Hide an HMI number when a 4-20 mA transmitter fails

Why the number stopped hiding

Object visibility on these panels is a discrete decision. The configuration help for the major HMI families describes the same input: pick a discrete tag, then choose whether the object shows when the tag is ON or OFF, and some panels accept a discrete tag or boolean expression in the same place. A Float32 process tag fits neither form, so the object loses its condition and stays visible permanently. The old program worked because the transmitter tags were integers and the visibility expression could evaluate a count. Converting the tags to Float32 for trending and datalogging in engineering units removed the input that expression needed.

Match your symptom to its cause before you change anything:

• The number stopped hiding the day the tag type changed. The visibility input lost its condition when the tag became Float32; the fix is the PLC-side bit described next.
• The number hides or flickers on a healthy loop parked near the bottom of range. The cutoff sits at exactly 4 mA, where a live reading and a dead loop meet; the threshold needs to move below that point.
• The number stays for a transmitter you have already unplugged. Either the new bit has not replaced the old expression in the visibility setting yet, or the compare reads a float that a clamp, a hold-on-fault, or a module open-circuit setting has altered; the wire-lift test below tells the three cases apart.
• The number reads wrong since the conversion. Scaling or tag mapping moved with the change; fix that first, because every threshold inherits the error.

Hiding is one choice. If operators should see straight away that a loop is dead, keep the object on screen and drive its color from the same health bit; changing a numeric display color from an alarm tag shows that exact pattern.

Build the bit in the ladder

One compare rung per transmitter does the whole job:

1. Find the scaling rung that turns the raw input word into engineering units and writes the Float32 tag.
2. Add a real-number comparison contact on that tag: value greater than the threshold you calculate below.
3. Drive an internal bit from that contact, one bit per transmitter. Keep it internal unless something outside the panel needs a field indication too.
4. Name the bit for what it means, so ON reads as transmitter healthy.
5. Add the bit to the panel's tag database as a discrete tag.
6. In the numeric object's visibility setting, select that tag and set the object visible when the bit is ON.
7. Leave the numeric display, trend and datalog objects on the Float32 tag; only visibility moves to the bit. If visibility also depends on a second condition such as the current screen or a machine mode, fold both into the one bit; HMI object visibility with two conditions covers that combination.

There is a shortcut when the raw input word still sits in an integer register and nothing overwrites it: add that word to the panel's tag database as an integer tag and point the visibility expression at it again, with no ladder change. Compare against the count form of a threshold below 4 mA, worked out in the example below, so a healthy loop at the bottom of its range keeps its number.

Set the threshold below the fault point

Convert the chosen fault current into engineering units with the same span the scaling rung uses:

EU_threshold = EU_low + ((I_threshold - 4 mA) / 16 mA) × (EU_high - EU_low)

Read EU_low and EU_high from your own scaling instruction. A dead 4-20 mA loop drives current toward 0 mA, so any threshold between 0 and 4 mA separates a low-but-alive process from a dead one. About half a milliamp of margin under the fault point is a sound starting place; widen it if the process is noisy. A healthy loop sitting at EU_low then stays visible, which is correct, and the object hides only when current falls below the threshold. If the process itself can run below EU_low, the transmitter is ranged wrong for the job; re-range it, because no threshold choice will behave.

The threshold inherits every scaling error, so if the displayed value and the loop current disagree, sort the scaling chain out first; why the scale value differs from the PLC analog input works through the usual causes.

Example: a 0 to 200 °C transmitter on a 0 to 4095 count input

The setup: a temperature transmitter ranged 0 to 200 °C, a scaling rung that maps 4 mA to 0 °C and 20 mA to 200 °C, and an analog input module that reports 0 to 4095 counts across 0 to 20 mA. The threshold current is 3.5 mA.

Engineering-unit threshold: 0 °C + ((3.5 - 4) / 16) × 200 °C = (-0.03125) × 200 °C = -6.25 °C. The compare rung is value > -6.25 °C.

The same point in counts: 4095 counts × (3.5 / 20) = 716.6 counts, so 717 counts is the integer form to use. The old cutoff, for reference, was 4095 × (4 / 20) = 819 counts.

Dead loop: the current falls toward 0 mA and the float falls with it, to 0 °C + ((0 - 4) / 16) × 200 °C = -50 °C. That is 43.75 °C below the trip point, the compare goes false, the bit drops OFF and the number leaves the screen.

Healthy loop anywhere in range: at 0 °C the float sits 6.25 °C above the trip point, and a transmitter accurate to 1% of span moves the reading by 2 °C, still leaving 4.25 °C of margin, so the bit holds ON and the number stays.

One compare and one bit, and the object disappears exactly when the loop dies, at any process temperature from 0 to 200 °C.

Three rung habits that defeat the compare

The compare sees only what the Float32 tag holds, so anything between the input and the compare decides whether a dead loop looks dead:

1. A clamp after the scaling. A limit instruction that holds the float at EU_low turns a dead loop into a healthy-looking bottom-of-range reading. Place the compare ahead of the clamp, or compare the raw counts.
2. Hold-on-fault or heavy filtering. Logic that freezes the float at its last good value keeps the bit ON and the number on screen indefinitely. Feed the compare from the unfiltered value ahead of the hold.
3. Module open-circuit configuration. Some analog input modules can be configured to substitute a fixed count or value when the loop opens. Read the module configuration and establish what an open loop produces before you fix the threshold, because that value has to land below it.

Prove it by lifting one wire

Follow your plant's lockout rules for the loop, and for anything interlocked to this transmitter, before touching a terminal. Then:

1. Watch three values live in the programming software: the raw count, the scaled float, and the health bit.
2. Lift one transmitter wire at the terminal.
3. Read the result. Float falls past the threshold, bit drops OFF, number disappears on the panel: the chain works. Float freezes at its last reading: hold logic ahead of the compare. Float parks at EU_low: a clamp after the scaling. Float jumps to a fixed value: the module's open-circuit substitution.
4. Land the wire and confirm the bit returns, the object reappears, and the value tracks the process again. If the object stays blank while the bit is ON, the visibility chain is fine and the display tag is the problem; HMI numeric display shows nothing: check the tag address first is the shortest route there.
5. Repeat on every transmitter whose rung you copied. A stale tag address in one copy hides the wrong number and still passes a test made on one screen.

The trend and datalog keep recording engineering units through all of this, including the samples the operator no longer sees; the bit governs the screen object only.

Recommendation

Recommendation: keep every display, trend and datalog object on the Float32 tag, and drive visibility from a PLC bit set by a real-number compare against a threshold roughly half a milliamp below the 4 mA fault point. Compare the raw counts against the count form of the same threshold when the scaled float is clamped or held downstream, keep the old 4 mA cutoff only where the process never sits near the bottom of its range, and drive the number's color from the same bit when operators need to see that a loop is dead.

A transmitter fault belongs in the PLC and on the HMI. The line-wide message that follows it belongs across the wall, on an andon-style board; we build shop-floor signage for exactly that job.

Questions