Modbus Rtu Protocol; Data Construction; Function Codes; Error Response - Toho Electronics TRM-10C Instruction Manual

8.3

Modbus RTU Protocol

The Modbus protocol is a communication protocol developed for PLCs by Modicon Inc. (AEG Schneider
Automation International S.A.S). It is described in the protocol specification (PI-MBUS-300 Rev. J). Refer to
this document for information on the Modbus protocol specification.

8.3.1 Data Construction

Slave address
(1 byte)

8.3.2 Function Codes

The following function codes are available on this product:
Code
03H Setting data read
04H Input data read
06H One-time data write
10H Continuous data writes

8.3.3 Error Response

In accordance with the communication protocol for the pertinent function code, if an error
occurs during command transmission, an error response is returned in the fixed format
described in this section.
■
Example of slave response (function code = 06H, command error = 10H)
Component
Slave address (only for Modbus RTU)
Function code + 80H
Error code
Error check (only for Modbus RTU)
Total number of bytes
■Error codes and their occurrence conditions
Error
code
01H Invalid function code
02H Invalid register address
03H
Invalid number of registers
04H Device error
10H Command error
Function code
(1 byte) (variable)
Function
Description
An unsupported function code is specified.
The relative address range exceeds "9999."
- The length of the accessed data is "0" or the sum of the
relative address and data length exceeds the limit.
- It is assumed that two or more function codes, each
executable for one area, are involved.
- Data longer than 2 words is specified for a one-time write
command.
- The data length exceeds 123 words.
The received data is shorter than the predefined data length.
An attempt at a write over an area exceeding the writable range.
―103―
Data CRC
(2 bytes)
Maximum data
Modbus-specific function (reference)
length
123 words Data read from holding register
123 words Data read from input register
1 word Data write to holding register
123 words Data write to holding register
Data
Data
length
1 -
1 86H
1 10H
2 CRC
5 -
Occurrence condition(s)
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