Exception Handling - Siemens SITRANS F Operating Instructions Manual

6.7

Exception handling

There is a defined set of exception codes to be returned by slaves in the event of problems.
All exceptions are signalled in the response from the slave by adding 80 hex to the function
code of the request and following this byte by an exception code.
Table 6- 16 Exception codes
Exception code Exception text
(dec)
01 Illegal function
02 Illegal data address
03 Illegal data value
04 Slave device failure
06 BUSY-acknowledge
If an exception code 04 is received, further requests have to be done to narrow down the
exact problem.
If any doubts about the address that failed, "Last Coil/HoldReg ErrorAddr" can be read. This
will return the faulty address.
Table 6- 17 Last Coil/HoldReg ErrorAddr
MODBUS MODBUS ad-
register dress
4:00681 680
4:00682 681
4:00683 682
4:00684 683
Reading "Last Coil/HoldReg ErrorNo" will return a detailed error reason. A list of error
numbers can be seen below.
Table 6- 18 Error numbers
Error Number (Dec) Error Number
(Hex)
3 3
4 4
5 5
127 7F
MAG 8000 Modbus RTU
Operating Instructions, 04/2016, A5E03409989-AB
Description
The function code received in the query is not an allowable action for the
slave
The data address received in the query is not an allowable address for the
slave.
A value contained in the query data field is not an allowable value for
the addressed location. This may indicate a fault in the structure of the
remainder of a complex request, such that the implied length is incorrect.
The request is by some other reason not acceptable. It may e.g. indicate
that the data value to write is evaluated to be beyond limits.
The slave is processing a long-duration command. The master should
retransmit the message later when the slave is free.
No. of Data type
bytes
2 Word
2 Word
2 Word
2 Word
Error name
Write access denied
Max. limit
Min. limit
No error
Description
Last Coil ErrorAddr (MODBUS cmd: 1 or 5)
Last Coil ErrorNo (127 = No error)
Last HoldReg ErrorAddr (MODBUS cmd: 3 or 16) R
Last HoldReg ErrorNo (127 = No error)
Reason
This parameter is in a state where it is not writable
The value was greater than the allowed maximum value
The value was less than the allowed minimum value
No problem
Function codes
6.7 Exception handling
Read/
write
R
R
R
35