Chapter 6 Modbus Protocol Description; Introduction To Modbus Rtu Protocol; Communication Format; Communication Information Transmission Process - Ebyte E90-DTU User Manual

Chengdu Ebyte Electronic Technology Co., Ltd.

Chapter 6 Modbus Protocol Description

E90-DTU (400SL30L) adopts the standard Modbus RTU protocol for configuration, and the Modbus RTU
communication protocol adopts the master-slave response method for data communication. First, the host (PC,
HMI, etc.) initiates a request through a unique slave address, and the slave (terminal device) responds according to
the host's request, that is, half-duplex communication. This protocol only allows the host to initiate a request and
the slave to respond passively, so the slave will not actively occupy the communication line to cause data conflict.
The device provides a host computer developed based on the Modbus protocol for users to use, and can also
be compatible with other Modbus host computers through the Modbus protocol. For detailed register descriptions,
see "Modbus Register Table".

6.1 Introduction to Modbus RTU protocol

6.1.1 Communication format

Information transmission is asynchronous, using hexadecimal for communication, information frame format:

address code

1 byte

6.1.2 Communication information transmission process

When the communication command is sent by the master to the slave, the slave that matches the address code
sent by the master receives the communication command. If the CRC check is correct, the corresponding operation
is performed, and then the execution result (data) is returned to the master. The returned information includes
address code, function code, executed data and CRC check code. If the address does not match or the CRC check
error, nothing will be returned.
6.1.3 Address code
The address code is the first byte of each communication frame, and supports 1 to 247. Each slave must have a
unique address on the bus, and only the slave that matches the address code sent by the master can respond to the
returned data.

6.1.4 Function code

The function code is the second byte of each communication frame. The host sends, and informs the slave to
perform the corresponding operation through the function code.
The device supports the following eight function codes:
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function code data area
1 byte N byte
E90-DTU(400SL30L)_UserManual_EN
CRC check code
2 byte
26