Analog signals 4–20 mA, 0–20 mA, 0–10 V

Standard signals

A current signal is insensitive to line resistance within the allowed load: the source maintains the current regardless of cable length. That is why field transmitters use 4–20 mA, while voltage is used for short connections inside a cabinet.

Live zero

With 4–20 mA the bottom of the measuring range is 4 mA, not 0. A current near zero does not mean "zero value" but a fault: broken wire, no transmitter supply, blown fuse. The wire-break diagnostic of Siemens S7-1500 analog modules can only be configured for the 4–20 mA and 1–5 V ranges — precisely because these have a live zero.

A 2-wire transmitter is powered by the loop itself: 4 mA is roughly the minimum current at which its electronics keep running.

NAMUR NE 43

The NAMUR NE 43 recommendation separates measurement information from a transmitter's failure signal:

The failure direction (down or up) is usually selectable in the transmitter. The PLC must distinguish "value out of range" from "transmitter reports a failure" and must not use a failure value in control or interlocks.

Wiring 2-, 3- and 4-wire transmitters

• 2-wire (passive) — supply and signal on one pair; the input must be configured to match (whether it powers the loop).
• 3-wire — separate supply wire, common negative.
• 4-wire (active) — own supply, current output acts as a source.

A common mistake is two supplies in one loop, or none: in the first case the current saturates, in the second the input sees 0 mA ("wire break").

Typical faults

Tools

• Analog Signal Calculator — signal ↔ engineering value and NE 43 status.
• PLC RAW Scaling — ADC counts ↔ engineering value.