Where a third-party server uses the Modbus protocol to retrieve data from Geo SCADA Expert, the Geo SCADA Expert system acts as a Slave PLC.
For this scenario, Geo SCADA Expert needs to run the Modbus Slave driver, and you need to configure suitable Slave items in the Geo SCADA Expert database (see Configuring Modbus Slave Devices in Geo SCADA Expert).
The Modbus Slave indexes into the Value Maps to extract the value(s) that are requested by the third-party server.
