Common MODBUS functions • 289
Function code FC7 (Read Exception Status)
Modular I/O System
ETHERNET TCP/IP
6.2.7
Function code FC7 (Read Exception Status)
This function reads the first 8 bits of the process output image.
Request
Byte
Field name
Example
Byte 0, 1
Transaction identifier
0x0000
Byte 2, 3
protocol identifier
0x0000
Byte 4, 5
length field
0x0002
Byte 6
unit identifier
0x01 not used
Byte 7
MODBUS function code
0x07
Response
Byte
Field name
Example
...
Byte 7
MODBUS function code
0x07
Byte 8
Reference number
0x00
Exception
Byte
Field name
Example
...
Byte 7
MODBUS function code
0x85
Byte 8
Exception code
0x01 or 0x02