Siemens SENTRON 3WA, System Manual

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Modbus TCP
5.6 Invalid values
3WA Air Circuit Breaker Communication
System Manual, 11/2021, 92310000004-02
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5.6 Invalid values
If data points which basically exist but which are not available temporarily are read using
Modbus function code 3 (hex: 0x03), the corresponding registers in the Modbus frame of the
COM190 are filled with the bit pattern 0xFF (all bits '1') to tag them as invalid.
The same fill pattern is used if a data point is only partially read or if reading takes place
across a gap that exists between two data points in the Modbus register map.
Possible reasons for temporary non-availability include:
• A communication fault is present on the CubicleBus² between the COM190
communication module and other 3WA components.
• The withdrawable circuit breaker is in the disconnected position in the guide frame, i.e.
communication via the CubicleBus² between the modules on the DIN rail or auxiliary
conductor contact system and the internal modules of the 3WA circuit breaker (ETU600,
BSS200, TUI600) has been interrupted.
If the fill pattern (all bits '1') is read for a data point, the contents of the data point are
considered to be invalid.
5.7 Difference between Modbus register number and register
address
Based on the Modbus Application Protocol Specification V1.1b3 Section 6.3, the Modbus
register number and register address in the 3WA system are different.
In order to obtain the register address, the register number must be reduced by the value 1:
register address = register number – 1
Example:
Modbus register address 0x3300 = register number 0x3301 – 1
5.8 Modbus TCP whitelist
The Modbus TCP whitelist is a security function of the COM190 communication module
which can prevent communication via Modbus TCP for addresses which have not been
approved. Only IP addresses which have been entered in the Modbus TCP whitelist and have
therefore been approved have access to the COM190.
Mode of operation
IP addresses which have not been entered in the whitelist are prevented from setting up a
TCP connection, making it impossible for them to access the COM190. In this way, the
Modbus TCP whitelist prevents not only reading of data and metering values but also changes
to parameters.