ICP DAS USA ET-7H16M Series User Manual page 79

Appendix A. What is Modbus TCP/IP?
Modbus is a communication protocol developed by Modicon in 1979.
Different versions of Modbus used today include Modbus RTU (based on serial communication like
RS485 and RS232), Modbus ASCII and Modbus TCP, which is the Modbus RTU protocol embedded
into TCP packets.
Modbus TCP is an Internet protocol. The protocol embeds a Modbus frame into a TCP frame so
that a connection oriented approach is obtained thereby making it reliable. The master query's the
slave and the slave responds with the reply. The protocol is open and hence highly scalable.
Modbus Message Structure
Modbus devices communicate using a master-slave (client-server) technique in which only one
device (the master/client) can initiate transactions (called queries). The other devices
(slaves/servers) respond by supplying the requested data to the master, or by taking the action
requested in the query.
A master's query will consist of a slave address (or broadcast address), a function code defining
the requested action, any required data, and an error checking field. A slave's response consists of
fields confirming the action taken, any data to be returned, and an error checking field.
Address
1 byte

Address specifies the address of the receiver.

Function Code specifies the message type.

Data is the data block.

Checksum specifies the numerical check value for testing the validity of the protocol.
PET-7H16M Series User Manual, version 1.0.1
Function Code
1 byte
Data
0 to 252 bytes
Checksum
2 bytes
P.79