TOP Server drivers provide several different scan rate modes for controlling how client request affect the polling of your configured devices.
The following modes are available for limiting the frequency that your devices are polled:
- Respect client specified scan rate (Default) - this mode is the default behavior of TOP Server and your devices are polled at the rate specified by the requesting client application (or at the rate specified in the TOP Server Alias Map).
- Request data no faster than - this mode sets a scan rate floor, allowing clients to poll your devices only as fast as the rate specified. Client updates are still allowed to occur as requested if they are slower than the specified rate.
- Request all data at - this mode fixes the scan rate for all client requests at the rate specified, essentially eliminating polling control by client applications.
- Do not scan, demand poll only - this mode provides full control of device polling from the client application, via writing to a trigger-based system tag named _DemandPoll. Polling of devices where this mode is selected will only ever occur upon a write by a client to the _DemandPoll tag for the device.
- Once triggered, the _DemandPoll item value will retain a value of 1 / True until polling is complete.
- After polling is complete, the _DemandPoll item value will revert to 0 / False.
- Respect tag specified scan rate – this mode forces clients to use the scan rate defined in the static tag properties, which allows a user to set scan rate limits per tag. Dynamic tags are scanned at the client-specified scan rate.
When enabled, the device-level setting “Provide initial updates from cache” forces values to items to be pulled from the server cache (if available) rather than requested from the device when a client initially connects.
These settings increase the flexibility for users who must have absolute control over the frequency of requests to their devices. This is useful for users with devices that require slow updates or who are using networks where excess data usage results in additional operational costs.
The Device Demand Poll mode is particularly helpful in SCADA industries like Oil & Gas, Water/Wastewater, Power and others where significant communication delays are common due to the distributed nature of the communications network.