Function Code Fc1 (Read Coils) - WAGO 750 Series Manual

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
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8, 9
Byte 10, 11 Bit count
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
.....
Byte 7
Byte 8
Byte 9
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:
Coil:
Exception
Byte
.....
Byte 7
Byte 8
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
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
MODBUS Functions
Description of the MODBUS Functions
Example
0x0000
0x0000
0x0006
0x01 not used
0x01
0x0000
0x0008
Example
0x01
0x01
0x12
Example
0x81
0x01 or 0x02
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