This section describes the data retrieval method used for PSTN outstations.
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.
The process of retrieving data from an outstation 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’). For information on the former type of connection, see Scanning Direct Outstations.
With a PSTN connection, communications are typically re-established each time the server requests data from the outstation or the outstation dials in to report an alarm. You can configure dial in limits for the outstation so that if it is dialing in too frequently, ClearSCADA will raise an alarm or generate an event to inform users that a potential problem has been detected. To configure the dial in limits, you need to use the Dial In Overactive feature (see Configuring an Outstation to Raise an Alarm if it Dials In Too Frequently).
When data has been retrieved from a PSTN outstation, communications typically cease until the next time that a connection is established.
When ClearSCADA requests data from a PSTN outstation, the server typically makes a ‘One Shot’ call to that outstation to retrieve the data from the outstation. If you want a ‘One Shot’ to occur automatically and on a regular basis, you should associate the One Shotpick action with a Schedule (see Using Schedules to Automate Regular Functions). You can use a Schedule, for example, to trigger the server to call a particular outstation or outstations at 04:00 each day and retrieve any data that has been generated since the outstations were last called.
(With systems on which telephone call duration costs do not apply, you might want to consider using an Open Line call instead of a One Shot call. Unlike a One Shot call, an Open Line call enables ClearSCADA to maintain communications with a PSTN outstation for a specified time period—for more information, see Configuring an Outstation, and see Define an Outstation’s PSTN Properties.)
A One Shot call is triggered whenever any pick action is executed that requires the ClearSCADA server to communicate with a PSTN outstation, or an outstation that has fallen back to using PSTN communications. Such actions include, for example, any Control, Inching, or One Shot pick actions.
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