AAVSP-04 discrete signal interfaces

What it is

AAVSP-04, the Allied Avionics Systems Publication on discrete signal interfaces, is NATO's electrical specification for how avionics equipment exchanges signals that carry only two states, a mode flag, a fault warning, a lamp command, a status report. It does not define what any of those signals mean, and it does not lay down a communications protocol. It defines the electrical envelope, voltage behaviour, timing and cabling, that a transmitter and a receiver have to share so equipment built by different designers, in different nations, interoperates correctly on the same aircraft wiring. Edition A, version 1, was promulgated in February 2017, and nothing in the document names an earlier edition it supersedes.

Where it sits among NATO's other agreements

AAVSP-04 does not stand alone. It requires the aircraft's own electrical supply to already meet STANAG 3456, aircraft electrical power system characteristics, wherever an interface draws power from it, and it is explicit that it does not replace or override STANAG 3837, the separate agreement covering aircraft/store electrical interconnection: the two sit side by side rather than one requiring the other. Two UK national documents are also named, UK Def Stan 59-41 on electromagnetic compatibility and UK Def Stan 00-18 (Part 1) Section 4, a guide to the discrete signal interfaces standard, though neither is a NATO agreement.

Four interface types, one electrical problem each

The document groups discrete signalling into four categories, each detailed in its own annex, and every one of them repeats the same point: it specifies the electrical interface, not the protocol or the function a signal performs. Choosing which type fits a given signal is a design decision the document leaves to the engineer.

Critical timing signalling (Annex A) covers functions where timing matters and rise time is significant, the document names event signalling and audio blanking as typical uses, using a balanced two-wire transmission scheme. The annex sets the transmitter's differential and common-mode voltage behaviour and output waveform, the receiver's threshold and common-mode rejection, and the cable's construction, a twisted, screened pair. It is a dedicated point-to-point link: where a signal has to reach more than one receiver, the annex calls for separate links rather than one transmitter feeding several.

Non-critical timing signalling (Annex B) applies where a response time in the tens of milliseconds is acceptable, and the document positions it as meeting most requirements where flexibility matters more than speed, mode selection, permanent flags, fault warning flags, status reporting and limit switches are its examples. An impedance-switching transmitter, built from contacts, logic gates or transistors, drives an active receiver, and up to three receivers can share one transmitter.

Low power switching (Annex C) is used where a signal also has to carry power, driving a lamp, a relay or an electromechanical indicator. It switches current from the aircraft supply to the load, and the annex covers fault propagation suppression and protection against the transients an inductive load produces. The document is unusually candid here: it expects future issues, of this or other standards, to specify a more complete range of power switching interfaces, treating this annex as a starting point rather than the final word.

Fault detecting, non-critical timing signalling (Annex D) adds the ability to detect a fault in the interface itself, on top of the same timing tolerance as Annex B. A sensor element and a switch element work together to produce separate SIGNAL and FAULT outputs, with up to three sensor elements able to share one switch element, one acting as master and the rest as slaves. The whole implementation has to meet national electromagnetic compatibility requirements.

None of the specific voltages, thresholds or timing values that make each interface work are reproduced here. They are the substance a design engineer opens the document for, and they sit in the annexes themselves, keyed to the test circuits each annex describes.

How it is checked

AAVSP-04 names no certification scheme, no accredited body and no audit or inspection regime. What it provides is a set of test circuits, one for the transmitter and one for the receiver in each annex, that define how a given electrical characteristic is measured. Whether those measurements feed a design qualification, a first-article acceptance test or ongoing production testing is decided by whatever contract or programme has invoked the standard, not by the document itself.

Standards it references

• STANAG 3456Binds
• STANAG 3837Background