Fieldbus
Communication
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219
MODBUS
Functions
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
4.3.3.2 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
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).
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 with 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
Exception
Byte
Field name
Example
.....
Byte 7
MODBUS function code
0x82
Byte 8
Exception code
0x01 or 0x02
