Function Code Fc1 (Read Coils); Request; Response - WAGO -I/O-SYSTEM 750 User's Installation And Configuration

6.2.1

Function code FC1 (Read Coils)

The function reads the status of the input and output bits (coils) in slave.

Request

The request determines the starting address and the number of bits to be read.
Example: An inquiry, with which the bits 0 to 7 are to be read.
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 inquired bits are packed in the data field. A 1 corre-
sponds 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 inquiry. 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
.....
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
Modular I/O System
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
Common MODBUS functions • 283
Function code FC1 (Read Coils)
Example
0x0000
0x0000
0x0006
0x01 not used
0x01
0x0000
0x0008
Example
0x01
0x01
0x12
Example
0x81
0x01 or 0x02