If the scan time is too long, a watchdog error will be declared.

Watchdog Error

An error condition triggered by a watchdog timer, which monitors system or process activity to ensure it is operating within expected norms, indicating malfunctions or failures.

Scan Time

The time required to read all inputs, execute the control program, and update local and remote input and output statuses. Scan time is, in effect, the time required to activate an output controlled by programmed logic.