Function Code Fc2 (Read Input Discretes) - WAGO 750 Series Manual

164 • MODBUS Functions
Description of the MODBUS Functions

6.3.2 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:
Byte
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8, 9
Byte 10, 11 Bit count
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).
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 with OFF-OFF-OFF-ON-OFF-OFF-
ON-OFF.
Bit:
Coil:
Exception
Byte
.....
Byte 7
Byte 8
Field name
Transaction identifier
protocol identifier
Length field
unit identifier
MODBUS function code
reference number
Field name
MODBUS function code
Byte count
Bit values
0 0 0 1 0 0 1 0
7 6 5 4 3 2 1 0
Field name
MODBUS function code
Exception code
Example
0x0000
0x0000
0x0006
0x01 not used
0x02
0x0000
0x0008
Example
0x02
0x01
0x12
Example
0x82
0x01 or 0x02
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