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4.3 Protocol Data Unit Formats by Functions
4
MODBUS(R) STANDARD FUNCTIONS
4.3 Protocol Data Unit Formats by Functions
This section describes MODBUS protocol data unit formats used in the QJ71MB91.
(1) Precautions
(a) Device number specified in messages
When specifying a device number in a message, specify it as "(Device number) -
1".
However, this does not apply to the file and device numbers specified for reading/
writing the extended file register.
(Example) When reading input 32 (100032) with Read Discrete Inputs (FC: 02)
The device number to be stored in the response message is "(Device number of
actually read/written device) - 1".
(b) When the QJ71MB91 receives a broadcast request message
Although the processing (read/write, diagnostics, etc.) requested by the request
message is performed, no response message is sent to the master.
(c) When the QJ71MB91 receives a request message in the listen only mode
The request message is discarded except for a particular case.
To receive the request message, change it to the online mode.
( Section 4.11.5)
Figure 4.6 Specifying the MODBUS device number
Data
(L) (L)
Function code
Function code
02
H
Head input number
001FH
Read points
0001
H
Specify 31 (001FH) for the head input number
to read the input 32 (100032) status.
(H)
(H)
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