How to correct the D6412/D4412 keypad display: System Busy

When multiple D623 or D625 keypd are in use and the display reads SYSTEM BUSY, it may be caused by a stuck key on one of the keypds. 

Inspect each keypad for a stuck key. If necessary leave only a single keypad connected and test for normal operation.  If the problem persists, bring a single keypad back to the panel and connect it while the field wiring is removed.