8 Communication
8.2 Modbus
140
SICAM I/O Unit, 7XV5673, Device Manual
E50417-H1040-C484-A3, Edition 05.2012
8.2.2
Exception Responses
The Modbus server performs a series of consistency checks of the Modbus client requests, and if errors (for
example request to read a nonexistent register) are detected, it generates Modbus exception codes which are
signaled to the Modbus client in exception responses messages.
The messages contain the following codes:
Exception-Code 01
ILLEGAL_FUNCTION
•
The Modbus client uses a function that is not supported by the Modbus server of the SICAM I/O Unit (the
supported Modbus functions are listed in chapter 8.2.1).
Exception-Code 02
ILLEGAL_DATA_ADDRESS
•
An attempt is made to read out or write to a nonexistent Modbus register (see chapter 8.2.8, Modbus
mapping for valid registers).
•
An attempt is made to read out or write to too many registers. A Modbus message enables reading out
125 holding registers and writing to 123 holding registers maximum.
•
The Modbus clients tries to write to a register in the Modbus server for which only read access is allowed
according to the Modbus mapping (see chapter 8.2.8).
Exception-Code 03
ILLEGAL_DATA_VALUE
•
The Modbus client addresses a register, for which the access to partial data was not released.
•
The Modbus client attempts to write to the Modbus server for which the access rights are set to "read only".
•
The Modbus client, the I/O mirror server, or the I/O mirror client (Ethernet or serial) tries to control the
writing of the
Indication from Remote
indication into the register of the SICAM I/O Unit. If the SICAM I/
O Unit is configured as I/O mirror server or I/O mirror client (apart from I/O mirror server, serial, unidirec-
tional), the
Indication from Remote
indication is only controlled via the I/O mirror.
Exception-Code 04
SERVER_FAILURE
•
Error during the time format conversion in the Modbus server because a faulty date/time format was
received via Modbus (for example month format > 12).
NOTE
The exception responses for Modbus TCP and Modbus UDP (Ethernet) and Modbus RTU (serial) are the
same.
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