Translate raw analog counts before migrating the channel.
This calculator converts a source analog-input raw count into engineering units, then calculates the equivalent raw count for a replacement PLC or DCS input range. It is intended for migration checks, commissioning records and scaling fault isolation.
Source raw count → engineering value → target raw count
Engineering value = EngLow + ((SourceRaw − SourceRawLow) / (SourceRawHigh − SourceRawLow)) × (EngHigh − EngLow). TargetRaw uses the same normalized fraction across the target raw span.
Engineering value → expected target count
Use this check during commissioning when a known process value or simulator test point is available.
Why migration scaling goes wrong
Nominal ranges differ
Two analog modules can both accept 4–20 mA while exposing different raw counts to the controller. The electrical signal range does not define the software count range.
Diagnostic counts are mistaken for process data
Some platforms expose underrange, overrange or fault states outside the nominal measurement span. Record those rules before translating logic.
Old logic scales twice
A replacement program may inherit an existing scale block while the new channel or driver already performs engineering-unit conversion.
Unipolar and bipolar modes are confused
Raw ranges can change with voltage/current mode and bipolar configuration. Verify the configured channel, not just the module family.
Clamping hides bad assumptions
Values that look stable at 0% or 100% can still be incorrectly scaled if software clamps the result before alarms or displays.
Integer rounding creates small offsets
Target counts may need integer rounding. Record both the calculated value and the observed raw count so tolerance can be evaluated explicitly.
Commissioning test points
| Percent of span | What to verify | Record |
|---|---|---|
| 0% | Low endpoint, underrange behavior, zero clamp | Injected signal, source raw, target raw, engineering value |
| 25% | Linear scaling away from endpoint | Calculated and observed target counts |
| 50% | Mid-span agreement and rounding | Deviation in counts and engineering units |
| 75% | Upper-span linearity | Calculated and observed target counts |
| 100% | High endpoint, overrange behavior, high clamp | Injected signal, raw counts and alarm behavior |
Migration record
Source channel
Record controller family, exact AI module catalog number, channel, signal mode, configured raw limits and existing scale logic.
Target channel
Record replacement module catalog number, channel mode, target raw limits, driver conversion and any hardware or software filtering.
Validation
Run several known test points. Preserve calculated values, observed target counts, display values, alarms and any accepted tolerance.
Reusable worksheet
Download analog scaling migration CSVRelated reference
Case 06 — 4–20 mA Loop Scaling & Troubleshooting. Use that case when the problem is on the field-current side rather than the PLC raw-count conversion.
Maintainer
Maintained by PLCProTech Ltd. as an open industrial automation engineering reference.