Here's a little tech for those interested. The Flight Deck Door was most
certainly assigned as a recorded parameter as per this chart:
A port that is not used looks like this in the documentation:
It is clear, without debate that Flight Deck Door was assigned and being
polled by the system.
These captures were taken from a Boeing 757 manual, document number:
D226A101-3, revision G.
As shown the flight data recorder receives a logic low (binary 0) when the
door is closed. With electronic circuits (specifically digital signals), you
must NEVER leave a pin open. It must be referenced to VSS (signal high),
or Ground (signal low) at all times. It CANNOT remain floating or the input
circuitry will receive noise, and/or an undetermined value.
For this reason, the following circuit is the standard for switched logic
circuits. There may be variations, however the signal input line will
ALWAYS sense Ground (logic 0), or VSS (logic 1)
So what does this mean? Well, according to the documentation, the door
is closed when a logic zero is received at Port D14, word 251, bit 1, subframe 3.
If this parameter was NEVER recorded the documentation would not assign
a port, and/or a word/bit position.
If the door was left open, the value would read logic 1 (VSS) as shown on the right side (Figure 2).
Parameters that are not recorded (IE: spares, or unused ports) are tied
to ground instead of VSS to reduce current draw and power consumption
in a circuit.
Summary:
Unused pins, spare ports, etc. are tied to ground and are labelled as spare
in the second chart from the top of this post.
Assigned parameters are never 'floating' and will either see a logic 1, or
logic 0. In the case of the Flight Deck Door, it was reading ground which
means it was closed (logic 0).
[edit on 28-11-2009 by turbofan]
