WAGO 750-830 Manual page 228

228 • Fieldbus Communication
MODBUS Functions
4.3.3.11 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 re-
turn 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) Register Values
Response
Byte
....
Byte 7
Byte 8
Byte 9-(B+1)
Exception
Byte
.....
Byte 7
Byte 8
Attention
If register areas for read and write overlap, the results are undefined.
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) 0x02
Field name
MODBUS function code
Byte Count (B = 2 x word count for read)
Register Values
Field name
MODBUS function code
Exception code
Example
0x0000
0x0000
0x000F
0x01 not used
0x17
0x0000
0x0002
0x0003
0x0001
0x0123
Example
0x17
0x04
0x0004, 0x5678
Example
0x97
0x01 or 0x02
WAGO-I/O-SYSTEM 750
BACnet/IP Controller