Communicating with Remote Devices via Modems

A dial-up remote I/O Device is one which is connected to CitectSCADA through a PSTN (Public Switched Telephone Network), and is accessible through pre-selected and pre-configured modems.

Once connected, CitectSCADA can write to, and read from, dial-up remote I/O Devices just as it does with any other I/O Device: local or remote.

Communications can be:

• On request: initiated by CitectSCADA using IODeviceControl() or by the remote I/O Device (for instance, to report an alarm condition).
• Periodic: for instance, to transfer the logged events for a period.
• Persistent: for instance, to monitor and control the water level at a remote dam.

The only limiting factor would be an inability to connect a modem to an I/O Device due to incompatible communications capabilities. Many I/O Devices have fixed settings and can only communicate at a pre-set rate determined by the manufacturer. If a modem cannot match these settings, communication cannot be established.

To make communications setup easier, you can connect dial-up remote I/O Devices with identical communications to the same modem and port. Where I/O Devices are connected to the same modem, CitectSCADA can communicate with each I/O Device one after the other using the same phone connection, rather than hanging up and re-dialing. This reduces the number of necessary telephone calls and increases the speed and efficiency of communications.

You need to have at least one modem at the I/O Server end, and at least one at the I/O Device end.

See Also