Modbus RTU Function Codes
3/07 UDC3500 Universal Digital Controller Product Manual 345
File Number
The file number word contains the register number from the register address structure
shown in Table 9-1 and Table 9-2. Although the register address structure tables indicate
up to 13 data registers are available for access, only register address 3 is currently
supported.
Register Address
The register address is used to designate the tag ID number for the parameter being
accessed. The register address is made up of two bytes. The LSB contains the RS422 tag
ID number. The tag ID numbers represent the parameter’s register address(es). See
Section 10 for the tag ID numbers. The MSB contains the control loop and database
extension using codes as shown below:
Modbus register address (High register, Low register)
00 h,xx - loop 1 basic data base registers
01 h,xx - loop 2 basic data base registers
40 h,xx - loop 1 extended data base registers
41 h,xx - loop 2 extended data base registers
xx = Modbus Parameter ID register address in hex– Implied Format
Table 9-4 Register Parameter ID Address Format for Function Code 21
Register
Address(es)
(Dec)
Register
Address(es)
(Hex)
Format
001 to 127 0001 to 007F analog formatted data
(2 registers – IEEE 32-bit floating point)
128 to 255
0080 to 00FF
integer formatted data
(2 registers – IEEE 32-bit floating point)
Unrestricted Registers
As mentioned previously, all register data is stored in the EEROM of this instrument with
some exceptions. These exceptions were made to allow write access to override
information. The registers, which are designated as Override values, are listed below.
These registers do not have restrictions on the number of writes.
ID Tag Register Number UDC Usage
125 (7Dh) Computer Setpoint
Restrictions on Parameter Numbers in One Message
The maximum number of writeable parameters per write request is 1.
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