Analog Input Reads 0 or Overrange: Troubleshooting a 4–20 mA Loop

The symptom

The local display on a pressure transmitter reads 3.2 bar, but the PLC tag shows 0, -32768, 32767, or the card raises a wire break / underrange diagnostic. Operators blame the program. In most cases the program is fine — the loop is not delivering current to the card.

Work through the checks below in order. Each one splits the problem in half, so you never guess.

Step 1 — Read the raw value, not the scaled tag

Open the online view of the raw input word (for example %IW64 on a Siemens S7-1500 or the .Data member on a Rockwell 1769-IF4). Note the number:

Step 2 — Measure the loop current, series not parallel

Put a multimeter on the mA range in series with the loop, or use a clamp-on mA meter if you cannot open the circuit. Many transmitters have TEST terminals across a diode, which let you read current without breaking the loop.

Step 3 — Who powers the loop?

This is the single most common commissioning mistake. A 2-wire transmitter needs 24 V from somewhere, and only one place:

If both the card and an external supply drive the loop, or neither does, you get 0 mA or a damaged channel. Check the terminal diagram for the exact card part number, not a similar one.

Step 4 — Check the voltage budget

Measure the voltage across the transmitter terminals with the loop running. A typical transmitter needs at least 10.5–12 V to operate. Add up everything in the loop at 20 mA: card input resistance (often 250 Ω = 5 V), any barrier (Ex isolators can drop 7–8 V), indicators and long cable. If the remaining voltage is below the minimum, the reading collapses as the process rises — it looks fine at low pressure and fails near full scale.

Step 5 — Confirm channel configuration

Open the hardware configuration. Confirm the channel type (current 2-wire, current 4-wire, voltage), the range (4–20 mA, not 0–20 mA), and that diagnostics such as wire-break are enabled where you expect them. A channel set to voltage with a current signal will read nearly zero.

Step 6 — Check the scaling arithmetic

With raw 0–27648 mapped to 0–10 bar:

Pressure := (REAL(Raw) / 27648.0) * 10.0;

Common errors: integer division (Raw / 27648 gives 0 for every value below full scale), the wrong raw span (Rockwell cards are configured per channel and may use engineering units directly), and forgetting a 4 mA offset when the raw span starts at a non-zero count.

What to remember

Measure before you edit code. Raw value, then loop current, then loop power, then configuration, then scaling — five checks that locate almost every analog fault. You can practise the scaling step with the analog scaling calculator and read more about signals in the hardware chapter on signals.