ZX Computings - OpenLibra

which the keyboard symbols
can be written. Eleven keys are
required, as the numbers 0-9
are not a lot of use if you cannot
RUBOUT any mistakes, except
by going back to the
Sinclair keyboard. The
RUBOUT key* requires the
pressing of two keys together,
0 and SHIFT. Therefore the
SHIFT key must be included on
the numeric pad. Pressing the
SHIFT key on its own does
nothing, so hitting it accidentally
does not give an error on
INPUT.
Having the SHIFT key on
the numeric pad also means
that all the cursor moving keys
are also available, SHIFT 5(*4,
SHIFT 6{ f }, SHIFT 7( I ) and
SHIFT 8(-H. These can be used
to quickly EDIT programs,
along with the EDIT key which
is SHIFT 1. As all of these keys
can be reached with one hand
if they are grouped in a square,
it means the other hand is free
to do other things, such as
follow a program in a book or a
set of data to be INPUT. This
can be very useful, as it is easy
to lose one's place when trying
to watch the screen and the
written program at the same
time.
As the keys 1-5, 6-0 and
SHIFT are all on different address
lines, all three must be
included on the numeric
keypad. These are A8 (SHIFT),
A11 (1-5) and A12(6-0). We
also need ALL of the
(K)EY(B)OARD-(D)ATA lines
(inputs to the computer)
KBDO-KBD4.
The keyboard port KBD0-4
is addressed by the ZX81
ROM as INPUT PORT 254 (FE
in HEXADECIMAL). BUT
because of the way Sinclair addresses
his ports, the keyboard
port appears at every EVEN IN-
PUT PORT address. That is
when address line AO is at
Binary 0, the IOREQ and the
WR are Binary 0.
The upper eight address
lines (A8-A1 5) reflect what was
in the B register at the time of
calling for an input from the
port. So the setting of a bit in
the "B" register to Binary 0 addresses
that key (the address
tine to 0 volts) and then looks at
the result on the data lines.
When a key is pressed, the appropriate
data line will also be
Binary 0.
These actions are all done
by the BASIC ROM when using
INPUT or INKEY$. This information
has only been included
for the machine code programmer.
We must open up the casing
of the ZX81 to get at the
connections on the printed circuit
board inside, and thereby
CUT SLOT
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A 1 1
the data lines.
If you turn the ZX81 upside
down, you will see four stuckon
rubber feet. Under three of
these feet are screws which
need to be removed before the
case can be opened. They are
under the front two feet and
the back left side foot. There
are a total of six screws to be
removed, ALL of them need to
be removed with a smallheaded
screwdriver, in order
not to damage the slot in the
screw. Once the screws are
taken out, the bottom half of
the casing can be removed and
the printed circuit board can be
seen in the top half, secured by
two more cross-cut screws in*
to the top casing. By the bottom
left hand side of the
printed circuit board you can
see the two white plastic strips
which connect the Sinclair
keyboard to the printed circuit
board. These must not be
damaged by dropping hot
solder on them, so cover them
up with a piece of paper. These
keyboard strips go into two
sockets on the underside of
the printed circuit board. The
solder strips on the top of the
printed circuit board which
connect the sockets to the rest
of the ZX81 is where we will
solder the wires, which we will
use to attach the numeric
keyboard.
These solder connections
consist of a group of eight address
strips and a group of five
KBD strips. Soldering onto
these strips will NOT discon-
A12
ZX81
PRINTED CIRCUIT
BOARD
KBDO - KBD4
QUANTITY COMPONENT
11 KEYBOARD 'PUSH-TO-MAKE' SW1TC HES
8 PIECES OF WIRE 18 INCHES LONG
1 BOX
1 2 3
4 5 6
7 8 9
SHIFT 0
nect any of Sinclair's keyboard
functions. None of the wires
connecting the ZX81 and the
numeric keypad must be over
1 8 inches long or this causes
problems in operating BOTH
keyboards. Also make sure
that no shorts are made between
the strips (see the
SOLDERING instructions).
A slot must be cut in the left
hand side of the bottom casing
to lead the wires out. This may
be done by making two saw
cuts Vi inch apart, % inch
deep, with a small hacksaw.
Then with a pair of pliers, grip
the area between the saw cuts
and bend the plastic
backwards and forwards until
the piece breaks off.
The wiring to the keys, in
comparison to the ZX81 's, is a
SUGGESTED LAYOUT
piece of cake. The connections
are shown in the circuit
diagram. The keys have only
two tags and these can be connected
either way round. The
address lines connect five keys
and must be wired from key to
key, using the wire now attached
to ttj^ZX81. There is
only one data line (KBD) to
each key and only one address
line to each key. The SHIFT
key only must be wired to address
line A8.
The keys can be arranged in
any order you like, but a suggested
layout is given.
"20 Simple Electronic Projects
for the ZX81" by
Stephen Adams is published
by Interface Publications. Contents
of this article ©
copyright S. Adams, 1982
GO ZX COMPUTING AUG/SEPT 1982