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In my project on the V120-22 I use 2 COM ports for RS485 Modbus communication. Port

2 - Master mode to communicate with another device, the Port 1 - Slave mode for

communication with the upper level.

After some time, the PLC stops responding via Port 1, while Port 2 is operating

normally and ladder is operating normally too. Helps only restart the PLC. If Port 2

is disabled, then the connection through Port 1 is without failing.

It seems, as if at the time of simultaneous access to the ports, there is a conflict.

Why lost communication with Port 1?

Ports settings:

Port 2: 115200, 8, none, 2

Port 1: 57600, 8, none, 1

The request to Port 1 is sent after a pause of 10 ms after the last response

Connection breaks after 2000-15000 requests

Best regards,


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Joe T.

Thanks for your response.

Sorry, I can't upload the program, because I'm not the author. And the program is awkward, this can be a cause of the error. I solved the problem as follows: set the check bit SB135 (COM Port 1, Data Receive) before sending the requests to Port 2 and reduced the baud rate to 9600.

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