Apply a scaling multiplier on an HMI numeric display

Decide by what happens to the digits

Compute the factor the operator's value needs at one instant: factor = expected display value ÷ raw tag value. The quotient decides the setting.

• The digits survive and only the point moves. Raw 873 must show 87.3, quotient 0.1. Set Fractional in the display format to 1 and leave scaling off. A tag in hundredths, quotient 0.01, takes 2 fractional digits.
• The numeral changes magnitude. Raw 87 must show 870, quotient 10, or raw 87 must show 8700, quotient 100. Fractional digits only add places after the point, so this is a job for the scaling option: check scaling Option on the object's Option tab and enter the multiplier.
• The quotient is not a power of ten. A transmitter that reports 0 to 4095 counts across a span of 0 to 100 °C needs 100 ÷ 4095 = 0.0244 °C per count. That is a real conversion with a real slope, so use the object's scaling fields or scale the value once in the PLC.

Most HMI design packages put decimal placement in the display format. At least one puts a Shift decimal places option under the field's General settings, and at least one adds a Rounding field that decides how the value appears when the tag carries more decimal places than the display shows. The names move between packages; the split between multiplying and placing the point does not.

What each symptom points to

The last row is common enough to have its own walkthrough: fixing an HMI value that shows ten times too small.

The two settings, field by field

Open the numeric display object, then its Option tab. Checking scaling Option turns on the multiplier; fill the scaling values beside it. For a straight factor the relationship is displayed value = raw tag × multiplier, and the entry checks by division: expected ÷ raw must give back the multiplier you typed.

Many packages express scaling as input and output endpoint pairs instead of a bare multiplier field. Set the pairs so a known reading maps to its known display value at both ends of the range, then divide each pair out: if both quotients give the same ratio, the scaling is a straight multiplier and holds between the ends.

The Display Format settings decide presentation. Number of Digits sets how many digit positions the object draws; Fractional sets how many of those sit after the decimal point. A reading of 165.0 needs four digit positions with one of them fractional; allow more if the value can grow past that. If the tag carries more precision than you show, set the Rounding field deliberately so 1649 and 1654 do not both read as 165 without anyone deciding that. The digit-count side is covered step by step in showing more decimal places on an HMI numeric display.

These fields change one object on one screen. A numeric entry object is a separate setting with its own scaling, and a value written back must arrive in the PLC's units; test the write-back at one known reading before operators use it.

Check it at three values before you download

1. Write the raw tag value and the expected display value side by side at a low, a typical and a high operating point. Take them from history or from a controlled run.
2. Divide expected ÷ raw at each point. Three equal quotients of 0.1, 0.01 or 0.001 mean decimal placement does the job. Quotients of 10 or 100 mean the scaling option. Quotients that differ from each other mean the conversion has an offset as well as a slope, and endpoint pairs must carry both.
3. Apply the change to the one object. Set Number of Digits wide enough for the largest value in the range and Fractional to the places the unit carries.
4. Run the simulator if the software has one. Otherwise force a test value at the source with the machine in a safe state. An operator acting on a screen that is off by a factor of ten is a hazard, so test values go in only where nobody is using that number to make a decision.
5. Compare displayed with expected at all three points and confirm rounding is not swallowing a change the operator must see.
6. Download through the plant's change process, then read the live tag and the screen again at the same three points.

Worked example: a tenths-of-a-degree oven tag

Inputs. The case is the standard implied-decimal tag this article is about: a tempering oven screen reads one integer tag that carries degrees Celsius in tenths, 0.1 °C per count, the common way PLCs hold an analog value without floating point. The operator must read the hold temperature to a tenth of a degree. The readings are illustrative numbers for the arithmetic, and they double as the low, typical and high check points from the section above: 0 counts at a cold start, 1650 counts at the normal hold, 2500 counts at the hot end of the batch.

Rule. The digits 1650 must reach the screen unchanged with the decimal point placed after the third digit. Placing the point is a display-format job, so Fractional gets 1 and scaling stays off. The value is already correct; a multiplier is the wrong tool here.

Arithmetic. Expected display = raw × 0.1 °C per count: 1650 × 0.1 = 165.0 °C, 2500 × 0.1 = 250.0 °C, 0 × 0.1 = 0.0 °C. The wrong path, for comparison: scaling Option with a x10 multiplier would show 1650 × 10 = 16500, a hundred times the true hold. The digit budget: 165.0 is four digit positions, one of them fractional, so Number of Digits is 4 and Fractional is 1.

Result. Scaling unchecked, Fractional 1, Number of Digits 4. The screen reads 0.0, 165.0 and 250.0 °C at the three readings.

Decision. When the digits survive and only the point moves, place the point with fractional digits and leave the multiplier off: 1 digit for a tag in tenths, 2 for hundredths (the x100-implied case), 3 for thousandths. Reach for scaling Option only when the numeral itself must change magnitude.

Make the change stick

These fields change what one object shows and nothing else. Alarms, trends, recipes and other screens still read the raw tag, and each needs the same treatment if operators read it; trend limits on a tag held in tenths or hundredths need their own pass for exactly this reason. When several consumers need the converted value, convert once at the source: scale the tag to engineering units in the PLC and leave every screen's factor at 1, so the HMI object, the alarm limits and any other screen on the shop floor read the same number.

Change only the setting that is wrong, and record the object's original settings next to the raw and expected readings so the next shift can tell a display-side conversion from a PLC-side one. Leave the project otherwise as you found it.

Recommendation: Decide from the quotient expected ÷ raw. When it comes out 0.1, 0.01 or 0.001, set Fractional to 1, 2 or 3 digits and leave the scaling option off. When it comes out 10 or 100, check scaling Option and enter the multiplier. When it comes out anything else, scale once in the PLC and leave the screen's factor at 1. In every case, confirm the screen at a low, a typical and a high tag value before the panel goes back in service.

Questions