Single packet:
Field
Description
Size
MBAP
Transaction ID
Packet number – for purpose
of simplification it can be equal to 0
2B
Protocol ID
For the Modbus protocol it is
always equal to 0
2B
Lenght
The number of subsequent data (after
this field) transmitted – in bytes
2B
Unit ID
It is always equal to 0 or 255
1B
PDU
Function Code
A Modbus command – supported
commands are set out in table
(a – commands)
1B
Address /
Action
For standard Modbus commands,
these are register addresses – table
(d – register addresses), for special
commands these are action numbers –
table (c – actions)
2B
Parameters
Command-dependent – available
in table (c – actions)
Size in the table
(c – actions)
42
MBAP Header
Command
Data
PDU
Single packet (ADU)
