How to Read an Analog Input in PLC Programming

How to Read an Analog Input in PLC Programming

Understanding analog inputs is one of the most important skills in industrial automation. Unlike digital signals that are simply ON or OFF, analog signals provide continuous measurements such as temperature, pressure, flow, level, weight, and speed.

This infographic explains the complete process of reading and using an analog input in Siemens PLC programming.

Step 1 – Connect the Sensor

Industrial sensors typically provide:

4–20 mA

0–10 V

These signals are connected to the PLC analog input module.

Step 2 – Read the Raw Value

The PLC does not initially see engineering units such as °C, bar, or %.

Instead, it reads a raw digital value.

Example for Siemens S7-1200 / S7-1500:

0 … 27648

The analog module converts the electrical signal into a digital value that the PLC can process.

Step 3 – Scale the Value

The raw value must be converted into engineering units.

Examples:

Tank Level → 0–100%

Pressure → 0–10 bar

Temperature → 0–200°C

Proper scaling allows operators and engineers to work with meaningful process values.

Step 4 – Store the Value

The scaled value is usually stored in a REAL variable for accurate calculations and monitoring.

Example:

TankLevel : REAL

Pressure : REAL

Temperature : REAL

Step 5 – Use the Value

Once scaled, the value can be used for:

HMI Displays

Trends

Alarms

Process Control

Reports

PID Control

Real-World Examples Included

Tank Level Monitoring

Pressure Measurement

Temperature Monitoring

Each example demonstrates how the same analog input principle applies to different industrial processes.

Key Principle

Sensor → Analog Input → Raw Value → Scaling → REAL Value → HMI

This simple sequence forms the foundation of nearly every industrial process control application.

Understanding analog inputs is essential for working with level transmitters, pressure transmitters, flow meters, temperature sensors, weighing systems, and advanced process automation.