App3-9 
4.4 LOOP BACK testing [08H] 
The function code checks communication between MASTER and SLAVE.   
The instruction message is returned as a response message without being changed.   
Any value can be used for test codes or data. 
ASCII Mode 
Instruction Message         Response Message (Normal)         Response(Fault) 
3AH  STX    3AH  STX    3AH  STX 
30H 
Node Address
  30H 
Node Address
  30H 
Node Address
31H    31H    31H 
30H 
Function 
Code 
  30H 
Function Code
  38H 
Function 
Code 
38H    38H    38H 
30H 
Test Code 
  30H 
Test Code 
  32H 
Error Code 
30H    30H    30H 
30H    30H    37H 
LRC CHEC 
30H    30H    35H 
41H 
DATA 
  41H 
DATA 
 
 
0DH 
END 
 0AH 
35H   35H      
33H   33H      
37H   37H      
31H 
LRC CHECK 
  31H 
LRC CHECK 
     
42H   42H      
0DH 
END 
  0DH
END 
     
0AH    0AH      
 
RTU Mode 
Instruction Message      Response Message (Normal)        Response(Fault)   
Node Address  01H   Node Address  01H    Node Address  01H 
Function Code  08H   Function Code 08H    Function Code  88H 
Test 
Code 
High
 
00H  
Test 
Code 
High
00H   Error Code  20H 
Low 
00H  
Low
00H  
CRC-16 
High
 
47H 
DATA 
High
 
A5H  
DATA 
High
A5H  
Low 
D8H 
Low 
37H  
Low
37H        
CRC-16 
High 
DAH  
CRC-16
High
DAH        
Low 
8DH  
Low
8DH