OK so here is the setup;
RedLion G3 HMI Panel is communicating over an RS232 port using Modbus RTU protocol to an AB Micrologix 1400 PLC. The communication is not a direct wire but via a radio link at 9600,8,N,1. All timeouts are set very high (like 5000ms) to give propagation time.
I found that;
1. Using a PC with a Modbus Slave simulator in place of the Micrologix PLC I can send data up and down the radio link with absolute reliability and the RedLion G3 requests and receives responses and puts data in the right tags etc.
2. Using a PC with a Modbus Master simulator, plugged directly into the MicroLogix PLC and simulating exactly the same Modbus sentence as the RedLion G3 the PLC responds and sends back the data requested.
3. All the cables used are those in the final installation to prove they are working.
BUT
Plug the radio directly into the Micrologix PLC and there is no communication Radio shows receive transmissions from the G3 but the PLC does not respond indicating that it thinks it is not receiving (anything?) a valid Modbus sentence.
I have tried swapping Tx and Rx on the cables just in case.
It is like the RS232 port on the Micrologix and the port on the Radio are incompatible. Is that possible? Anyone got any other ideas I could try.
Thanks
Adrian
RedLion G3 HMI Panel is communicating over an RS232 port using Modbus RTU protocol to an AB Micrologix 1400 PLC. The communication is not a direct wire but via a radio link at 9600,8,N,1. All timeouts are set very high (like 5000ms) to give propagation time.
I found that;
1. Using a PC with a Modbus Slave simulator in place of the Micrologix PLC I can send data up and down the radio link with absolute reliability and the RedLion G3 requests and receives responses and puts data in the right tags etc.
2. Using a PC with a Modbus Master simulator, plugged directly into the MicroLogix PLC and simulating exactly the same Modbus sentence as the RedLion G3 the PLC responds and sends back the data requested.
3. All the cables used are those in the final installation to prove they are working.
BUT
Plug the radio directly into the Micrologix PLC and there is no communication Radio shows receive transmissions from the G3 but the PLC does not respond indicating that it thinks it is not receiving (anything?) a valid Modbus sentence.
I have tried swapping Tx and Rx on the cables just in case.
It is like the RS232 port on the Micrologix and the port on the Radio are incompatible. Is that possible? Anyone got any other ideas I could try.
Thanks
Adrian