Technical data
218 • Fieldbus Communication
MODBUS Functions
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
4.3.3.1 Function Code FC1 (Read Coils)
This function reads the status of the input and output bits (coils) in a slave de-
vice.
Request
The request specifies the reference number (starting address) and the bit count
to read.
Example: Read output 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 0x01
Byte 8, 9 reference number 0x0000
Byte 10, 11 Bit count 0x0008
Response
The current values of the response bits are packed in the data field. A binary 1
corresponds to the ON status and a 0 to the OFF status. The lowest value bit of
the first data byte contains the first bit of the request. The others follow in as-
cending 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 0x01
Byte 8 Byte count 0x01
Byte 9 Bit values 0x12
The status of the inputs 7 to 0 is shown as 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 0x81
Byte 8 Exception code 0x01 or 0x02