Input Port
Connections
to the
input port
are
shown
in tig 5.
Again
there
are 8
sep-
arate channels,
and
they
may be
read
with
a
single
PEEK
statement.
The
comm-
and
PHIHT
PEEK
(25000)
on the
2X80,
or
PBIST
PEEK
11000
on the 81
will
print
a
value
between
0 AND 255
representing
the
state
of the 8
lines.
If any
line
la
held
at
logic
low it
vill
contribute
a
zero
to
this
figure.
If it is at
logic
high
it
vlll
contributs
a
value corresponding
to the
data
on the
POKE
table
above.
Thus
if
channels zero
and 7 are
high,
bat the
rest
low,
the
value
printed
by the
PEEK
statement
will
ba 129
(=128+1),
and BO on.
The
circuit
of
fig.6
shows
a
single
push
button eonneated
to
channel
7.
When
the
button
Is
pressed, channel
7
will
go
high
-
otherwise
it is
kept
low
by the
resistor
to
ground.
To test the stats of the switch, use an expression such B.BJ
IP PEEK (25000)>127 TEHH GO TO 100 (ZX80)
or IP
PEEK
11000
T 127
THHS
GO TO 100
(zxai)
This
will
cause
a
jump
to
line
100 if the
switch
is
closed.
Note
that
if
nothing
ia
connected
to any
channel,
it
will
resume
a
Jjigh
state
BO
that
PEEK-
ing the port with nothing connected should produce a value of 255. If it
does not, then all board connections should be checked.
Further applications detaila are given in the applications booklet.
BAM Pack Extender Card
Once
the
port
is
working
satisfactorily
on
both
Input
and
output,
the
extender card should be soldered in. Shis allows the simultaneous use of
the RAM
pack
and
port boaid
on the
ZX81,
though
this
IB not
possible
on the
ZX80.
The extender card solders to the rear of the edge connector to effectively
extend the extension plug at the rear of the 2X81. See Figs 7 t 8. to wire
up the
card,
first
bend
the
edga
connector pins towards each other
so
that
the
card Just
fits
between
them.
Then
position
the
card
between
them
as in
Figa
7 and 8. The card should he positioned with notch outwards (i.e away from the
main port board), and exactly at ri^at angles to the port board. How care-
fully solder the 22 upper and 22 lover pins to the extender card, keeping the
card
at
right
angles
to the
main
board.
When
you
have
checked
that
there
are no
solder
bridges,
you
should
be
able to plug the RAH pack into the extender card, and the port onto the ZX81.
To use
this
tandem
arrangement
satisfactorily,
the
ZX01
and
extensions should
be kept on a flat surface to avoid poor contacts through flexing of the board.
If the
system
fails
to
operate
with
the
HAM
connected,
try
flexing
the
arrange-
ment
and
reinserting
the
paver
plug.
All
boards
should
be
inserted
as far as
they will go.
Hote
when
using
the
RAM
pack
aa
well
aa the
port board
you
will
not be
able
to
drive
so
many
external
devices
(lamps,
relays
eto) with
the
output
port.
If in doubt, check that the ZX81 ia not getting too hot.
Errata
There are two rranor corrections to the circuits issued In the applications
booklet and in PCW.
1. In all applications using ths high impedance loudspeaker, this should
be connected via a 10 mfd capacitor to earth as shown in fig 4 of the
attached sheet, and not directly to the positive supply as in PCW.
2, In all applications of the solid state buzzer, this should be connected
between the particular channel that it la used with, and the positive
supply (pin 16), rather than earth (pin 9). The polarity of the buzzer should
also be reversed, so that its red lead la on the positive supply. It will
function aa connected in the applications circuit, but it works more effectively
as described here.
Loudspeaker
A suitable loudspeaker for audio output may be obtained from Technomatio
Limited at SOp + P4P + TAT.
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