Priority input scan

For any writable point, the effective input value is determined by a priority scan, looking for a non-auto action at level 1 (emergency), then the value at the highest valid input, going from level 2, to 3, and so on to level 16. (At level 8, any non-auto action is evaluated as valid).

Like almost all control execution, this priority scan is event-driven, meaning it occurs when any input value changes. An input’s value typically comes from a link, however, for most inputs, you may enter a value directly in the point’s property sheet (as an alternative source).

A valid input is one with none of the following status bits set:

  • down

  • fault

  • disabled

  • null

  • stale

If all 16 priority levels are evaluated without a valid input (and without an action at levels 1 and 8), then the fallback value is used.

You can configure the writable point’s Fallback property to be null, so that the point’s Out has a null status in this condition. Depending on the specific control sequence and usage of the writable point, this may be an effective solution

However, by default, a set action exists on any writable point, which writes directly to the Fallback value. If you want a writable point to always have a Fallback of null, go to its slot sheet and set the Hidden config flag on the slot. Otherwise, a user can invoke a right-click command to set Fallback to any value.