Fault Finding Guides · Free PDF
PLC Analog Input Fault Finding Guide
A one page guide to PLC and DCS analog input problems: scaling, wiring type, noise, ground loops and channel faults.
One page A4Vendor neutralFree, no sign up
Download
A one page A4 guide to print for the workshop or keep on your phone in the field. The full content is also shown below.
Download PDF A4 · 53 KBBefore you start
- Permit, isolation and control room informed.
- Put the loop in manual or inhibit trips as authorised.
- Use certified test equipment in hazardous areas.
- Record as found readings before changing anything.
Quick checks
- Measure loop current at the card.
- Compare raw counts with expected.
- Check card diagnostics and forces.
- Check shield grounding.
Advertisement
Symptoms, causes and fixes
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| Value wrong but mA correct | Scaling, raw range or units wrong | Check raw limits (e.g. 0 to 27648) and LRV, URV |
| Overrange or underrange alarm | Signal outside card limits, open wire | Measure mA; check wiring |
| Two channels interfere | Mixed 2 and 4 wire devices on shared common, ground loop | Wire per card manual; isolate |
| Noisy value | Shield earthed both ends, VFD cables nearby, no filter | Single point shield; segregate; filter |
| Stuck value | Channel or module fault, forced value | Check module status and forces |
| HART not passing | Card not HART capable, low loop resistance | Check card type; add resistance |
Advertisement
Formulas
- Value = LRV + (raw − rawmin) ÷ (rawmax − rawmin) × span.
- mA = 4 + 16 × %.
- Size loops for 22 mA.
Related Tools, Procedures and Calculators
Frequently Asked Questions
Why is my PLC value wrong when mA is right?
The scaling or raw count range in the program does not match the card.
Can I mix 2 and 4 wire transmitters?
Only if the card allows it; wrong wiring on a shared common causes errors.
Where should the shield be earthed?
Usually at the control system end only.
You May Also Like
Learn more on the blog
Analog input stability: cable length problems
A practical guide on instrumentationblog.in with more background on this equipment.
Read Full ArticleMore Fault Finding Guides
Pressure Transmitter Fault Finding GuidePrintable A4 PDFDP Flow Measurement Fault Finding GuidePrintable A4 PDFMagnetic Flowmeter Fault Finding GuidePrintable A4 PDFCoriolis Flowmeter Fault Finding GuidePrintable A4 PDFVortex and Ultrasonic Flowmeter Fault Finding GuidePrintable A4 PDFRadar and Ultrasonic Level Fault Finding GuidePrintable A4 PDFDP Level and Guided Wave Radar Fault Finding GuidePrintable A4 PDFTemperature Measurement Fault Finding GuidePrintable A4 PDFControl Valve and Positioner Fault Finding GuidePrintable A4 PDFOn Off and ESD Valve Fault Finding GuidePrintable A4 PDFpH and Conductivity Analyser Fault Finding GuidePrintable A4 PDFFixed Gas Detector Fault Finding GuidePrintable A4 PDFModbus and HART Communication Fault Finding GuidePrintable A4 PDFVFD Fault Finding Guide (Vendor Neutral)Printable A4 PDFNAMUR NE107 Diagnostic Status GuidePrintable A4 PDF
Advertisement