Using Value Maps to Represent Modbus Data Address Ranges

The Modbus Slave driver allows a Geo SCADA Expert system to act as a Modbus PLC and report any data changes back to its Master. To facilitate this, the Geo SCADA Expert server uses Value Maps to map ‘point’ properties into memory, so that they can be accessed by the Modbus protocol.

You need to configure a separate Value Map for each type of Modbus data—Input Register, Holding Register, Discrete, and Coil—that Geo SCADA Expert is to map to memory. You then need to specify which type of Modbus data each Value Map represents, by assigning each Value Map as an Input Register, Holding Register, Discrete, or Coil Map, on the Modbus Slave Source Form (see Specify the Value Maps that Store the Source Data). This sets the addresses that are populated in each Value Map to appear as Modbus Register, Discrete, or Coil addresses to the Modbus Master.

The table below indicates which type of Value Map you need to configure to represent each type of Modbus data. It also indicates the Modbus address ranges that each Value Map might represent, once that Value Map has been assigned as representing a particular type of Modbus data. (Modbus addressing can start at 0 or 1—the table below indicates the Modbus equivalent for Modbus addressing that starts at 1.)

Thus, to represent Modbus Holding Register 400010, you would populate address 9 of a WORD Value Map with the database item that represents that Holding Register. You would then specify the WORD Value Map as the Holding Register Map property on the Modbus Slave Source Form.

For a more detailed example of how you might use a Value Map to emulate Modbus data, see Example Configuration.