VEGA VEGACOM 557 Modbus protocol - Manual (Page 42)

42 VEGACOM 557 Modbus
Data transmission with Modbus RTU
Error messages in RTU mode
Errors in the telegram of function code 04 (Read Input Register) are commented by an error
message (Exception Response). The most significant bit of the function code is set to 1 (cor-
responds to function code + 80H). In the byte Exception Code, the error is named in coded
form (see error overview).
Error overview:
Exception Code Meaning
1 unpermitted function
2 unpermitted start register
3 unpermitted number of registers
Example for error message:
A DCS value should be collected from a VEGAMET 513. VEGACOM 557 operates in mode
"Sort measured values acc. to VEGAMET addresses“. The Slave-address of VEGACOM 557
is set to 1.
In the enquiry telegram, 240 was inadvertently enquired as start register. The highest possible
register in this case, is 222. Therefore the following answering telegram appears:
Start
Slave
ad-
dress
Func-
tion
code
Ex-
ception
code
Error
Check
End
No. of
Bytes
Value
range
Meaning
Structure of the answer VEGACOM 557 to Modbus-Master
R TU answer telegram (function code 04) to faulty enquiry
3 ... 4 1 1 1 2 3 ... 4
Closed level 1 .. 247 0x84
1 ..
3
CRC Closed level
0x01 0x84 0x02 0xXX 0xXX
Exception Code = 2