Protocol Description; Mbap - ICP DAS USA ALM-06-WF User Manual

5.2 Protocol Description

The Modbus protocol defines a simple protocol data unit independent of the
underlying communication layers. The mapping of Modbus protocol on network can
introduce some additional fields on the application data unit.
Transaction ID
(2 bytes)

5.2.1 MBAP

The Modbus/TCP extension includes 7 additional bytes to the original Modbus
protocol, which allows for transport over the TCP/IP layers.
A dedicated header is used on TCP/IP to identify the Modbus Application Data
Unit. It is called the MBAP Header (MODBUS Application Protocol Header). The
MBAP Header consists of 7 bytes of information:
Fields
Transaction Identifier
Protocol Identifier
Length
Unit Identifier
ALM-06-WF User's Manual
Modbus/TCP Application Data Unit
Protocol ID Length
(2 bytes) (2 bytes)
MBAP Header
Figure 5-1: Modbus/TCP Application Data Unit
Length Description
Identification of Request/Response transaction – Copied
2 bytes
from request to response
2 bytes 0 = Modbus protocol
2 bytes Number of following bytes - Includes the Unit Identifier
1 byte Identification of remote slave
Table 5-1: MODBUS Application Protocol Header
ALM-06-WF
User's Manual (Rev1.0, Feb./2019) ------------- 32
Unit ID FCode
(1 bytes) (1 bytes)
Protocol Data Unit
Data
(0 to 252 bytes)