<s>
Gillham	B-Device
code	I-Device
is	O
a	O
zero-padded	O
12-bit	O
binary	O
code	O
using	O
a	O
parallel	O
nine	O
-	O
to	O
eleven-wire	O
interface	B-Application
,	O
the	O
Gillham	B-Device
interface	I-Device
,	O
that	O
is	O
used	O
to	O
transmit	O
uncorrected	O
barometric	O
altitude	B-Application
between	O
an	O
encoding	O
altimeter	O
or	O
analog	O
air	O
data	O
computer	O
and	O
a	O
digital	B-General_Concept
transponder	B-Architecture
.	O
</s>
<s>
It	O
is	O
a	O
modified	O
form	O
of	O
a	O
Gray	B-Device
code	I-Device
and	O
is	O
sometimes	O
referred	O
to	O
simply	O
as	O
a	O
"	O
Gray	B-Device
code	I-Device
"	O
in	O
avionics	O
literature	O
.	O
</s>
<s>
The	O
Gillham	B-Device
interface	I-Device
and	O
code	O
are	O
an	O
outgrowth	O
of	O
the	O
12-bit	O
IFF	O
Mark	O
X	O
system	O
,	O
which	O
was	O
introduced	O
in	O
the	O
1950s	O
.	O
</s>
<s>
The	O
civil	O
transponder	B-Algorithm
interrogation	I-Algorithm
modes	I-Algorithm
A	O
and	O
C	B-Architecture
were	O
defined	O
in	O
air	B-Application
traffic	I-Application
control	I-Application
(	O
ATC	O
)	O
and	O
secondary	O
surveillance	O
radar	O
(	O
SSR	O
)	O
in	O
1960	O
.	O
</s>
<s>
He	O
was	O
the	O
UK	O
's	O
representative	O
to	O
the	O
International	O
Air	O
Transport	O
Association	O
(	O
IATA	O
)	O
committee	O
developing	O
the	O
specification	O
for	O
the	O
second	O
generation	O
of	O
air	B-Application
traffic	I-Application
control	I-Application
system	I-Application
,	O
known	O
in	O
the	O
UK	O
as	O
"	O
Plan	O
Ahead	O
"	O
,	O
and	O
is	O
said	O
to	O
have	O
had	O
the	O
idea	O
of	O
using	O
a	O
modified	O
Gray	B-Device
code	I-Device
.	O
</s>
<s>
The	O
exact	O
timeframe	O
and	O
circumstances	O
of	O
the	O
term	O
Gillham	B-Device
code	I-Device
being	O
coined	O
are	O
unclear	O
,	O
but	O
by	O
1963	O
the	O
code	O
was	O
already	O
recognized	O
under	O
this	O
name	O
.	O
</s>
<s>
By	O
the	O
mid-1960s	O
the	O
code	O
was	O
also	O
known	O
as	O
MOA	B-Device
–	I-Device
Gillham	I-Device
code	I-Device
or	O
ICAO	B-Device
–	I-Device
Gillham	I-Device
code	I-Device
.	O
</s>
<s>
ARINC	B-Device
572	I-Device
specified	O
the	O
code	O
as	O
well	O
in	O
1968	O
.	O
</s>
<s>
Once	O
recommended	O
by	O
the	O
ICAO	O
for	O
automatic	O
height	O
transmission	O
for	O
air	B-Application
traffic	I-Application
control	I-Application
purposes	O
,	O
the	O
interface	B-Application
is	O
now	O
discouraged	O
and	O
has	O
been	O
mostly	O
replaced	O
by	O
modern	O
serial	O
communication	O
in	O
newer	O
aircraft	O
.	O
</s>
<s>
An	O
altitude	B-Application
encoder	O
takes	O
the	O
form	O
of	O
a	O
small	O
metal	O
box	O
containing	O
a	O
pressure	O
sensor	O
and	O
signal	O
conditioning	O
electronics	O
.	O
</s>
<s>
As	O
a	O
twelfth	O
bit	O
,	O
the	O
Gillham	B-Device
code	I-Device
contains	O
a	O
D1	O
bit	O
but	O
this	O
is	O
unused	O
and	O
consequently	O
set	O
to	O
zero	O
in	O
practical	O
applications	O
.	O
</s>
<s>
Different	O
classes	O
of	O
altitude	B-Application
encoder	O
do	O
not	O
use	O
all	O
of	O
the	O
available	O
bits	O
.	O
</s>
<s>
All	O
use	O
the	O
A	O
,	O
B	O
and	O
C	B-Architecture
bits	O
;	O
increasing	O
altitude	B-Application
limits	O
require	O
more	O
of	O
the	O
D	O
bits	O
.	O
</s>
<s>
Up	O
to	O
and	O
including	O
30700ft	O
does	O
not	O
require	O
any	O
of	O
the	O
D	O
bits	O
(	O
9-wire	O
interface	B-Application
)	O
.	O
</s>
<s>
Up	O
to	O
and	O
including	O
62700ft	O
requires	O
D4	O
(	O
10-wire	O
interface	B-Application
)	O
.	O
</s>
<s>
Up	O
to	O
and	O
including	O
126700ft	O
requires	O
D4	O
and	O
D2	O
(	O
11-wire	O
interface	B-Application
)	O
.	O
</s>
<s>
Bits	O
D2	O
(	O
msbit	O
)	O
through	O
B4	O
(	O
lsbit	O
)	O
encode	O
the	O
pressure	O
altitude	B-Application
in	O
500ft	O
increments	O
(	O
above	O
a	O
base	O
altitude	B-Application
of	O
−1000±250ft	O
)	O
in	O
a	O
standard	O
8-bit	O
reflected	O
binary	O
code	O
(	O
Gray	B-Device
code	I-Device
)	O
.	O
</s>
<s>
Bits	O
C1	O
,	O
C2	O
and	O
C4	O
use	O
a	O
mirrored	O
5-state	O
3-bit	O
Gray	O
BCD	O
code	O
of	O
a	O
Giannini	O
Datex	O
code	O
type	O
(	O
with	O
the	O
first	O
5	O
states	O
resembling	O
O'Brien	O
code	O
type	O
II	O
)	O
to	O
encode	O
the	O
offset	O
from	O
the	O
500ft	O
altitude	B-Application
in	O
100ft	O
increments	O
.	O
</s>
<s>
Specifically	O
,	O
if	O
the	O
parity	O
of	O
the	O
500ft	O
code	O
is	O
even	O
then	O
codes	O
001	O
,	O
011	O
,	O
010	O
,	O
110	O
and	O
100	O
encode	O
200	O
,	O
100	O
,	O
0	O
,	O
+100	O
and	O
+200ft	O
relative	O
to	O
the	O
500ft	O
altitude	B-Application
.	O
</s>
<s>
The	O
Gillham	B-Device
code	I-Device
can	O
be	O
decoded	O
using	O
various	O
methods	O
.	O
</s>
