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An altitude encoder takes the form of a small metal box containing a [[pressure sensor]] and signal conditioning electronics.<ref name="Ameriking_2004"/><ref name="ACK Encoder"/> The pressure sensor is often heated, which requires a warm-up time during which height information is either unavailable or inaccurate. Older style units can have a warm-up time of up to 10 minutes; more modern units warm up in less than 2 minutes. Some of the very latest encoders incorporate unheated 'instant on' type sensors. During the warm-up of older style units the height information may gradually increase until it settles at its final value. This is not normally a problem as the power would typically be applied before the aircraft enters the runway and so it would be transmitting correct height information soon after take-off.<ref name="Shadin_2016"/>
The encoder has an [[open-collector]] output, compatible with 14 V or 28 V electrical systems.{{cn|date=August 2022|reason=Possible original research as open-collector interfaces are often specified in terms of drain current rather than voltage.}}
== Coding == The height information is represented as 11 binary digits in a parallel form using 11 separate lines designated D2 D4 A1 A2 A4 B1 B2 B4 C1 C2 C4.<ref name="Phillips_1998_MODEC"/> As a twelfth bit, the Gillham code contains a D1 bit but this is unused and consequently set to zero in practical applications. Different classes of altitude encoder do not use all of the available bits. All use the A, B and C bits; increasing altitude limits require more of the D bits. Up to and including 30700 ft does not require any of the D bits (9-wire interface<ref name="Honeywell_2002"/>). This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4 (10-wire interface<ref name="Tooley-Wyatt_2009"/>). Up to and including 126700 ft requires D4 and D2 (11-wire interface<ref name="Tooley-Wyatt_2009"/>). D1 is never used.<ref name="Phillips_1998_ASCII"/><ref name="DFS_2000"/>
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