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A practical 4-20 mA reference for linear scaling, Allen-Bradley and Siemens raw values, live-zero behavior, and underrange or overrange troubleshooting.
A 4-20 mA loop represents a process value with a live zero: 4 mA is the configured low endpoint, 20 mA is the configured high endpoint, and the 16 mA span carries the usable measurement. The general conversion is EU = EU low + (raw - raw low) × (EU high - EU low) / (raw high - raw low).
The current signal and the PLC integer are separate representations. A transmitter can be healthy at 12 mA while the controller displays the wrong engineering value because the input range, raw-count endpoints, data type, or scaling block does not match the installed module. Preserve the measured current and unscaled controller value before changing logic.
Counts outside the nominal range should remain visible to diagnostic logic. A negative engineering result or a value above span may be useful evidence of a configured fault band, wire problem, or transmitter condition; it should not be silently clamped until the site alarm policy is understood.
Original public reference
Use this as a screening reference. Confirm the installed equipment, configuration, and site requirements before making a field change.
| Representation | Low | Midpoint | High | Boundary |
|---|---|---|---|---|
| Loop current | 4 mA | 12 mA | 20 mA | Configured fault bands vary |
| Siemens nominal | 0 | 13,824 | 27,648 | Check module representation |
| Allen-Bradley | Module/configuration specific | Calculate from endpoints | Module/configuration specific | Do not assume one universal range |
At 12 mA, a correctly configured linear 4-20 mA signal is at 50% of span.
Siemens commonly uses 0-27648 as the nominal analog representation; use the exact module manual and channel configuration.
Allen-Bradley raw ranges vary by module family, input format, and configuration. Do not assume one universal endpoint.
Validate low, midpoint, and high values while trending measured current, raw counts, and final engineering units together.
Supporting guides
Use the linear 4-20 mA scaling equation without losing the live-zero offset, engineering units, or fault-range behavior at the ends of the signal.
Identify the actual Allen-Bradley module data representation before scaling raw integer counts, floating-point channel data, alarms, and overrange indicators.
Work from Siemens analog integer representation to normalized and engineering-unit values while keeping unipolar, bipolar, overflow, and underflow ranges distinct.
Separate valid extended measurement, NAMUR-style fault indications, open-loop behavior, saturation, and PLC clamping when a 4-20 mA value leaves its nominal span.
Normalize the signal inside its raw range, then apply the engineering span: EU = EU low + (raw - raw low) × (EU high - EU low) / (raw high - raw low). For current in milliamps, raw low is normally 4 and raw high is normally 20.
Dividing measured current by 20 treats zero milliamps as the process minimum. A 4-20 mA loop has a 4 mA live-zero offset, so subtract 4 before dividing by the 16 mA span.
Not automatically. Preserve a raw diagnostic value and apply the transmitter, module, and site fault-band policy. An unexplained clamp can hide wiring faults or configured underrange and overrange indications.