Signal Scaling (4–20 mA)
Convert between the analog signal (mA/V), PLC raw counts and the engineering value, and spot broken-wire and over-range states.
Enter any one of the three fields, the others update
- Percent of span
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- Resolution (1 count)
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| Span | Signal | Raw counts | Value |
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How does linear scaling work?
An analog transmitter maps the measured quantity linearly onto an electrical signal. For a 4–20 mA, 0–10 bar pressure transmitter, 4 mA is 0 bar and 20 mA is 10 bar; 12 mA is exactly halfway, 5 bar. The general formula:
value = loweng + (signal − lowsig) × (higheng − loweng) ÷ (highsig − lowsig)
The PLC’s analog input module first converts the signal to a raw count. On Siemens modules the nominal range is usually 0–27648 (for 4–20 mA, 4 mA = 0 and 20 mA = 27648); other vendors commonly use 12, 15 or 16-bit counts. Use the range from the device and module documentation.
Broken wire and over-range
The “live zero” of 4–20 mA is a key advantage: current still flows at a measurement of 0, so a current near 0 mA means a broken wire. The NAMUR NE 43 recommendation treats roughly 3.8–20.5 mA as valid measurement and below 3.6 mA or above 21 mA as a failure signal. The tool’s warnings follow these thresholds; rely on the values in your own device’s documentation.
Frequently asked questions
Why is 4–20 mA used more than 0–20 mA?
Because of the “live zero”: current still flows at 4 mA when the measurement is zero, so 0 mA can be told apart as a broken wire or loss of power, and two-wire transmitters can power themselves from the loop current.
Why is the raw count 0–27648?
Siemens analog input modules map the nominal range to 27648 counts, with headroom above that for over-range. Other vendors use 12, 15 or 16-bit ranges. Confirm the range in your hardware documentation.
Are the values I enter stored?
No, the calculation runs entirely in your browser.
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