Technical data

Fieldbus Communication • 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