Ethernet I/O Modules
ICP DAS CO., LTD.
PETL/tET/tPET DIO Series User Manual, Ver. 2.2, Aug. 2017, Page: 85
The Leading 6 bytes of a Modbus/TCP Protocol Query
Byte 00
Byte 01
Byte 02
Byte 03
Byte 04
Byte 05
Transaction identifier
Protocol identifier
Length Field
(upper byte )
Length Field
(lower byte)
Transaction identifier
= Assigned by the Modbus/TCP master (client)
Protocol identifier
= 0
Length field (upper byte)
= 0 (since all messages are smaller than 256)
Length field (lower byte)
= The number of following RTU data bytes
RTU Data Structure
Byte 06
Byte 07
Bytes 08-09
Bytes 10-11
Net ID
(Station Number)
Function Code
Data Field
Reference Number
(Address Mapping)
Number of Points
1. Net ID
specifies the address of the receiver (Modbus/TCP slave).
2. Function Code
specifies the message type.
3. Data Field
is the data block.
