Acrel APM Series User Manual

47
Figure 3: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)
Figure 4:The diffrences of modbus frame between Modbus TCP/IP and Modbus RTU
3. MBAP Header descriptio
The MBAP Header contains the following fields:
Fields
Length
Description
Client
Server
Transaction
Identifier
2Byte
Identification of a
MODBUS Request /
Response transaction
Initialized by the
Client
Recopied by the
server from the
received request
Protocol
Identifier
2Byte
0x00:MODBUS protocol
Initialized by the
client
Recopied by the
server from the
received request
Length
2Byte
Number of following bytes
Initialized by the
client ( request)
Initialized by the
server (Response)
Device ID
1Byte
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:It is used for transaction pairing, the MODBUS server copies in
the response the transaction identifier of the request.

Acrel APM Series Specifications

General

Brand	Acrel
Model	APM Series
Category	Measuring Instruments
Language	English