Technical data

148 • MODBUS Functions
Description of the MODBUS Functions
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
6.3.1 Function Code FC1 (Read Coils)
This function reads the status of the input and output bits (coils) in a slave
device.
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
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 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: 0001 0010
Coil: 7654 3210
Exception
Byte Field name Example
.....
Byte 7 MODBUS function code 0x81
Byte 8 Exception code 0x01 or 0x02