Table of Contents
Advertisement
•
The Modbus RTU message has a maximum size of 256 bytes:
–
1 byte server address
–
253 bytes for data
–
2 bytes for CRC
Exception Responses for Modbus TCP Server and Modbus RTU (slave)
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 Q200. The
supported Modbus functions are listed in the next chapter.
Exception Code 02 - ILLEGAL_DATA_ADDRESS
•
An attempt is made to read out or write to a nonexistent Modbus register.
•
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 client tries to write to a register in the Modbus server for which only read access is allowed
according to the Modbus mapping.
Exception Code 03 - ILLEGAL_DATA_VALUE
•
The redundancy client addresses a register that does not allow access to partial data. The register has a
complex data structure and is distributed across several registers. The register can be read or written only
as a whole.
•
The Modbus client tries to write to the Modbus server for which the access rights are set to "read only".
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
i
i
The parameterization of the Ethernet communication via the Ethernet interface are described in detail in
the SICAM Q200 device manual.
SICAM, Multifunctional Recorder, Manual
E50417-C1040-C606-A3, Edition 03.2017
2.1 Functions, Properties and Exception Responses for Modbus TCP Server and Modbus RTU (Slave)
Communication Modbus
15
Hide quick links:
Advertisement
Table of Contents
