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Modbus RTU Function Codes 
186 
UDC2500 Universal Digital Controller Product Manual 
4/07 
9.4   Function Code 21 (15h) - Write Configuration Reference Data 
Introduction 
Function Code 21 (15 Hex) is used in this instrument to allow writes of integer and 
floating point values to the configuration database and override values. 
The configuration database of this instrument is located in EEROM. The override values 
are stored in RAM. 
Integer format is used to write to “Digital” configuration items. Floating Point format is 
used to write to “Analog” configuration items as defined by the configuration ID tags. 
Write Restrictions 
Care should be taken not to exceed the 100,000 write limit of the EEROM. 
Request and Response Formats 
The Request and Response formats for Function code 21 (15 Hex) are shown below. 
Details for each block reference follow. 
Request Message Format 
Slave 
Address
Byte 
Count
Function 
Code 15
Reference 
Type
File 
Number
Register 
Count
Register 
Address
CRC 
Data
CRC 
Data
Data
Data
Data
Data
File 
Number
 
 
Response Message Format (echo back of request) 
Slave 
Address
Byte 
Count
Function 
Code 15
Reference 
Type
File 
Number
Register 
Count
Register 
Address
CRC 
Data
CRC 
Data
Data
Data
Data
Data
File 
Number
 
The register address is interpreted as the tag ID configuration number. 
 
For Infrared Transactions, add three BOFs (C0hex) at the beginning of each message and 
one EOF (FFhex) at the end of each message. 
Reference Type Definitions 
The Reference Type definition is always 06. 
See examples in Subsection 9.4.1