MODBUS RTU Commands

The following commands (functions) were implemented in the MODBUS RTU AMB-12I-4O.

• 01 (01 hex) Function code – Read Coils
• 02 (02 hex) Function code – Read Discrete Inputs
• 05 (05 hex) Function code – Write Single Coil
• 15 (0F hex) Function Code – Write Multiple Coils

Coils are mapped to outputs and discrete inputs are mapped to inputs. Coils read/write output status and discrete inputs read Input status. There are 12 inputs for reading-input 0 to input 11 which refer to input 1 to input 12 and coil 0 to coil 3 which refer to output 1 to output 4.

Note: For all below example, consider slave address – 0x01 will change with 8bit DIP switch.

01 (01 hex) Function Code – Read Coils

Read Coils command is requesting the ON/OFF status of coils. Read Coils command will return individual current outputs status. Also Read Coils Command will return sequentially outputs current status. 

Example of how to read output status are given below:

REQUEST FRAME –

01 01 0000 0004 3DC9
01 – The Slave Address (01 hex = 01)
01 – The Function Code 
0000 – The Data Address of first coil to read (0000hex = coil 0)
0004 – The total number of coils requested (0004hex = 4 coils – outputs))
3DC9 – The CRC for error checking

RESPONSE FRAME – 

01 01 01 0A D18F
01 – The Slave Address (01 hex = 01)
01 – The Function Code 
01 – The number of data bytes to follow (4 coils / 8 bits per byte = 1 byte)
0A – Coils 0 to 3(Output), 0A hex = 0000 1010 (0 – ON, 1 – OFF)
D18F – The CRC for error checking

02 (02 hex) Function Code – Read Discrete Inputs 

Read Discrete Inputs Command is requesting the ON/OFF status of Inputs. Read Discrete Inputs command will return individual current Input status. Also this Command will return sequentially Input current status. 

Example of how to read Inputs status are given below:

REQUEST FRAME -  

01 02 0000 000C 780F
01 – The Slave Address (01 hex = 01) 
01 – The Function Code  
0000 – The Data Address of first Input to read (0000hex = Input 0) 
000C – The total number of Inputs requested (000Chex = 12 Inputs)) 
780F – The CRC for error checking 

RESPONSE FRAME -  

01 02 02 5A09 3FD3 
01 – The Slave Address (01 hex = 01) 
02 – The Function Code  
02 – The number of data bytes to follow (12 coils / 8 bits per byte = 2 byte) 
5A – Input 7 to 0(Input), 5A hex = 0000 1010 (0 – ON, 1 – OFF) 
09 – Input 11 to 8(Input), 09 hex = 0000 1010 (0 – ON, 1 – OFF) 
3FD3 – The CRC for error checking

(05hex) Function Code – Write Single Coil

Write Single Coil (0x05) command will set single output(relay) ON or OFF. 

Example of how to set Relay ON is given below:  

REQUEST FRAME – 

01 05 0003 FF00 7C3A
01 – The Slave Address (01 hex = 01)
05 – The Function Code 
0003 – The Data Address of coil to change status (0003hex = coil 3)
FF00 – The Status to write (FF00 – ON relay, 0000 – OFF relay)
7C3A – The CRC for error checking

RESPONSE FRAME – 

01 05 0003 FF00 7C3A
01 – The Slave Address (01 hex = 01)
05 – The Function Code 
0003 – The Data Address of coil to change status (0003hex = coil 3)
FF00 – The Status written (FF00 – ON relay, 0000 – OFF relay)
7C3A –  The CRC for error checking

15 (0Fhex) Function Code – Write Multiple Coil

Write Multiple Coils command will set multiple sequential relays ON or OFF. 

Example of how to set Relay1 to Relay4 with single command is given below:

REQUEST FRAME – 

01 0F 0000 0004 01 0A BE91
01 – The Slave Address (01 hex = 01)
0F – The Function Code 
0000 – The Data Address of first coil(0000hex = coil 0)
0004 – The number of coils to written(04hex = Output 1 to 4)
01 – The number of data bytes to follow
0A – 4 space holders & coils 0 – 3 (0000 1010)(0 – OFF, 1 – ON)
BE91 – The CRC for error checking

RESPONSE FRAME –

01 0F 0000 0004 5408
01 – The Slave Address (01 hex = 01)
0F – The Function Code 
0000 – The Data Address of first coil(0000hex = coil 0)
0004 – The number of coils to written(04hex = Output 1 to 4)
5408 – The CRC for error checking

MODBUS Exception Codes

Exception responses from either the master or a slave can result from data processing errors.
One of the following events can occur after a request from the master:

• If the slave receives the request from the master without a communication error and can handle the request correctly, it returns a normal response. 
• If the slave does not receive the request from the master due to a communication error, it does not return a response. The master program eventually processes a timeout condition for the request.
• If the slave receives the request from the master but detects a communication error, it does not return a response. The master program eventually processes a timeout condition for the request.
• If the slave receives the request from the master without a communication error, but cannot handle it (for example, the request is to read a register that does not exist), the slave returns an exception response to inform the master of the nature of the error.

Exception Frame:

Exception Codes: