Modbus RTU Function Codes 
9.4.1 Write Configuration Examples 
Example #1 
The following is an example of a request to write the Gain 1 value using Function code 
21 (15 Hex). 
Request Message (Write Gain  1= 1.5 “ID Tag 1”) 
  02 15 0B 06 00 03 00 01 00 02 3F C0 00 00 (CRC16) 
Where: 
 02  = Address 
  15  =  Function Code 21 (15 Hex) 
 0B  = Byte Count 
  06  =  Reference Type (IEEE Floating Point) 
  00 03  =  File Number (Access Data Value) 
  00 01  =  Register Address (Standard Access - Gain 1 - ID Tag 1)  
  00 02  =  Register Count (Floating Point Data) 
  3F C0 00 00  =  1.50 
 (CRC16) 
 
This is the response to the above request. 
Response Message (The response is an echo of the request)  
  02 15 0B 06 00 01 00 02 00 02 3F C0 00 00 (CRC16) 
184  UDC3200 Universal Digital Controller Product Manual  12/04