Function Code Fc2 (Read Discrete Inputs); Table 93: Request Of Function Code Fc2; Table 94: Response Of Function Code Fc2; Table 95: Assignment Of Inputs - WAGO -I/O-SYSTEM 750 Manual

WAGO-I/O-SYSTEM 750
750-882 Media Redundancy ETHERNET Controller
12.2.3.2

Function Code FC2 (Read Discrete Inputs)

This function reads the input bits from a slave device.
Request
The request specifies the reference number (starting address) and the bit count
to be read.
Example: Read input bits 0 to 7

Table 93: Request of Function Code FC2

Byte
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8, 9
Byte 10, 11
Response
The current value of the requested bits are packed into the data field. A binary 1
corresponds to the ON status and a 0 the OFF status. The lowest value bit of the
first data byte contains the first bit of the inquiry. The others follow in an
ascending order. If the number of inputs is not a multiple of 8, the remaining bits
of the last data byte are filled with zeroes (truncated).

Table 94: Response of Function Code FC2

Byte
...
Byte 7
Byte 8
Byte 9
The status of the inputs 7 to 0 is shown as a byte value 0x12 or binary 0001
0010. Input 7 is the bit having the highest significance of this byte and input 0 the
lowest value. The assignment is thus made from 7 to 0 as follows:

Table 95: Assignment of Inputs

OFF
Bit
Coil
Field name
Transaction identifier
Protocol identifier
Length field
Unit identifier
MODBUS function code
Reference number
Bit count
Field name
MODBUS function code
Byte count
Bit values
OFF OFF ON
0 0 0 1
7 6 5 4
Fieldbus Communication 231
Example
0x0000
0x0000
0x0006
0x01 not used
0x02
0x0000
0x0008
Example
0x02
0x01
0x12
OFF OFF ON OFF
0 0 1 0
3 2 1 0
Manual
1.5.0