Issues with the Modbus slave function of Siemens CP341
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When using the CP341 as a MODBUS Slave for communication in the S7-300, Siemens’ documentation states that one should call FB7 and FB8 in OB1, then delete them from OB1 and subsequently call FB80. I followed these instructions, but when uploading the program to the PLC, it kept showing an error indicating that the blocks FB7 and FB8 could not be copied, and the CPU entered STOP mode. I would like to know how to resolve this issue.
I don’t know what document you’re looking at; you only need to call FB7 (RECEIVE) and FB8 (SEND) when your CP341 is in MASTER mode. As a slave, directly call FB80
I checked the documentation; FB7 and FB8 must be installed in the controller, as FB80 needs to call on FB7 and FB8 internally. So, you should still download FB7 and FB8 separately.
Later, I found out that it was because my CPU did not support FB7/FB8; by using FB107/FB108 and FB81 instead, I was able to download the files to the CPU successfully. But there was a new problem: when I plugged in the RS485 serial connector, the SF red light turned on.
Check the hardware diagnostic information; it’s possible that the cp341 has detected a disconnected connection