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Instrumentation

Analog Inputs

4–20 mA loops, scaling, and signal integrity.

Analog I/O

Overview

Analog inputs digitize continuous process signals; the 4–20 mA current loop dominates because its live zero exposes broken wires and its current mode resists voltage drops.

Engineering purpose

Bring temperatures, pressures, levels, and flows into control with known accuracy, scaling, and fault detection.

How it works

Transmitters regulate loop current proportional to the measured variable; the input card digitizes across its range; scaling to engineering units happens in exactly one defined layer; values below 3.6 mA signal faults, not low readings.

  • The 4–20 mA live-zero enables broken-wire detection: below 3.6 mA means fault, not a low reading.
  • Scale raw counts to engineering units in exactly one layer — PLC or SCADA, never both.
  • Use shielded twisted pair grounded at one end; route away from VFD output cables.

Common faults

Double scaling in both PLC and SCADA; ground loops drifting readings; noise coupled from power cables; under-range fault currents displayed as plausible low values.

Diagnostic checks

  1. 1Inject a calibrated signal and verify the value through PLC and SCADA.
  2. 2Suspect ground loops when readings drift or oscillate.
  3. 3Inject a calibrated mA signal at the field end and verify the displayed value end-to-end through PLC and SCADA.

Safety notes

Define and test the control response to bad-quality analog values — a loop frozen at last-good value can quietly drive a process into trouble.

Commissioning notes

Document loop power architecture (2/4-wire), verify shield grounding at one end only, and record as-left calibration for every loop.

Related concepts

Live zero, loop powering, single-point scaling, shielding and ground loops, burnout detection.

analog · 4-20 · 4–20 · ma loop · 0-10v · scaling · آنالوگ · میلیامپر · میلیآمپر · مقیاس

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When Hermes Brain uses this article

Cited for readings stuck at zero or drifting, scaling discrepancies between layers, and loop fault diagnosis.

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