Protocol:Modbus:Buffers

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Buffer support in Modbus is implemented via a new function code pair:


  • FC107 (hex 0x6B) for BUFFER READ
  • FC108 (hex 0x6C) for BUFFER WRITE


The packet format looks like this:


BUFFER READ

Master Command:

Byte 0: 0xXX // slave address

Byte 1: 0x6B // function code for buffer read

Byte 2,3: 0xYYYY // address of buffer


Slave Response:

Byte 0: 0xXX // slave address

Byte 1: 0x6B // function code for buffer read

Byte 2: 0xXX // number of bytes in buffer up to 0xF9 max

Byte 3…. <buffer data>


BUFFER WRITE

Master Command:

Byte 0: 0xXX // slave address

Byte 1: 0x6C // function code for buffer write

Byte 2,3: 0xYYYY // address of buffer

Byte 4: 0xXX // number of bytes in buffer up to 0xF9 max

Byte 5…. <buffer data>


Slave Response:

Byte 0: 0xXX // slave address

Byte 1: 0x6C // function code for buffer write

Byte 2,3: 0xYYYY // address of buffer