Siemens SIMATIC S7-1200 System Manual page 1286

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Communication processor and Modbus TCP

13.9 Legacy Modbus RTU communication (CM/CB 1241 only)

Network 3: Move the first 3 words of the holding register data that has been read to some

other location, and set a DONE history bit. This network also sets an ERROR history bit and

saves the STATUS word to another location in the event of an error.

Network 4: Write 64 bits of data from MW600-MW607 to output bit locations 00017 to 00081

on slave #2.

Network 5: Set a DONE history bit when the write is complete. If an error occurs, the

program sets an ERROR history bit and saves the STATUS code.

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK