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In the Modbus communications all these register addresses are not active and don't
contain valid information. A read or write to an unused register address will produce an
exception error.
7.6 Check the data access level and the limited range of data
To ensure that a write message to a Modbus address will produce a normal response,
check that the data value and access level is valid.
Writings in the controller memory though Modbus are restricted to the setting of some
access levels which need to be set. Look at:
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SET MODE
-
LOCKING SWITCH
-
BANK SETTINGS
Writings in the controller memory through Modbus are restricted to a limited range
applied on that data. An exception error is sent in case of a writing exceeding the allowed
limited range of a data.
7.7 Counters and Loopback diagnostics
Modbus offers some interesting functions to help the user to locate errors:
Fetch comm event counter (function 11) to catch successful messages.
Fetch comm event log (function 12) to look at the history of errors.
Diagnostics function and subfunctions (function 8)
Loopback diagnostics (subfunction 0) returns a response identical to the query
Restart communication (subfunction 1)
Force Listen only mode (subfunction 4)
Read various diagnostic counters
7.8 Debugging tool and documents
The user can find much application on the internet to scan Modbus devices and test
communication.
Look in the documentation of the controller and the corresponding Modbus data table
for appropriate information.
Look in the Modbus protocol for more information on Modbus.
18 – Troubleshooting ç RS-485
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