ZX Computings - OpenLibra

0 VOLTS- PIN 7
Construction
GATES 1-4 IN 7400 IC
+5 VOLTS- PIN 14 CIRCUIT DIAGRAM
when both inputs are binary 1,
otherwise the output is binary
1. Figure 1 shows the connections
to the IC.
The RAM CS line is connected
to both inputs of gate
one, so that its output of gate
one is constantly binary 0,
unless the correct 1 6K section
is decoded by the ULA. If the
RAM CS line is changed to
binary 0 by the ULA, one of the
inputs to gates two and three is
+5 VOLTS
TO PAD 2A
ON EDGE
CONNECTOR
13
binary 1. The other input to the
gate depends on A1 0. It must
be binary 1 to operate gate two,
and binary 0 for gate three to
operate.
As A10 changes'every 1K,
starting as binary 0, then
changing to binary 1, then back
to binary 0 again, the ORAM
will be first turned on by gate
three going to binary 0, and
then the ERAM (Extra RAM),
The next RAM to come on in the
A10
7400 IC (UPSIDE DOWN}
third change will be ORAM, as
A10 is now binary 0 again. This
sequence will repeat itself all
the way through the 16K section.
Construction and
Alterations
The first step is to wire up the
7400 IC as shown in Fig. 2,
remembering that it is shown
upside down. This is because
ORAM CS
0 VOLTS
ERAM CS
we are going to mount it on tN je
ZX81 's PCB using a piece a p
Blu-tack or a double side<
sticky pad, so the legs of the IC
are sticking upwards, and cfe
not make contact with tto
ZXB1 's PCB. The only connec
tions that are made are to th»
PCB at the points shown. Th
connection to the ORAM C!
lines is made underneath th
PCB.
The RAM CS line appears o
the edge connector pad 2 A, Wcan
solder our connection n
the 7400 IC (pins one and twc
to the hole connected to th
o
h
u
c
$
c
ir
lr
C
edge connector pad 2A. Th g
printed circuit track leadini „
from this pad to pin eight c v
lC4b must be cut. Pick a spo £
half-way along the track an t)
make two small cuts about a a
eighth of an inch apart, using f(
craft knife. Then, still using th g
craft knife, scrape away all c
the track in between, sotherei n
no connection between th t)
two. Attach the wire from pi e
eight of the 7400 IC to pin eigf c
of IC4b, whether fitted or n