Function Code Fc23 (Read/Write Multiple Registers) - WAGO 750 Series Manual

6.3.11 Function Code FC23 (Read/Write multiple registers)

This function performs a combination of a read and write operation in a single
request. The function can write the new data to a group registers, and then
return the data of a different group.
Request
The reference numbers (addresses) are zero-based in the request message;
therefore, the first register is at address 0.
The request message specifies the registers to read and write. The data is sent
as 2 bytes per register.
Example: The data in register 3 is set to value 0x0123, and values 0x0004 and
0x5678 are read out of the two registers 0 and 1.
Byte
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8-9
Byte 10-11
Byte 12-13
Byte 14-15
Byte 16
Byte
17-(B+16)
Response
Byte
....
Byte 7
Byte 8
Byte 9-
(B+1)
Exception
Byte
.....
Byte 7
Byte 8
Note
If register areas for read and write overlap, the results are undefined.
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
Description of the MODBUS Functions
Field name
Transaction identifier
protocol identifier
length field
Unit identifier
MODBUS function code
reference number for read
Word count for read (1-125)
reference number for write
Word count for write (1-100)
Byte Count
(B = 2 x word count for write)
Register Values
Field name
MODBUS function code
Byte Count
(B = 2 x word count for read)
Register Values
Field name
MODBUS function code
Exception code
MODBUS Functions
Example
0x0000
0x0000
0x000F
0x01 not used
0x17
0x0000
0x0002
0x0003
0x0001
0x02
0x0123
Example
0x17
0x04
0x0004
0x5678
Example
0x97
0x01 or 0x02
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