A controller that trips silently helps no one at 3 a.m. FactoryTalk Alarms & Events turns conditions into named, prioritised alarms with a full lifecycle (raise → acknowledge → return-to-normal) and a time-stamped audit trail of exactly what tripped and when. Drive the cell below and work the alarms the way an operator would.
Two alarms watch this cell: an ALMA on the analog Tank_Level (trips at the HI limit) and an ALMD on the digital Motor_01.Fault. Fill the tank past the limit, trip the motor, then acknowledge, and watch each alarm walk through the ISA-18.2 states while the event log writes an audit record for every transition.
| Alarm | Type | State | Ack |
|---|
Notice the four states: Normal (mint), Active · unacked (red, blinking on the banner), Active · acked (it's still in alarm, but someone has seen it), and RTN · unacked (amber: the condition cleared on its own but still needs an acknowledgement so that nobody misses it). An alarm only goes back to Normal once it is both clear and acknowledged.
🔔 ALMD: digital alarm. You use one instruction per discrete condition (a fault bit, an E-stop, a door). It carries severity and the in-alarm/acked state, and it latches the time it tripped. Drop it on the rung next to the logic that detects the condition.
📈 ALMA: analog alarm. It watches a real value against LL / L / H / HH limits, with a deadband so a noisy signal near the limit doesn't chatter. One instruction covers a level, a pressure or a temperature across its whole range.
🖥️ Tag-based vs device-based. Modern Logix alarms live in the controller (the ALMD/ALMA tags) and FactoryTalk subscribes to them, so there is no separate alarm database to keep in sync. FactoryTalk Alarms & Events can also run server tag-based alarms, defined in its own Tag Alarm and Event Server instead. That is the route for PLC-5, SLC 500 and third-party controllers that have no alarm instructions.
🗂️ Events & the historian. Every raise, ack and return is logged with a timestamp, the operator and any comment, first to the FactoryTalk Alarm & Event log and then on to a historian. That record is your audit trail and your post-mortem.
Severity (1–1000) sets the sort order and colour on the banner and summary; shelving lets an operator silence a known-nuisance alarm for a set time without disabling it; and latched alarms hold until acknowledged even if the condition flickers. Good alarm design mostly comes down to fewer, better alarms, rationalised so that every one means "do something now."
An alarm's condition clears on its own before anyone acknowledges it. In an ISA-18.2 lifecycle, what state is it in?