Protocol:Modbus:Buffers

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 strings are null terminated, and 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