Analog I/O — 4–20 mA and 0–10 V Signals

Why Analog Signals Matter in Mechatronics

Most real-world process variables — pressure, temperature, flow, position — are continuous quantities, not simple on/off states. Analog signals carry that continuous information between field devices (sensors and actuators) and the PLC. Two signal ranges dominate industrial practice: 4–20 mA current loops and 0–10 V voltage signals. Understanding how to wire, scale, and configure these signals is a core technician skill covered on the PMMI Industrial Electricity and PLC exams and applied daily on the shop floor.

The 4–20 mA Current Loop

A 4–20 mA loop-powered transmitter converts a physical measurement into a current that travels around a series loop. Because current is the same everywhere in a series circuit, the signal is immune to voltage drops caused by wire resistance — a major advantage over voltage signals in noisy industrial environments over long cable runs.

Why 4 mA, not 0 mA? The live-zero at 4 mA lets the PLC distinguish a healthy transmitter reading zero from a broken wire (0 mA). Any reading below 4 mA indicates a fault.

A typical loop-powered transmitter wiring diagram has three components in series:

1. 24 VDC loop supply (positive terminal)

2. Transmitter (two-wire, draws 4–20 mA from the supply)

3. Analog input module (internally terminates the loop at the negative terminal)

Always wire per the manufacturer's diagram and verify polarity before energizing. Use a DMM in series (current mode) to measure loop current, or a clamp meter on the signal wire if the meter has sufficient resolution for milliamp ranges.

The 0–10 V Voltage Signal

Analog output channels often command drives, proportional valves, or positioners using a 0–10 V signal. The PLC converts a scaled integer in the output register to a proportional voltage. The load must be connected between the AO+ terminal and the common (0 V) reference. Verify that the load impedance meets the module's minimum specification (typically ≥ 1 kΩ) to prevent loading errors.

Safety Before You Wire

• Follow OSHA 29 CFR 1910.147 lockout/tagout: de-energize and lock out the 24 VDC supply before making or changing connections.
• Follow NFPA 70E: verify absence of voltage with a DMM before touching terminals; know the shock boundary even at 24 VDC (stored energy in capacitors and inductors can cause injury).
• Follow NFPA 79: use wire gauges and termination methods appropriate for industrial control panel wiring.
• Inspect wire insulation for nicks before inserting into terminals. Torque terminal screws to the manufacturer's specification.
• Have your wiring checked by the instructor before energizing any circuit — this is a required hold point.

How Analog Input Modules Represent the Signal: Raw Counts

The analog input module's analog-to-digital converter (ADC) converts the incoming current into a digital integer the PLC can read. A common 15-bit unipolar module uses the range 0 to 32,767 counts, where:

• 0 counts = 4 mA (live zero)
• 32,767 counts = 20 mA

The PLC program must convert those raw counts into meaningful engineering units (EU) — for example, pounds per square inch (psi) for a pressure transmitter.

The Linear Scaling Formula

Because the relationship between raw counts and engineering units is linear, use the following interpolation formula:

$$EU = \frac{raw - raw_{min}}{raw_{max} - raw_{min}} \times (EU_{max} - EU_{min}) + EU_{min}$$

Where:

• raw = the value read from the PLC input register
• raw_min = raw count at 4 mA (typically 0)
• raw_max = raw count at 20 mA (typically 32,767)
• EU_min = engineering unit at 4 mA (e.g., 0 psi)
• EU_max = engineering unit at 20 mA (e.g., 100 psi)

Many PLCs have a built-in scaling instruction (SCL, SCALE_X, etc.) that applies this formula automatically. Even so, a technician must supply the correct parameters, so you must understand the formula manually.

Configuring the Analog Output for 0–10 V

In the PLC software, configure the analog output channel for 0–10 V mode and note the output integer range (for example, 0 = 0 V, 32,767 = 10 V). Load a scaled integer into the output register using a MOV (move) instruction. To send 5 V, calculate the required integer: (5 / 10) × 32,767 = 16,384. Verify with a DMM across the output terminals and the common reference. Accuracy within ±0.2 V of the target confirms correct configuration.

Checking Your Work

Record every measurement — loop current, raw counts, calculated EU, and output voltage — in your lab log. Compare scaled values to DMM readings. Results should be within ±2% of expected values for the input scaling exercise. If they are not, recheck wiring polarity, module configuration range, and formula parameters before assuming the equipment is faulty.