1. What the Program Does
This example combines two level-fault conditions. An active Level_High or Level_Low sets a latched alarm. Even after the sensor clears, the alarm stays active until the operator requests reset.
Level_High OR Level_Low → Alarm_Latch → Level_Alarm2. Example Signals
| Signal | Type | Purpose |
|---|---|---|
| Level_High | Input | High-level fault |
| Level_Low | Input | Low-level fault |
| Manual_Reset | Input | Manual alarm reset |
| Alarm_OR | Block | Combines fault conditions |
| Alarm_Latch | Block | Retains the alarm state |
| Level_Alarm | Output | Final level alarm |
3. Main Logic
OR activates when either fault sensor is active. Its result enters Set on an SR latch; Manual_Reset enters Reset:
Alarm_Request = Level_High OR Level_Low
Level_Alarm = SR(Alarm_Request, Manual_Reset)4. Testing in the Simulator
- Activate Level_High: Level_Alarm becomes TRUE.
- Clear Level_High: Level_Alarm stays TRUE because it is latched.
- Activate Manual_Reset: the alarm clears if no fault is active.
- Repeat with Level_Low.
- Activate Level_High and Manual_Reset together to observe SR's Set priority: the alarm remains TRUE.
5. What to Watch For
The latch separates the occurrence of a fault from the current sensor state. Even a brief level excursion remains visible to the operator.
A real program usually permits reset only after the cause has been removed. This learning diagram keeps reset as a separate input so latch behaviour is easy to test.
Check in editor
Open the ready-made FBD diagram, toggle inputs, and compare the simulation behavior with the program notes. The JSON is also available as a direct download.