MESSAGE FORMAT
FUNCTION CODES
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1TGC908004
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8.
Function Codes
The MOBDUS protocol implemented in Digital Gateway is using the MODBUS standard func-
tion codes (FC). Those are applicable for Modbus RTU as well as for Modbus TCP communica-
tion. The standard function codes supported are as follows:
Function Code
Function
Description
FC02
Read Input Status
Bit-orientated reading from regis-
ter file
FC03
Read Holding Registers
Word-orientated reading from reg-
ister file
FC04
Read Input Registers
Word-orientated reading from reg-
ister file
FC06
Preset Single Register
Writing of a word into register file
FC08
Diagnostics
Check communication between
master and slave, (loop back)
FC16
Pre-set multiple Registers
Write of several successive words
into register file
Table 15 Function Codes
The Digital Gateway is a ‘standard MODBUS slave device. The PLC or PCS master initiates the
communication by sending the 'Query Messages' and the Digital Gateway replies the re-
quested information in 'Response Messages'.
8.1.
Message Format
Query Messages
The MODBUS query messages have the standard query structure as below.
•
The slave address
•
Function code for Read or Write operation
•
Start address of the desired information
•
Register length or data code to be read
•
CRC-Error checking field
Response Messages
The MODBUS query messages have the standard query structure as below.
•
The slave address
•
Applied function code
•
Length of response (byte)
•
Requested information/Action performed
•
CRC-Error checking field
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