Sinclair ZX81, Assembly Instructions Manual, Page: 22 / 43

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.