Data access using the Modbus RTU protocol
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isoRW425_D00052_04_M_XXEN/03.2019
7.2.2
ISOMETER® answer to the master
7.3
Exception code
If a request cannot be answered for whatever reason, the ISOMETER® will send
a so-called exception code with which possible faults can be narrowed down.
7.3.1
Structure of the exception code
Byte Name Example
Byte 0
ISOMETER® Modbus address
0x03
Byte 1
Function code
0x10
Byte 2, 3
Start register
0x0BBB
Byte 4, 5
Number of registers
0x0001
Byte 6, 7
CRC16 Checksum
0x722A
Exception
code
Description
0x01 Impermissible
function
0x02
Impermissible data access
0x03
Impermissible data value
0x04
Internal fault
0x05 Acknowledgement
of receipt (answer will be time delayed)
0x06
Request not accepted (repeat request, if necessary)
Byte Name
Example
Byte 0
ISOMETER® Modbus address
0x03
Byte 1
Function code (0x03) + 0x80
0x83
Byte 2
Data (exception code)
0x04
Byte 3, 4
CRC16 Checksum
0xE133