is actually one of the devices we use to test Modbus
Test Pro against. Download it for free (shareware) at

http://www.rawiq.com/mbtestpro.php

Hope this helps!

Tim Kirk
tkirk@rogue-engr.com
www.rogue-engr.com