Download Ethernet Communication Functions

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3.3
3-3
< 3. MODBUS/TCP >
Network Frame Structure
The MODBUS/TCP frame structure is as follows:
MODBUS TCP/IP ADU
MBAP Header
Function code
Data
PDU
MBAP Header (MODBUS Application Protocol Header) : Header used to identify the MODBUS/TCP protocol
PDU (Simple Protocol Unit): Body of the data communication
3.3.1
MBAP Header Structure
The MBAP Header (MODBUS Application Protocol Header) consists of the following seven
bytes.
Byte No
Description
0
1
Transaction ID
2
3
Protocol ID
4
5
Number of bytes
6
Unit ID
Transaction ID: The host device specifies an arbitrary value to identify a transaction. The UT351 or UT551 returns the value it
received from the host device as its response.
Protocol ID: This parameter is set to ‘0’ to indicate the MODBUS/TCP protocol.
Number of bytes: The number of bytes from the Unit ID (byte number 6) byte on.
Unit ID: For the communication with the UT351 or UT551 itself, specify “1” for the host device. The UT351 or UT551 returns
‘1’ as its response. For the communication with the device connected to the RS485 communication terminals of the
UT351 or UT551 using the Ethernet-serial gateway function, specify its communication address (2 to 99). The device
returns the same value as its response.
3.3.2
PDU Structure
The PDU (Simple Protocol Unit) consists of the following n bytes.
Byte No
0
1 to (n-1)
Description
Function code
Data
Function code: The command specified from the host device
Data: Depending on the function code, D register addresses, the number of individual D registers, or parameter values are
specified in this position.
IM 05G01B52-01E
4th Edition : 2005.03.25-00