226 Fieldbus Communication
WAGO-I/O-SYSTEM 750
750-880, 750-880/025-000 ETHERNET Programmable Fieldbus Controller
Manual
Version 1.0.1
12.2.3.2 Function Code FC2 (Read Input Discretes)
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 95: Request of Function code FC2
Byte
Field name
Example
Byte 0, 1
Transaction identifier
0x0000
Byte 2, 3
Protocol identifier
0x0000
Byte 4, 5
Length field
0x0006
Byte 6
Unit identifier
0x01 not used
Byte 7
MODBUS function code 0x02
Byte 8, 9
Reference number
0x0000
Byte 10, 11
Bit count
0x0008
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 96: Response of Function code FC2
Byte
Field name
Example
...
Byte 7
MODBUS function code 0x02
Byte 8
Byte count
0x01
Byte 9
Bit values
0x12
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 97: Assignment of inputs
OFF OFF OFF
ON OFF OFF ON OFF
Bit
0 0 0 1 0 0 1 0
Coil
7 6 5 4 3 2 1 0
