Wiring APROL GLOBAL_ALARM_SIGNAL to a Physical Alarm Horn

Overview

A common APROL requirement: energize a physical digital output to sound a buzzer or horn whenever an unacknowledged alarm exists anywhere in the system — the same condition that makes the AlarmMonitor line display flash with a red frame and triggers the operator station's audible horn.

The naive implementation is a large OR network that collects the outputs of every alarm block in every CFC. That approach is unmaintainable: every new alarm block must be manually wired into the OR tree, and any omission produces a silent horn for that alarm.

APROL already maintains this aggregate condition internally as the system variable GLOBAL_ALARM_SIGNAL. The AlarmServer sets it to 1 when one or more unacknowledged alarms are present in the AlarmMonitor, and it returns to 0 when all alarms have been acknowledged. The point that costs engineers hours is that this variable is not exposed in the normal variable browser — a wildcard search on GLOBAL* in "All variables" returns nothing. Access is provided exclusively through a dedicated function block.

Root Cause: Why the Variable Search Returns Nothing

GLOBAL_ALARM_SIGNAL is an AlarmServer-owned system variable, not a process variable published into the normal engineering variable namespace that the CaeManager variable browser indexes. Consequences for the engineer:

• Name-wildcard searches (GLOBAL, *ALARM_SIGNAL*) in the variable list produce zero hits — this is expected behaviour, not a broken installation or a missing option package.
• You cannot drag-and-drop the variable onto a CFC connector, and you cannot reference it directly by typing the name into a connector field.
• B&R Online Help contains wording that discourages direct use of the variable. Read this as: the direct-reference path is deprecated in favour of the function-block path, not as "the signal no longer exists." The AlarmServer still maintains the signal in APROL 4.x.

Solution: AprFuAlarmSignal

The function block AprFuAlarmSignal, supplied in the APROL library, is the sanctioned interface. Per its block description, it makes the GLOBAL_ALARM_SIGNAL system variable available to a logic (CFC/IEC task) for further processing.

Because the block reflects the AlarmServer's aggregate state, it automatically includes every alarm the AlarmServer manages. No maintenance is required when alarm blocks are added or deleted later.

Step-by-Step: Driving a Physical Horn Output

1. Choose the host task. Place the logic in a CFC that runs on the controller that owns the physical DO channel. Note the architectural constraint: GLOBAL_ALARM_SIGNAL is an AlarmServer/runtime-system signal, so the consuming logic depends on the connection to that server. Plan the failure behaviour accordingly (see the note below).
2. Insert the block. In the CaeManager CFC editor, open the block catalog, select library APROL, and drag AprFuAlarmSignal onto the sheet. If it is not visible, verify the APROL library is included in the project/task's library list.
3. Read the block's output. Connect the block output (the BOOL carrying the global alarm state) to a named connector, e.g. GlobalAlarmActive, so it can be reused by several consumers (horn, beacon, remote annunciator, SMS gateway trigger).
4. Add horn conditioning logic. A raw connection to the DO is rarely acceptable in the field. Typical conditioning:
   HornOut := GlobalAlarmActive AND NOT HornSilenced AND HornEnableKeyswitch;
   Latch HornSilenced on a "Silence/Mute" pushbutton and reset it on any rising edge of GlobalAlarmActive so that a new alarm re-sounds a silenced horn. Without this reset, one silence press mutes the plant indefinitely.
5. Add a pulse/ring-back timer (optional). If continuous sounding is undesirable, gate the output with an on-delay/off-delay pair or a periodic pulse so the horn beeps rather than sounds continuously while the condition persists. Keep the pulse period well above the DO module and relay switching lifetime constraints.
6. Map to the physical channel. Assign the resulting BOOL to the digital output channel in the I/O configuration. Confirm the channel's electrical rating covers the horn's inrush; for inductive or high-inrush sounders, drive an interposing relay or contactor from the DO rather than the field device directly.
7. Protect the output. Fit a flyback diode (DC coil) or RC snubber (AC coil) across an interposing relay coil, and fuse the horn branch separately from the I/O supply so a shorted sounder cable cannot drop the DO module supply.
8. Download and commission. Build the project, download to the target, and verify with the runtime online view that the block output tracks the AlarmMonitor state.

Verification

1. Force an alarm. Trigger a low-consequence alarm (simulate a process value across a limit, or use an alarm block with a test input). Confirm all three indications appear together: AlarmMonitor red flashing frame in the line display, operator-station horn "honking" (if the horn is enabled on that station), and the physical DO channel LED illuminating.
2. Check the block output online. Open the CFC in online/watch mode and confirm the AprFuAlarmSignal output = 1 concurrently with the flashing frame.
3. Acknowledge and confirm reset. Acknowledge the alarm in the AlarmMonitor. The output must fall to 0 once no unacknowledged alarms remain. If it stays at 1, an unacknowledged alarm still exists somewhere — filter the AlarmMonitor for unacknowledged entries across all areas, not just the current view, before suspecting the block.
4. Test the silence latch. With an alarm active, press Silence and confirm the DO drops. Trigger a second, different alarm and confirm the DO re-energizes on the rising edge of the aggregate signal.
5. Measure the horn branch. With the horn sounding, check the DO channel/interposing relay coil voltage and the actual load current against the module's per-channel and per-group ratings from the module datasheet. Verify total group current with all annunciation outputs simultaneously on — the worst case is an alarm flood.

Design Notes and Pitfalls

Confirm the exact input/output pin names, data types, and any enable/reset pins for AprFuAlarmSignal against the block description in the B&R Online Help for your installed APROL version before wiring the CFC, since pin sets can differ between library revisions.

FAQ