Tag linking (PLC to HMI) in PLC Programming

Introduction

Tag linking connects data points in a PLC to display or control elements in an HMI. It helps operators see and change machine status easily.

You want to show sensor values on a screen for monitoring.

You need to start or stop a machine from the HMI panel.

You want to display alarms or warnings from the PLC on the HMI.

You want to log data from the PLC to the HMI for reports.

You want to change setpoints or parameters from the HMI.

Syntax

PLC Programming

HMI_Tag = PLC_Tag

HMI_Tag is the name of the variable on the HMI side.

PLC_Tag is the variable name in the PLC program.

Examples

Link the temperature sensor value from PLC to show on HMI.

PLC Programming

HMI_Temperature = PLC_Temperature_Sensor

Link a start button on HMI to control motor start in PLC.

PLC Programming

HMI_StartButton = PLC_Motor_Start

Show alarm status from PLC on HMI screen.

PLC Programming

HMI_AlarmStatus = PLC_Alarm_Flag

Sample Program

This example shows a temperature sensor value linked from PLC to HMI display. It also links a start button on HMI to control the motor start in PLC. When the start button is pressed, the motor start command updates.

PLC Programming

PLC_Tag Temperature_Sensor : REAL := 25.5;
PLC_Tag Motor_Start : BOOL := FALSE;

// HMI links
HMI_Tag Display_Temperature : REAL;
HMI_Tag Start_Button : BOOL;

// Linking logic
Display_Temperature := Temperature_Sensor;
Motor_Start := Start_Button;

// Simulate pressing start button on HMI
Start_Button := TRUE;

// Output current states
PRINT("Temperature on HMI: " + TO_STRING(Display_Temperature));
PRINT("Motor start command from HMI: " + TO_STRING(Motor_Start));

Important Notes

Tag names must match exactly between PLC and HMI for linking to work.

Data types should be compatible (e.g., BOOL to BOOL, REAL to REAL).

Some HMI software requires manual tag configuration or import from PLC.

Summary

Tag linking connects PLC variables to HMI elements for monitoring and control.

It allows real-time data exchange between PLC and HMI.

Correct naming and data types are important for successful linking.

Practice

1. What is the main purpose of tag linking between a PLC and an HMI?

easy

A. To allow real-time data exchange for monitoring and control

B. To program the PLC logic

C. To design the HMI screen layout

D. To compile the PLC code

Solution

1. Step 1: Understand tag linking function

   Tag linking connects PLC variables to HMI elements so data can be shared live.

2. Step 2: Identify the purpose

   This connection allows operators to monitor and control processes via the HMI using PLC data.

3. Final Answer:

   To allow real-time data exchange for monitoring and control -> Option A

4. Quick Check:

   Tag linking = real-time data exchange [OK]

Hint: Tag linking means live data sharing between PLC and HMI [OK]

Common Mistakes:

• Confusing tag linking with programming PLC logic
• Thinking tag linking designs the HMI screen
• Assuming tag linking compiles code

2. Which of the following is the correct way to link a PLC tag named MotorSpeed to an HMI element?

easy

A. Link the HMI element to the exact tag name MotorSpeed

B. Link the HMI element to a tag named motor_speed (different case)

C. Link the HMI element to any random tag name

D. Link the HMI element without specifying a tag name

Solution

1. Step 1: Check tag name matching

   Tag linking requires exact matching of tag names including case sensitivity.

2. Step 2: Choose correct linking method

   Only linking to the exact tag name MotorSpeed ensures proper data exchange.

3. Final Answer:

   Link the HMI element to the exact tag name MotorSpeed -> Option A

4. Quick Check:

   Exact tag name match = correct linking [OK]

Hint: Match tag names exactly, including case [OK]

Common Mistakes:

• Ignoring case sensitivity in tag names
• Using incorrect or random tag names
• Not specifying any tag name

3. Given the PLC tag Temperature with value 75, what will the HMI display if linked correctly?

medium

A. 0

B. 75

C. Temperature

D. Error

Solution

1. Step 1: Understand tag linking output

   The HMI shows the current value of the linked PLC tag.

2. Step 2: Apply to given value

   Since Temperature is 75 in PLC, HMI displays 75.

3. Final Answer:

   75 -> Option B

4. Quick Check:

   PLC value 75 = HMI display 75 [OK]

Hint: HMI shows the PLC tag's current value [OK]

Common Mistakes:

• Expecting tag name instead of value on HMI
• Assuming default zero if not linked
• Thinking HMI shows error without reason

4. You linked a PLC tag Pressure to an HMI element but the HMI shows no data. What is the most likely cause?

medium

A. The PLC program is running fine

B. The HMI screen design is incorrect

C. The HMI device is powered off

D. The PLC tag Pressure is not defined or has a different name

Solution

1. Step 1: Check tag definition

   If the PLC tag Pressure is missing or named differently, linking fails.

2. Step 2: Identify cause of no data

   No data on HMI usually means tag mismatch or missing tag in PLC, not design or power issues.

3. Final Answer:

   The PLC tag Pressure is not defined or has a different name -> Option D

4. Quick Check:

   Missing or wrong tag name = no data [OK]

Hint: Check tag names first if HMI shows no data [OK]

Common Mistakes:

• Blaming HMI design instead of tag mismatch
• Ignoring tag existence in PLC
• Assuming power issues without checking tags

5. You want to display a PLC boolean tag AlarmActive on the HMI as a red light when true and green light when false. How should you link and configure the HMI element?

hard

A. Create a new tag in HMI without linking to PLC

B. Link AlarmActive tag but use only one color for both states

C. Link AlarmActive tag and set HMI element colors for true (red) and false (green)

D. Link a numeric PLC tag instead of boolean

Solution

1. Step 1: Link the boolean PLC tag

   Link the AlarmActive boolean tag directly to the HMI element to get live status.

2. Step 2: Configure HMI element colors

   Set the HMI element to show red light when true and green light when false for clear status indication.

3. Final Answer:

   Link AlarmActive tag and set HMI element colors for true (red) and false (green) -> Option C

4. Quick Check:

   Boolean tag + color config = correct display [OK]

Hint: Use boolean tag with color settings for status lights [OK]

Common Mistakes:

• Using one color for both states causing confusion
• Not linking the PLC tag to HMI
• Linking wrong data type tag