How long will it take for a MNB controller to drop to 'Read-Only' mode when the AC power supply voltage drops below 20.4 Vac?

Issue

How long will it take for a MNB controller to drop to 'Read-Only' mode when the AC power supply voltage drops below 20.4 Vac?

Product Line

TAC I/A Series

Environment

MNB controllers

Cause

Site installation using one AC transformer for multiple controllers.  Site is also suffering brownout conditions.

Resolution

Factory testing confirms that it takes only one AC cycle of power (i.e. one complete sine wave) of voltage below 20.4 Vac for the controller to drop into 'Read-Only' mode.

It is difficult to view with a standard digital multimeter.  A scope may be required to capture the voltage dip.