Modbus RTU Baud rate issue .

Dear All,

I am facing the issue for Modbus RTU RS-485 Devics communication with Modbus RTU Master .
BPS 2400 is working ok, but change BPS 9600 showing communication issue with Master & Slave .
how to resolved this issue .Please help
customer test with modscan software working ok 9600 bps

Sandip Ahire


There was a very similar issue encountered not too long ago in the following thread. Please follow the suggestions there first.

If ModScan is working successfully at 9600 with the Modbus Slave, then it is likely an issue or misconfiguration of your Modbus Master. Make sure to power cycle devices after changing the baud rate and confirm that the other settings such as parity and number of stop bits match.

Now, your packet capture images do seem to both show complete, valid Modbus packets. So judging by that alone, communication seems to be working successfully. What error(s) or indication(s) are you seeing that leads you to believe there are communication issues at 9600 baud?
Hi Sandip,
As per jschulze post - check all the basics, etc.
Not all modbus clients are created equal.
Depending on your modbus master you may be able to preconfigure delays between transmissions. I have found a couple of modbus clients that will not communicate consistently unless there is a 20-50ms delay.
Your Modscan test may be a single transmission request & response only - so delays are not a factor.
Cheers, PB
It -can- also be a hardware issue. If it is a short distance (1 meter or so) it is ok, but if you have more slave(s) a proper 120ohm impedance corrected cable with 120ohm terminator resistor might help. A normal 120ohm 1/8W resistor between the A and B line where the modbus ends will do.
This is a very good 120ohm impedance corrected cable from Lapp we use for all our applications and all our customers also use this one for Modbus. At Conrad you can even order it per metre: