6. MODBUS COMMUNICATION PROTOCOL
IMR01J08-E1
32
6.2 Function Code
Function code contents
Function code
(Hexadecimal)
Function Contents
03H
Read holding registers
Measured value (PV), alarm status, etc.
06H
Preset single register
Set value (SV), alarm set value, PV bias, etc.
(For each word)
08H
Diagnostics (loopback test) Diagnostics (loopback test)
Message length of each function (Unit: byte)
Function code
Function
Query message
Response message
(Hexadecimal)
Min Max Min Max
03H
Read holding registers
8
8
7
255
06H Preset
single
register
8 8 8 8
08H Diagnostics
(loopback
test)
8 8 8 8
6.3 Communication Mode
Signal transmission between the master and slaves is conducted in Remote Terminal Unit (RTU)
mode.
RTU mode
Items Contents
Data bit length
8 bit (Binary)
Start mark of message
Unused
End mark of message
Unused
Message length
See
6.2 Function Code (P. 32)
Data time interval
24 bit’s time or less *
Error check
CRC-16 (Cyclic Redundancy Check)
* The data time intervals in one query message from the master must be 24 bit’s time or less. If the
data time interval exceeds 24 bit’s time, the slave regards the transmission as ended and because the
message format is incomplete, the slave does not respond.
