01 Modbus Data Struture

Modbus DataStructure

Author: Fengbing References:

Read Coil Status (FC=01) .................................................................................................................. 1 Read Input Status (FC=02) ............................................................................................................... 2 Read Holding Registers (FC=03) ....................................................................................................... 3 Read Input Registers (FC=04) ........................................................................................................... 4 Force Single Coil (FC=05) .................................................................................................................. 5 Preset Single Register (FC=06) ......................................................................................................... 6 Force Multiple Coils (FC=15) ............................................................................................................ 7 Preset Multiple Registers (FC=16) .................................................................................................... 8 Modbus ASCII vs Modbus RTU ......................................................................................................... 9 Modbus TCP/IP .............................................................................................................................. 13 Exception Responses...................................................................................................................... 16

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Read Coil Status (FC=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 Slave Address (17 = 11 hex) 01: The Function Code (read Coil Status)

0013: The Data Address of the first coil to read. (Coil 20 - 1 = 19 = 13 hex) 0025: The total number of coils requested. (coils 20 to 56 = 37 = 25 hex) 0E84: The CRC (cyclic redundancy check) for error checking. Response

11 01 05 CD6BB20E1B 45E6

11: The Slave Address (17 = 11 hex) 01: The Function Code (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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Read Input Status (FC=02)

Request

This command is requesting the ON/OFF status of discrete inputs # 10197 to 10218 from the slave device with address 17. 11 02 00C4 0016 BAA9

11: The Slave Address (17 = 11 hex) 02: The Function Code (read Input Status)

00C4: The Data Address of the first input to read. (10197 - 10001 = 196 = C4 hex) 0016: The total number of coils requested. (197 to 218 = 22 = 16 hex) BAA9: The CRC (cyclic redundancy check) for error checking. Response

11 02 03 ACDB35 2018

11: The Slave Address (17 = 11 hex) 02: The Function Code (read Input Status)

03: The number of data bytes to follow (22 Inputs / 8 bits per byte = 3 bytes) AC: Discrete Inputs 10204 -10197 (1010 1100) DB: Discrete Inputs 10212 - 10205 (1101 1011)

35: 2 space holders & Discrete Inputs 10218 - 10213 (0011 0101) 2018: The CRC (cyclic redundancy check).

The more significant bits contain the higher Discrete inputs. This shows that input 10197 is off (0) and 10204 is on (1). Due to the number of inputs requested, the last data field 35 contains the status of only 6 inputs. The two most significant bits in this data field are filled in with zeroes.

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Read Holding Registers (FC=03)

Request

This command is requesting the content of analog output holding registers # 40108 to 40110 from the slave device with address 17. 11 03 006B 0003 7687

11: The Slave Address (17 = 11 hex)

03: The Function Code (read Analog Output Holding Registers)

006B: The Data Address of the first register requested. (40108-40001 = 107 = 6B hex) 0003: The total number of registers requested. (read 3 registers 40108 to 40110) 7687: The CRC (cyclic redundancy check) for error checking. Response

11 03 06 AE41 5652 4340 49AD

11: The Slave Address (17 = 11 hex)

03: The Function Code (read Analog Output Holding Registers)

06: The number of data bytes to follow (3 registers x 2 bytes each = 6 bytes) AE41: The contents of register 40108 5652: The contents of register 40109 4340: The contents of register 40110

49AD: The CRC (cyclic redundancy check).

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