Mark

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The only time I've seen a chain is in reference to a programming pin adapter plug that must stay with the aircraft when a component is changed.

Two examples are the engine FADEC units and the newer Standby (Attitude, Heading, Altimeter, Airspeed) Indicators in the cockpit.

Engine specific strapping (or coding) is on one plug that attaches to the FADEC. It is chained to the engine.

Aircraft specific data is coded to a plug that connects to the Standby Indicator. A chain connects it to the aircraft wiring harness and not the instrument.

Here's a picture of a FADEC unit. I'll have to take a pic of one (with the programming plug) on an actual engine.