9492600990 25-3
DECS-150 Modbus
®
Communication
Modbus on TCP/IP
Application Data Unit
The following describes the encapsulation of a Modbus request or response when it is carried on a
Modbus TCP/IP network. See Figure 25-2.
Figure 25-2. Modbus Request/Response Over TCP/IP
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).
This header provides some differences compared to the Modbus RTU application data unit used on a
serial line:
• The Modbus ‘slave address’ field usually used on Modbus Serial Line is replaced by a single byte
‘Unit Identifier’ within the MBAP header. The ‘Unit Identifier’ is used to communicate via devices
such as bridges, routers, and gateways that use a single IP address to support multiple
independent Modbus end units.
• All Modbus requests and responses are designed in such a way that the recipient can verify that
a message is finished. For function codes where the Modbus PDU has a fixed length, the function
code alone is sufficient. For function codes carrying a variable amount of data in the request or
response, the data field includes a byte count.
• When Modbus is carried over TCP, additional length information is carried in the MBAP header to
allow the recipient to recognize message boundaries even if the message has been split into
multiple packets for transmission. The existence of explicit and implicit length rules and use of a
CRC-32 error check code (on Ethernet) results in an infinitesimal chance of undetected corruption
to a request or response message.
MBAP Header Description
The MBAP Header contains the fields listed in Table 25-1.
Table 25-1. MBAP Header Fields
Fields Length Description Client Server
Transaction
Identifier
2 Bytes Identification of a Modbus
request/response transaction.
Initialized by
the client.
Recopied by the
server from the
received request.
Protocol
Identifier
2 Bytes 0 = Modbus protocol. Initialized by
the client.
Recopied by the
server from the
received request.
Length 2 Bytes Number of following bytes. Initialized by
the client
(request).
Initialized by the
server (response).
Unit Identifier 1 Byte Identification of a remote
slave connected on a serial
line or on other buses.
Initialized by
the client.
Recopied by the
server from the
received request.
The header is 7 bytes long:
• Transaction Identifier – Used for transaction pairing, the Modbus server copies in the response
the transaction identifier of the request.
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