Scan Class Name

Unique name of the scan class.

Mode

Direct
The scan class executes at a fixed rate, defined by the slow rate setting.

Leased
The scan class executes at the fast rate when any of the tags it contains are subscribed and visible in a Client window. If no tags are subscribed, the scan class runs at the slow rate.

Driven
The rate of the scan class is based on the value of a driving tag. The condition is a simple comparison between a tag value and a number. If the condition is true, the scan class will execute at the fast rate. If false, it will run at the slow rate. There are two exceptions to this: the Any Change operator, and One-shot mode. Using either of these, the scan class will not run at a rate. Instead, it will be triggered by a change in the driving tag's value. Keep in mind that the driving tag can be an Expression tag that performs complex calculations and references other tags. In this way, it's possible to create robust scan class triggering.

Slow Rate

Base update rate, specified in milliseconds, at which tags will be executed.
Note: If the rate is set to 0, the scan class will not be executed. It is common for leased and driven modes to use 0 as a slow rate in order to achieve an on/off effect.

Fast Rate

Used by the Driven and Leased modes, this is the faster rate that the tags will be executed at when those modes are active.
Note: If the rate is set to 0, the scan class will not be executed.

Stale Timeout

How long to wait before the tags in the scan class are determined to be stale (not running). This is calculated off of the last expected execution time of the scan class, and is particularly important for scan classes executed by other drivers through the external Tags provider. This property is not used by internal providers.

Driving Tag

Which tag will determine when the rate of the scan class should be changed.

The Driving Tag should not use this same scan class. The Driving Tag should be on a separate scan class that runs at a faster rate than the Driven scan class. This means that changes to the Driving Tag will be recognized by the system faster, and the change in rate on the Driven scan class will occur faster.

Operator

How the Value property should be compared to the Driving Tag's value. If the comparison is true, then the Fast Rate will be used by the Scan Class. Otherwise, the Slow Rate will be used.

The Any Change operator works differently than the other operators: The scan class will execute immediately whenever the driving tag changes value. Using the Any Change operator means that the scan class no longer uses the Slow Rate or Fast Rate properties.

Value

Used by the Operator property to determine if the Scan Class should execute at the slow or fast rate.

One-shot execution

One-shot will execute once when the comparison condition is true, and not again until the condition become false, and subsequently true.

OPC Data Mode

This mode dictates how OPC values are obtained. The default mode, Subscription, is preferred because it is much more efficient than a read.

Subscribed
All OPC tags in the scan class will be subscribed according to the scan class rate. Values will come in asynchronously as they change.

Read
Tags will not be subscribed, but will instead be synchronously read each time the scan class executes. This operation is less efficient, but allows more precise control over when values are obtained. This mode is particularly useful when collecting data over a slow or expensive connection for display. When combined with the one-shot execution mode above, and a static tag tied to a momentary button, it's easy to create a manual refresh button on a screen that pulls data on-demand.

OPC Read After Write

When enabled, a read request will be sent immediately after a write request. This means that the value on the tag will be updated much quicker to reflect the latest written value.

Enabling this property is less efficient as a single write to a tag becomes two separate requests. This is especially helpful with slower scan classes as the tags will show the latest value quicker than the normal execution would allow.