Direct Answer
The most common mistake is misunderstanding how power, signal and PLC COM are related.
A 2-wire pressure transmitter uses the same two wires for both power and 4–20mA signal transmission.
A 3-wire pressure transmitter separates power positive, power negative and signal output. The signal usually shares the same reference as the power negative.
Fluke explains that many 4–20mA systems use a two-wire current loop where a single twisted pair supplies power to the transmitter and also carries the output signal.
2-wire Transmitter Wiring
A 2-wire pressure transmitter is loop-powered. The PLC analog input must be part of the current loop.
Typical wiring:
24VDC+ → Transmitter+ → Transmitter- → PLC AI+ → PLC AI-/COM → 24VDC-
| Point | Connection |
|---|---|
| 24VDC + | Transmitter + |
| Transmitter – | PLC AI+ |
| PLC AI- / COM | 24VDC – |
Common mistakes:
- wiring the transmitter directly to 24VDC and leaving PLC AI outside the loop;
- setting PLC input to voltage mode instead of current mode;
- reversing AI+ and AI-;
- not checking loop voltage and load resistance.
3-wire Transmitter Wiring
A 3-wire pressure transmitter normally has V+, GND and OUT.
| Transmitter Terminal | Connection |
|---|---|
| V+ | 24VDC + |
| GND / V- | 24VDC – / PLC COM |
| OUT / Signal | PLC AI+ |
| PLC AI- | PLC COM / 24VDC – |
The key point is that signal output and power negative normally share a common reference. If COM or GND is not connected correctly, the PLC reading may drift, jump or stay at zero.
Main Difference
| Item | 2-wire Transmitter | 3-wire Transmitter |
|---|---|---|
| Power and signal | Same loop | Separate power and signal |
| Number of wires | 2 | 3 |
| PLC connection | AI is in series loop | OUT to AI+, GND to COM |
| Main risk | PLC AI not included in loop | COM / GND connection error |
| Typical use | Industrial loop-powered signal | Control cabinet applications |
Common Field Problems
| Problem | Possible Cause |
|---|---|
| PLC reads zero | No power, broken loop, AI not in loop |
| PLC reads full scale | Wrong input type or short circuit |
| Reading jumps | Grounding issue, VFD noise, poor shielding |
| Signal cannot reach 20mA | Insufficient loop voltage or high load |
| Transmitter powered but no PLC value | OUT or COM not wired correctly |
| PLC value differs from gauge | Scaling error, unit error or different tapping point |
AutomationDirect recommends separating the transmitter from the PLC input during troubleshooting to confirm whether the transmitter is outputting the correct current signal before blaming the PLC input.
NOIKE-AH Application Solution
NOIKE-AH pressure transmitters can provide 4–20mA pressure feedback for PLC, VFD and control panel applications. NOIKE-AH’s product range includes Pressure Series products such as pressure gauges, pressure switches, pressure sensors and pressure transmitters, and its pressure sensors and transmitters support outputs such as 4–20mA / HART / Modbus. (Noike-AH)
Recommended configuration:
- use a pressure transmitter for continuous pressure feedback;
- use an intelligent pressure switch for alarm and protection;
- use a digital pressure gauge for local pressure verification;
- use shielded cable, correct grounding and signal isolation when VFD interference is present.
Conclusion
2-wire and 3-wire pressure transmitter wiring is easy to confuse because the power and signal structures are different.
2-wire: power and 4–20mA signal share the same loop, and the PLC AI must be wired in series.
3-wire: V+ and GND power the transmitter, OUT sends the signal, and PLC COM must be connected correctly.
NOIKE-AH recommends confirming wiring type, output signal, PLC input mode, terminal definition and scaling before commissioning. In pump control panels and constant pressure systems, correct wiring reduces zero signal, full-scale readings, signal jumping, PLC display errors and commissioning time.
About NOIKE-AH
Nickel Electronics (Anhui) Co., Ltd (NOIKE-AH) is a global manufacturer of high-precision sensors for measuring and monitoring flow, pressure, temperature, level etc,.
With decades of engineering experience, NOIKE-AH provides reliable sensing solutions that ensure accuracy, efficiency, and safety across industries such as chemical processing, water treatment, food & beverage, pharmaceuticals, and energy.
Our intelligent measurement technology enables customers to gain real-time insights, improve process optimization, and achieve predictive maintenance through data-driven control.
Driven by innovation, NOIKE-AH continues to advance industrial automation with smart digital instruments, IIoT-ready solutions, and customized system integration, empowering industries worldwide to reach new standards in process reliability and performance.
