How to Create Alarm in HMI: Simple Steps for PLC Programming

To create an alarm in an HMI, define an alarm tag linked to a PLC variable and configure the alarm conditions and messages in the HMI software. Use alarm blocks or alarm tags to monitor values and trigger visual or audible alerts when conditions are met.

Syntax

Creating an alarm in HMI typically involves these parts:

• Alarm Tag: A variable linked to a PLC value to monitor.
• Condition: The rule that triggers the alarm (e.g., value > threshold).
• Message: Text shown when the alarm activates.
• Action: What happens when alarm triggers (e.g., sound, flash).

Each HMI software has its own way to define these, but the core idea is to watch a PLC variable and alert when a condition is true.

plaintext

AlarmTag: BOOL
Condition: AlarmTag == TRUE
Message: "Temperature High"
Action: Flash, Sound

Example

This example shows how to create a simple high temperature alarm in an HMI linked to a PLC variable TempSensor. The alarm triggers if temperature goes above 75 degrees.

structured_text

VAR
  TempSensor: INT; // PLC temperature input
  TempHighAlarm: BOOL; // Alarm tag
END_VAR

// Alarm condition logic in PLC
TempHighAlarm := TempSensor > 75;

// In HMI software, create alarm linked to TempHighAlarm
// Alarm message: "Temperature exceeds safe limit!"
// Alarm action: Flash red indicator and sound buzzer

Output

When TempSensor > 75, TempHighAlarm becomes TRUE, triggering the HMI alarm with message and actions.

Common Pitfalls

• Not linking the alarm tag correctly to the PLC variable causes alarms never to trigger.
• Using wrong condition logic (e.g., TempSensor < 75 instead of > 75) triggers alarms incorrectly.
• Forgetting to configure alarm actions (sound, flash) means alarms show no alert.
• Not testing alarms with real PLC values can miss errors.

structured_text

/* Wrong condition example */
TempHighAlarm := TempSensor < 75; // This triggers alarm when temperature is low, which is wrong

/* Correct condition example */
TempHighAlarm := TempSensor > 75; // Triggers alarm only when temperature is high

Quick Reference

Remember these quick tips when creating alarms in HMI:

• Always link alarm tags to correct PLC variables.
• Define clear and correct alarm conditions.
• Set meaningful alarm messages for operators.
• Configure visible and audible alarm actions.
• Test alarms with real or simulated PLC data.

Key Takeaways

Link alarm tags directly to PLC variables to monitor real-time data.

Use clear conditions to trigger alarms only when needed.

Provide meaningful messages and configure alert actions for operator awareness.

Test alarms thoroughly to ensure they work as expected.

Avoid common mistakes like incorrect conditions or missing alarm actions.