A practical field guide to industrial I/O
Small signals.
Big consequences.
Clear notes on sensors, PLC inputs, current loops, connectors, and the faults that only appear once the machine is already running.
Written for maintenance techs controls engineers curious builders
FIELD NOTE 001REV. 2.4 / 2026
CURRENT LOOP04 — 20
01Signal path
02Return to common
18ORIGINAL
DIAGRAMS
DIAGRAMS
NO STOCK PHOTOS NEEDED JUST THE PARTS THAT MATTER ↓
04–20mA in plain English
24 Vfield-ready examples
01symptom at a time
Technical notes without the fog.
THE SHORT VERSION
Start with what
the meter says.
Industrial signals look tidy on a drawing. In the cabinet, they pick up noise, lose a reference, meet the wrong input, and still get blamed on the sensor.
Signal Field Guide is a growing reference for understanding what the signal is doing before you replace the part.
Why this site exists →THE FIELD GUIDE
Four ways to find
the useful bit.
01 / SIGNAL BASICS Read the loop
Follow the fault
Turn a number
Read the loop
before the manual.
Current, voltage, resistance, and what each one is really telling you.
Explore → 02 / TROUBLESHOOTINGFollow the fault
without the drama.
Meter-first sequences for symptoms that feel intermittent, impossible, or both.
Find a symptom → 03 / BENCH TOOLSTurn a number
into a decision.
Small calculators for the numbers you actually meet in the cabinet.
Use the tools →RECENT FIELD NOTES
Useful answers to
specific questions.
01 SIGNAL BASICS 4–20 mA vs 0–10 V: the practical difference Two signals can describe the same pressure. Their failure modes, wiring, and distance limits are not the same. › 02 WIRING PNP vs NPN sensors without the guessing A visual guide to sourcing, sinking, and the PLC input that decides whether your sensor behaves. › 03 TROUBLESHOOTING Why does a 4–20 mA loop read 0 mA? A calm, meter-first sequence for separating power, polarity, load, and configuration faults. ›
THE 30-SECOND CHECK
What should
12 mA mean?
Put a current reading and transmitter range into the converter. Get the engineering value before you touch the wiring.
Open 4–20 mA converter → LIVE EXAMPLE ENGINEERING VALUE
12.00 mA
4
20
0–100 bar range 50.0 bar