Prevent Unnecessary Polling of Your Devices with Scan Modes

Safeguard against clients requesting data too frequently

TOP Server drivers provide several different scan rate modes for controlling how any client requests affect the polling frequency for your configured devices. This applies to all client connection types including OPC UA, OPC DA and Wonderware SuiteLink interfaces.

These settings increase the flexibility for users who need or prefer to have granular 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.

Choose a scan mode that works best for each device

With the scan mode being configurable at the device level in TOP Server, you can select a different scan mode for each device, if you need to.  Set a scan rate floor or fix the scan rate at a specific frequency for certain devices or for all devices - the choice is yours based on the unique needs of your project.

Have a subset of devices on a network that you know has bandwidth issues but another group of devices that are on fiber with no such restrictions?  You can choose a different scan mode for each based on those architectural limitations.

Five scan modes enable different functionality for each scenario

The following modes are available for controlling the frequency at which your devices are polled:

  1. Respect client specified scan rate (Default) - This mode has historically been 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 or at the static tag level for non-OPC clients).
  2. 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.
  3. 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.
  4. 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.
    1. 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.
    2. Once triggered, the _DemandPoll item value will retain a value of 1 / True until polling is complete.
    3. After polling is complete, the _DemandPoll item value will revert to 0 / False.
  5. 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.

Device scan modes are useful for many different scenarios

Device scan modes allow the user to to pick and choose which methods to user per device. Typically, it is customary to define the scan rate at the client level, which is why that is the default behavior.

However, for bandwidth sensitive applications, having more control is typically desired. Setting a scan rate floor protects the control network from unruly client applications that might inadvertently request data updates far more quickly than necessary, generating excess traffic.

Going even further, restricting polling to demand polls only gives the user the peace of mind that a request to a particular device will only happen when they have expressly asked TOP Server to do so. This can be 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.

For those users seeking even more finite control over polling frequency, though, you may want to explore whether the TOP Server Scheduler Plug-in is the best fit for your application.

Important Notes:

  • The Scan Mode related device setting "Provide initial updates from cache" forces initial value updates for items to be pulled from the server cache (if available) rather than explicitly requested from the device when either a new client initially connects or when new items are added for an existing client connection.
  • It is always recommended to evaluate your polling frequency needs on a per device basis and to request only the updates you need for your tags for the best performance.
    • For example, if you only need updates for certain tags every 5 seconds but others are needed every second, you should not request all tags every second.
    • Knowing the polling frequency requirements of your project upfront will assist as you decide which scan mode is best for each device.
  • The TOP Server Scheduler Plug-in provides the most granular control over which devices and which tags for those devices are and are not polled by the driver.

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The demo is the full product once licensed. Once a client application connects to the  TOP Server, the TOP Server runtime will operate for 2 hours at a time. At the end of the 2 hour demo period, the demo timer must be reset by restarting the TOP Server runtime service.

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