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CE-05171N page 41 of 44
7.3 Modbus Protocol
The Centurion configurable controller was programmed with the Modbus protocol which is a system is based on
a “master” and “slave” relationship. With Modbus protocol, the master and slave are able to continue to com-
municate with each other through defined messages over a variety of network types. The master initiates the
queries or commands, and the slave responds to the query with a message or takes action based on the query.
In this case, the master is either MConfig (or another Modbus client software), or C3-3 Display, but never both
simultaneously as there can only ever be a single Modbus master.
The Centurion system communicates through Modbus using Remote Terminal Unit (RTU) transmission mode to
maximize data processing.
As with all numeric data defined within Modbus, the programming is limited to accepting integers (whole num-
bers only, no decimals). This is important whenever a decimal point is defined for analog inputs.
For example, for the controller to properly read “100.0” with an implied decimal point of 1, the user would need
to enter “1000” and 1000 would be stored in the appropriate Modbus register. From the C3-3 Display, the value
will be represented with a decimal in the fixed position.
For additional information, please refer to the Centurion Configurable Controller Programming Manual.
7.4 Features of Transferring Data in Modbus
Note that text visible in the MConfigPro software and on the C3-3 Display is not stored in the Centurion con-
troller. Except for the version description in the Version and Security dialog for each configuration file, descrip-
tions assigned in the configuration are not stored in the Centurion controller. Descriptions for various I/O and
processes are downloaded to the C3-3 Display and reside as a configuration in that device only. Configurations
of the C3-3 are one way only.
Parameter changes made in the C3-3 Display are actually communicated to the Centurion controller where the
logic resides. No changes made through the C3-3 Display affect the C3-3 Display configuration as the display
merely reads from, and writes to the Centurion main I/O module. As such, the display can write numeric para-
meters to the controller.
7.5 Modbus Register Address Listings
For a complete MODBUS address map, please refer to The Centurion MODBUS reference document.