89-16 Signal Processing
89.5.5 Digital Input and Output
ese cards provide I/O lines, normally in groups of eight, which may be used to sense or generate
di
gital
sig
nals
fo
r
de
vices
ou
tside
th
e
co
mputer
. A gr
oup
of ei
ght
in
put
li
nes
is re
ferred
to as an (8 bi
t)
in
put
po
rt
an
d
a gr
oup
of ei
ght
ou
tput
li
nes
as an (8 bi
t)
ou
tput
po
rt
. In
put
an
d
ou
tput
le
vels
va
ry
fr
om
ca
rd
to ca
rd
an
d
it is be
st
to co
nsult
th
e
ap
propriate
da
ta
sh
eet
. Ty
pically,
vo
ltages
be
tween
2.5 an
d
5.0V
ar
e
co
nsidered
as hi
gh
lo
gic
le
vels,
wh
ereas
vo
ltages
be
tween
0.0 an
d
0.5 ar
e
co
nsidered
as lo
w
lo
gic
levels. ese levels are sometimes referred to loosely as transistor–transistor logic (TTL) levels. Note
th
at
th
e
ac
tual
lo
gic
le
vels
us
ed
by th
e
va
rious
TT
L
fa
milies
di