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 or 32767, or the card raises a wire break or underrange diagnostic. Operators blame the program. In most cases the program is fine and the loop is simply not delivering current to the card.

Work through the checks below in order. Each one splits the problem in half, so you never have to 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) and note the number:

Step 2: measure the loop current in series, not in parallel

Put a multimeter on its 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 that let you read the current without breaking the loop. What you read tells you where to go next:

Step 3: find out what powers the loop

Getting this wrong is one of the most common commissioning mistakes. A 2-wire transmitter needs 24 V from somewhere, and from exactly one place.

With a card-powered (active) input, the card supplies the loop voltage, and you wire the transmitter between the card's supply terminal and its input. With an externally powered (passive) input, a separate 24 V supply drives the loop and the card only measures; the card has to be configured as a 2-wire or 4-wire input to match.

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

Step 4: check the voltage budget

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

Step 5: confirm the channel configuration

Open the hardware configuration and check three things: the channel type (current 2-wire, current 4-wire or 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 on it reads nearly zero.

Step 6: check the scaling arithmetic

With raw 0–27648 mapped to 0–10 bar:

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

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

The five checks in order

Measure before you edit code. Check the raw value, then the loop current, then the loop power, then the channel configuration, and only then the scaling. Those five checks find almost every analog fault. You can practise the scaling step with the analog scaling calculator and read more about signal types in the hardware chapter on signals.