Writing to Siemens PLC via modbus using Function Code 6 breaks modbus communication
I am stablished modbus communication with siemens plc. Reading holding registers works fine , but while writing there register address 40401 , modbus communications goes down and comes back up once i stop writing.
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From the limited information provided, i would like to ask few more questions 1. Between what systems you have established communication? Understand that Siemens PLC is Modbus Slave here and Which system is acting here as a Modbus Master? 2. Some PLC will allow only the Group Write (FC - 16) for few set of registers, pls refere it's respective modbus manual 3. By how you are declaring that the communication goes down and comes back? Are you getting any exception response for particular MB Write FC6? 4. Can you able to test the same with any third party simulator and observe the response? 5. If possible you can capture the modbus frames/packets being sent out and received to analyse further
Your question is very limited. Is that serial Modbus RTU or Modbus TCP? I have a successful Modbus RTU communication with Simatic 1200. Writing is done with MBWrite and start address %MW10#400 for 40401. It is very important to set Poll Time =0 in your Modbus protocol.
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