PLC analog input scaling: raw counts, 4–20 mA and diagnostics

Analog scaling is an affine mapping between two known endpoints. The difficult part is usually confirming what the raw endpoints represent in the configured input channel. A number valid for one module and range can be wrong for another, even within the same controller family.

FIG. 04A measurement becomes a signal

Write the mapping explicitly

Engineering value = lower engineering limit + (raw − raw lower limit) × engineering span / raw span. Raw span is raw upper limit minus raw lower limit. Validate that both spans and the chosen input representation match the channel documentation.

In an illustrative module configuration where 4 mA maps to raw 0 and 20 mA maps to raw 27,648, a 0–100 °C transmitter gives 50 °C at raw 13,824. That raw mapping is conditional on the module and configured range; it is not a universal PLC convention. For the electrical side, the 4–20 mA scaling tool maps 12 mA to 50% of span.

Check five points and the endpoints

Record expected current, raw reading and engineering display at 0%, 25%, 50%, 75% and 100%. For a 0–100 °C linear transmitter, these are 4, 8, 12, 16 and 20 mA with values 0, 25, 50, 75 and 100 °C. Use suitable calibrated equipment and the approved loop-check procedure.

A transmitter configured for 0–200 °C but software scaled for 0–100 °C can look correct at the low endpoint while being wrong everywhere else. Record both device and software ranges. Check whether a square-root extraction is already applied for differential-pressure flow before adding another transform.

Do not clamp away signal faults

A clamped display can show a plausible minimum during a broken loop. Keep the numeric value and quality status as separate signals. Use the module's documented diagnostics for wire break, overflow, underrange and channel faults. Define how downstream logic handles bad quality, stale values and startup; the correct response depends on the process design.

Preserve the commissioning record

Attach channel identity, module model, range configuration, scaling parameters, calibration reference and observed points to the FAT/SAT worksheet. Include diagnostic tests only where authorized and controlled. Continue with RTD versus thermocouple selection and the instrumentation learning path.

Manufacturer reference

Siemens S7-1200 current input representation illustrates why configured ranges and raw representations must be checked in the applicable manual. Use the documentation for the exact installed hardware.