Scale a 4–20 mA Signal So 11.4 mA Represents 0 Percent

If the measured input is in milliamps, map 11.4 mA to 0% and 20 mA to 100% with output% = (mA − 11.4) × 100 / 8.6. If the PLC value is already scaled to 0–100% for a 4–20 mA range, the equivalent transform is output% = (input% − 46.25) × 100 / 53.75. Clamp or alarm outside the intended operating range according to the process requirements; do not apply both formulas in sequence.

Identify the value and units

The ticket describes a 4–20 mA input that has already been converted to a 0–100% value. In that representation, 11.4 mA corresponds to (11.4−4)/16 × 100 = 46.25%. The desired output span is 100−46.25 = 53.75 percentage points.

Scale an input already in percent

Use the two endpoint pairs: input 46.25% → output 0%; input 100% → output 100%. Linear interpolation gives output% = (input% − 46.25) × 100 / 53.75. For values below 46.25%, clamp at 0 only if that matches the control requirement; decide explicitly what values above the upper endpoint should do.

Scale directly from milliamps

If the source variable is current in mA rather than a scaled percentage, use output% = (mA − 11.4) × 100 / (20 − 11.4). This form maps 11.4 mA to zero and 20 mA to 100. Confirm that the analog channel is configured for the correct current range and that its raw-to-engineering conversion is correct.

Verification

Check the computed endpoints: 11.4 mA → 0%, 20 mA → 100%, and the midpoint 15.7 mA → 50%. For a 0–100% input representation, confirm 46.25% → 0%, 100% → 100%, and 73.125% → 50%. Test lower and upper out-of-range and wire-break behavior separately; scaling alone does not distinguish a legitimate low process value from a sensor fault.

Limitations and Operational Precautions

The derivation assumes a linear input and the stated endpoints. The ticket does not identify the analog module, raw input format, process safety role, or desired behavior above 20 mA. Set limits, diagnostics and fail-safe response from the instrument and project requirements. Do not let a display clamp hide an under-range or over-range fault.

Technical References

Siemens, S7-400 Module Data, Reference Manual (official documentation for analog module ranges and diagnostics; PDF access returned 403): https://support.industry.siemens.com/cs/attachments/1117740/module_data_en_en-US.pdf
Siemens, SIMATIC PCS 7 Compendium Part B – Process Safety V10.0, section 7.4 (official search result identifies selectable 0–20 mA and 4–20 mA ranges; PDF access returned 403): https://support.industry.siemens.com/cs/attachments/109990271/pcs7_compendium_part_b_en-US_en-US.pdf

Scale a 4–20 mA input in a PLC
Analog input under-range diagnostics

Engineer FAQ

What output corresponds to 15.7 mA? It is the midpoint of 11.4–20 mA, so the linear mapping gives 50%.

What is 11.4 mA as a percentage of a standard 4–20 mA range? (11.4−4)/(20−4) × 100 = 46.25%.

Why can't I subtract 46.25 and multiply by 11.628? For a percentage input spanning 46.25–100%, the full output span is 53.75 points, so the multiplier is 100/53.75 ≈ 1.86047. The factor 11.628 applies to a current in milliamps using an 8.6 mA span.

Should negative calculated values be clamped? Clamp only when the intended engineering behavior requires a zero minimum, and retain separate under-range or sensor-fault diagnostics.

What if the signal exceeds 20 mA? Define whether the application should clamp, report an over-range condition or apply another documented behavior; the ticket does not specify it.

Can FC105 perform this mapping directly? A standard 4–20 mA scaling block can map configured raw input to engineering limits, but this offset target requires the lower engineering endpoint to represent the desired 0% point, subject to the installed STEP 7/PCS 7 version and block parameters.

Does the equation handle a wire break? No. Scaling maps numerical values; wire-break detection and module diagnostics must be configured and handled separately.

What should be tested at commissioning? Verify endpoint and midpoint values, plus under-range, over-range and fault cases against the instrument and project acceptance criteria.

Does the formula depend on current being linear? Yes. It assumes a linear mapping between the specified endpoints.

Can the same equation be used if the variable is raw counts? Only after the raw counts are correctly related to current or engineering units for the exact analog module and configured range.

Scale a 4–20 mA Signal So 11.4 mA Represents 0 Percent