PLC Programming · Analog Input · 4-20 mA · Siemens S7-1500 · Process Control
PLC Analog Input Simplified: 4 Essential Steps Every Beginner Must Master First
Most beginners struggle to understand how a real-world signal actually enters a PLC. A PLC analog input takes a continuous physical measurement, such as tank level or pressure, converts it to a 4-20 mA electrical signal, and reads it through an analog input module for control, monitoring and alarms. This guide breaks down the complete signal path from sensor to PLC logic, explains why 4-20 mA is the industry standard, and shows a real Siemens S7-1500 example with a live scaling calculator.
PLC Analog Input: The 4-Step Signal Path Every Beginner Must Master
A sensor (level, pressure, flow, temperature transmitter) measures the physical process variable continuously.
→The transmitter converts the measured value into a standardised 4-20 mA analog electrical signal.
→The PLC analog input (AI) module reads the current, converts it to a digital raw value (e.g. 0-27648 for Siemens).
→The PLC scales the raw value to engineering units (m, bar, °C) and uses it for control, monitoring and safety logic.
How a PLC Analog Input Module Receives the Signal
A PLC analog input module contains an Analog-to-Digital Converter (ADC) that samples the incoming 4-20 mA current at a fixed resolution, typically 12, 14 or 16 bits depending on the module. The PLC scan cycle reads this digital value every scan (typically every 10-100 ms) and the user program scales it from the raw integer to engineering units using linear scaling formulas.
Why 4-20 mA Is the Standard for PLC Analog Input Signals
4 mA represents 0% of range, not 0 mA. If the cable breaks or the sensor fails, the PLC reads 0 mA, which is clearly below the valid range, immediately flagging a fault rather than a false zero process value.
Current signals are far less affected by electrical noise than voltage signals. In a plant full of VFDs, motors and switching equipment, 4-20 mA remains accurate over long cable runs without screening complications.
Two-wire transmitters draw their own power from the same 4-20 mA loop. No separate power cable is needed to the field device, simplifying wiring and reducing the cabling cost in large plants.
Current does not drop with cable resistance the way voltage does. A 4-20 mA signal is just as accurate over 500 metres of cable as it is over 5 metres, making it ideal for field instruments far from the control room.
4-20 mA Scale Explained for PLC Analog Input Programming
(below 4 mA)
(above 20 mA)
Real-World PLC Analog Input Example: Tank Level to Siemens S7-1500
Siemens S7-1500 Analog Input Wiring for 4-20 mA Transmitters
The AI module's internal 250 ohm shunt resistor converts the 4-20 mA current into a 1-5 V signal that the ADC digitises. The PLC programmer never sees this voltage stage : TIA Portal presents it directly as a scaled raw integer.
PLC Analog Input Scaling Reference Table (Siemens S7 Normalised Values)
PLC Analog Input Scaling Formula Used in Ladder Logic and TIA Portal
Where:
EU = engineering value (m, bar, °C etc.)
LRV = Lower Range Value (process value at 4 mA)
URV = Upper Range Value (process value at 20 mA)
mA = measured current
Siemens S7 raw value scaling (0-20mA card, normalised 0-27648): Raw value = (mA - 4) / 16 x 27648
EU = LRV + (Raw value / 27648) x (URV - LRV)
Example: mA = 16, LRV = 0 m, URV = 5 m EU = 0 + [(16-4)/16] x (5-0) = 0.75 x 5 = 3.75 m In TIA Portal, this scaling is built into the NORM_X (normalise 0.0-1.0) and SCALE_X (scale to engineering range) function blocks. The programmer configures LRV and URV once and the PLC performs this calculation automatically on every scan cycle, typically every 10-100 milliseconds.
PLC Analog Input Scaling Calculator
Enter your transmitter range and measured current to calculate the engineering value and the Siemens S7 raw value, exactly as the PLC would compute it internally.
Quick FAQs: PLC Analog Input
- 4-20 mA Current Loop: Complete Wiring and Troubleshooting Guide
- 4-20 mA Percentage to Current Conversion: Formula and Calculator
- Why 4-20 mA Is Still the Best Signal for Industrial Automation
- Ultrasonic Level Transmitter: Real Sensor Example for PLC Analog Input
- Process Variables: What Sensors Connect to PLC Analog Inputs
External References
- Siemens: SIMATIC S7-1500 Analog Input Module Technical Specifications
- IEC 61131-3: PLC Programming Languages Standard
- NAMUR NE43: Fault Signal Levels for Analog Input Diagnostics
What we learn today
- A PLC analog input follows 4 steps: sensor measures the process variable, transmitter converts it to 4-20 mA, the AI module reads the current and digitises it, and the PLC scales this to engineering units for control and alarm logic. The scaling formula is EU = LRV + [(mA-4)/16] x (URV-LRV), implemented in Siemens TIA Portal using NORM_X and SCALE_X function blocks.
- 4-20 mA is the standard because of the live zero (4 mA = 0%, enabling fault detection below 4 mA), noise immunity from using current instead of voltage, loop-powered simplicity (no separate power cable needed), and accuracy independent of cable length. Siemens S7 PLCs normalise the signal to a raw integer range of 0-27648.
- Real example: a tank level transmitter ranged 0-5 m outputs 4-20 mA to a Siemens S7-1500 AI module. The PLC scales the raw value (0-27648) into a level reading in metres, then uses this scaled value for pump start/stop control and high-high level alarm logic : exactly the kind of application every beginner should master before moving to more advanced PLC topics.