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Tuesday, August 21, 2018
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MODBUS RTU Slave Communication Issue
MODBUS RTU Slave Communication issue with MODBUS Master
By Praveen Gonsalves on 21 February, 2018 - 4:43 am


I'm working on a project using Texas Instruments TM4C123 Launchpad which will act as a slave. The firmware is written and tested using Master simulators.

The code is working fine for the following Master simulators:
1. Modscan32
2. ModbusMat
3. Modbus Poll
4. Simply Modbus Master
5. Mdbus Master and Slave by Farrellton Solar

My issue is, I purchased a Modbus Master Device which polls data from my Modbus Slave. When I connect my Modbus Slave to the Master, I don't get data. The Same Modbus Master works well with other Modbus Slave devices.

When I do step by step debugging, I see that my code get stuck in loop where it wait for data to come on Modbus. But when I run using simulator, it never gets stuck over there.

Also I've measured the voltages on both the devices which are as follows:
Master Device
TX+ to GND - 1.7V
TX- to GND - 1.5V

Slave Device
TX+ to GND - 0.728V
TX- to GND - 1.727V

Is the difference in voltage related to the problem?

Can somebody explain why does the my board respond to Master simulators perfectly, but not to the actual devices.

Thank you for your time.


Hi Praveen,

Did you verify if the baud rate/parity bits/stop bits are fine?

Is your Modbus Master device querying the slave with supported function code?