A central SCADA server utilizes a data communication network to exchange data such as settings, configuration, sensor data and energy consumption data with the electronic controllers installed in the district heating systems.
The SCADA system is compatible with the protocols Modbus/RS-485 (for serial line data communication) and Modbus/TCP (for Ethernet or internet data communication)
It is possible to operate the SCADA system from an unlimited number of workstations by using an internet browser (multi-user capability)
When an electronic controller is added to the SCADA system the graphical user interface of the SCADA system is generated automatically to reflect the end user’s control application in the electronic controller
***Free programmable controller(s) can be added to the SCADA system by using OPC-UA server software (Unified Architecture)
В каком значении здесь может быть "free"?

Спасибо.

т.е. те, которые не входят в комплект оборудования и их докупили, но нужно программировать индивидуально и сопрягать с помощью открытого ПО