Read Coil Status (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: The Slave Address (11 hex = address17 )
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 Slave Address (11 hex = address17 )
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 field1B 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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