23
01
Illegal function
The function code is not supported by Meter
02
Illegal Data Address
Attempt to access an invalid address or an
attempt to read or write part of a floating point value
03
Illegal Data Value
Attempt to set a floating point variable to an invalid value
Exception Cases : An exception code will be generated when meter receives ModBus query with valid parity
& error check but which contains some other error ( e.g. Attempt to set floating point variable to an invalid value)
The response generated will be “Function code” Red with HEX (80H ). The exception codes are listed below.
16
Presets Multiple Registers
Set the content of read / write locations ( 4X )
04
Read input Registers
Read content of read only location ( 3X )
Read Holding Registers
Read content of read /write location ( 4X )
03
Communication Baud Rate is user selectable from the front panel between 4800,9600,19200,38400,57600
bps.
Function code :
8-bit binary, hexadecimal 0-9, A-F
2 hexadecimal characters contained in each 8-bit field of the message
4 bytes (32 bits) per parameter.
Floating point format ( to IEEE 754)
Most significant byte first (Alternative least significant byte first)
Format of Data Bytes
2 byte Cyclical Redundancy Check (CRC)
Error Checking Bytes
1 start bit,
8 data bits, least significant bit sent first
1 bit for even/odd parity
1 stop bit if parity is used; 1 or 2 bits if no parity
Byte format
The each byte in RTU mode has following format:
Two consecutive 16 bit registers represent one parameter. Refer table 4 for the addresses of 3X registers
(Parameters measured by the instruments). Each parameter is held in the 3X registers. Modbus Code 04 is used
to access all parameters.
Example :
To read parameter ,
Volts 3 : Start address = 04 (Hex) Number of registers = 02
Accessing 3 X register for reading measured values
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