In order to perform some pick actions, such as a point Control, the ClearSCADA server needs to be in communications with the outstation.
When such a pick action is requested for an item on a direct outstation on which communications is set to ‘Always’ and not ‘On Demand’, ClearSCADA will immediately send that request to the outstation. If ClearSCADA has detected that the outstation’s direct line(s) have failed and the outstation has PSTN fallback enabled, ClearSCADA will execute a One Shot call to the outstation. The One Shot call sends the request to the outstation via PSTN.
With direct or PSTN outstations on which communications is set to ‘Incoming Only’ or ‘Periodic’, if such a pick action is requested outside of the available communications period, ClearSCADA will automatically queue that request until communications is next established. (If the pick action is requested during a ‘Periodic’ outstation’s available communications period, but that outstation’s direct line(s) are down, ClearSCADA will execute a One Shot call to the outstation. This will only occur if the outstation has PSTN fallback enabled and the direct line(s) are down during the availability period.)
With a PSTN outstation, if a call is in progress at the time, ClearSCADA will immediately send the request to the outstation. If a call is not in progress, ClearSCADA will execute a One Shot call to that outstation in order to send the request to the outstation.
With PSTN outstations and ‘On Demand’ outstations, it may be preferable to queue certain requests rather than have ClearSCADA contact the outstation as soon as such a request is triggered. The same applies to direct outstations on which PSTN fallback is enabled and ClearSCADA has detected that the direct channels have failed. For more information, see Optionally Queue Pick Action Requests on PSTN and ‘On Demand’Outstations.
Further Information
Direct and PSTN Communications.
‘Incoming Only’ and ‘Periodic’ communications: see Specify the Availability of Communications Between the Outstation and ClearSCADA in the ClearSCADA Guide to Advanced Drivers.