Modbus Read Coils (FC01)

Function Code 01

Request

This command is requesting the ON/OFF status of discrete coils # 20 to 56
from the slave device with address 17.

11 01 0013 0025 0E84

11: The SlaveID (11 hex = address 17 )
01 : The Function Code 1 (read Coil Status)
0013 : The Data Address of the first coil to read.
( 0013 hex = 19 , + 1 offset = coil #20 )
0025: The total number of coils requested. ( 25 hex = 37, inputs 20 to 56 )
0E84 : The CRC (cyclic redundancy check) for error checking.

Response

11 01 05 CD6BB20E1B 45E6

11: The SlaveID (11 hex = address 17 )
01 : The Function Code 1 (read Coil Status)
05 : The number of data bytes to follow (37 Coils / 8 bits per byte = 5 bytes)
CD : Coils 27 - 20 (1100 1101)
6B : Coils 35 - 28 (0110 1011)
B2 : Coils 43 - 36 (1011 0010)
0E : Coils 51 - 44 (0000 1110)
1B : 3 space holders & Coils 56 - 52 ( 0001 1011)
45E6 : The CRC (cyclic redundancy check).

The more significant bits contain the higher coil variables. This shows that coil 36 is off (0) and 43 is on ( 1). Due to the number of coils requested, the last data field 1B contains the status of only 5 coils. The three most significant bits in this data field are filled in with zeroes.

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