The process of scanning (‘polling’) direct outstations for data differs with each advanced driver. This section explains the basic principles that apply to most advanced drivers. Use this section in conjunction with the relevant driver-specific documentation for comprehensive information on the scanning regime used by the driver(s) on your system, (Outstation Types and Communications Set-ups).
This section describes the scanning method used for direct outstations on many advanced drivers.
Many outstations on advanced drivers support current data—the present state or value of a point at the instance that the ClearSCADA server requests current data from the outstation. Many outstations also support logged data—time stamped data that is stored at the outstation.
Scanning is the process of checking each of the outstations on a channel to assess whether any of those outstations have new current data or logged data. The data is then retrieved, processed and stored in the database, ready for use in ClearSCADA. Each advanced driver on a ClearSCADA system undertakes the scanning of the outstations that are associated with that driver. For example, the IEC 60870-5 driver on a ClearSCADA system scans each of the IEC 60870-5 outstations that are active.
The process of retrieving data differs, depending whether the outstation has a permanent connection to the server (‘direct communications’), or uses a dial-up modem and telephone line for all communications (‘PSTN communications’). With a PSTN outstation, communications are typically re-established each time the server requests data from the outstation, or the outstation dials in to report an alarm. Once data has been retrieved from a PSTN outstation, communications then typically cease until the next time that a connection is established. For information on this type of connection, see Retrieving Data from PSTN Outstations.
For information on the process used to scan direct outstations on many advanced drivers, see Scanning Process and Independent Connection Scanning Process.