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ljean / modbus-tk

Create Modbus app easily with Python
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How to use READ_WRITE_MULTIPLE_REGISTERS？ #121

Open CN-YongJun opened 4 years ago

CN-YongJun commented 4 years ago

master.execute(slave=1, function_code=cst.READ_WRITE_MULTIPLE_REGISTERS, starting_address=0, quantity_of_x=64, output_value=self.file[0:64], data_format="", expected_length=64)

I want to use the 0x17 instruction to make Modbus send and receive data at the same time, but the host will send packets as follows:

01 17 00 00 00 40 00 17 00 40 80 00 50 00 16 00 00 00 20 00 C9 00 00 00 00 00 08 00 E5 00 20 00 00 00 08 00 61 00 17 00 00 00 08 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 3D 00 23 00 00 00 08 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 00 21 00 00 00 08 00 3F 00 23 00 00 00 08 DC E3

The first few bytes are:01 17 00 00 (read addr) 00 40 (read num) 00 17 (write addr) 00 40 (write num).

The starting_address I set is obviously 0, but the write address in the 0x17 instruction somehow becomes 0x17. And I can't find execute other parameters that can be written to set this address.

CN-YongJun commented 4 years ago

I try to use pymodbus,done.

MattBrittan commented 4 years ago

This issue appears to be due to line 262 in modbus.py:

du = struct.pack(
                ">BHHHHB",
                function_code, starting_address, quantity_of_x, defines.READ_WRITE_MULTIPLE_REGISTERS,
                len(output_value), byte_count

This looks wrong to me (defines.READ_WRITE_MULTIPLE_REGISTERS will always be 23). The code was changed to this in commit [dcb0a2f115d7a9d63930c9b4466c4501039880a3][3]; previously it was:

pdu = struct.pack(">BHHHHB", function_code, starting_address, quantity_of_x, starting_addressW_FC23, len(output_value), byte_count)

This makes more sence (you need a way to pass in the address to start writing and the current interface does not seem to provide this).

(looked into this as part of answering a stackoverflow question)

jacksonmatheson commented 4 years ago

See pull request #125 Ready for merge if everyone is happy and @ljean approves of changes.

@MattBrittan Thank you very much for your work on this.