2014-
02
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Commands
If the master is scheduled to transfer the read/write command to the IDENTControl Compact a
second time, the toggle bit must be inverted. This enables cyclic data communication with the
PLC.
Command: read holding registers (03h)
This MODBUS command (request) reads the input register. When data becomes available, the
IDENTControl Compact writes it to the input register. If the data is not retrieved, up to 32 data
sets can be stored in the cache. If the cache is full, the data is lost. If the register is empty of
data, the value of the register is 0.
You must know the following parameters to make the request:
The IDENTControl Compact sends a response that includes the content of the requested
register. A read/write command is executed from the first address of the relevant channel.
Example
In this example, the data generated in the previous example is read out.
Prerequisite: The command
write multiple registers
was executed successfully.
Settings in the MODBUS master
Address
(0-based)
Register division
Byte number of the
identification
telegram
Use
Type Meaning
2000d
High byte
Reserved
00h -
Low byte
Reserved/Deletion bit (LSB)
00h No delete operation
2001d
High byte
Byte 0
Length of the identification
telegram from this byte
onwards
00h -
Low byte
Byte 1
Length of the identification
telegram
06h 6 bytes long
2002d
High byte
Byte 2
Command code
19h
Enhanced read
Low byte
Byte 3
Word number/Channel/Toggle
bit
40h Read 4 words, no
channel specification
required
2003d
High byte
Byte 4
Word address
High byte
00h Read from tag
address 0
Low byte
Byte 5
Word address
Low byte
00h Same as previous
byte
Start address (depending on channel)
Channel 0
0d
Channel 1
1000d
Channel 2
2000d
Number of registers to be read:
Maximum 125d
Slave IP address:
IP address of the IDENTControl Compact
Timeout:
1000 ms
Device ID:
1
Start address:
2000d
Number of registers to be read:
12d
