ALM-04-MRTU User’s Manual
ALM-04-MRTU
User’s Manual (Rev1.0, Jan./2020) ------------- 23
4.
Modbus RTU Protocol
The Modbus protocol is developed by Modicon Inc., originally developed for
Modicon controllers. Detailed information can be found at
http://www.modicon.com/techpubs/toc7.html
. You can also visit
http://www.modbus.org
to find more valuable information. ALM-04-MRTU modules support the Modbus RTU
protocol. The communication Baud Rates range from 300bps to 115200bps. The
following Modbus functions are supported.
4.1
Function Code
Function Code
Descriptions
01 (0x01)
Read Coils
02 (0x02)
Read discrete Inputs
03 (0x03)
Read multiple Output registers
05 (0x05)
Write Single Coil
06 (0x06)
Write single Output register
Table 4-1: Supports Function Codes of ALM-04-MRTU
If the function specified in the message is not supported, then the module responds as
follows.
4.2
Error Response
Byte Index
Field Name
Byte count
Description
00
Address
1 Byte
1 to 247
01
Function code
1 Byte
Function code + 0x80
02
Exception code
1 Byte
01
Table 4-2: Error response of ALM-04-MRTU
If a CRC mismatch occurs, the module will not respond.