ZX Computings - OpenLibra

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left is the ERAM CS line. The
position you fix this will depend
on what sort of ORAM you
have. Stick the 7400 IC (legs
upwards) onto the printed circuit
board in the position
shown. The A10 connection
can be made by pushing a wire
into the hole next to L2 and
soldering it.
TWO 221S as ORAM
In this case, remove the two
ORAM ICs and mount two
more 2114s on top of them.
ER AM 2114s should have their
pin eights cut short, so that
when they are mounted on the
ORAM the two pin eights do not
touch. Solder this 'piggy back'
arrangement together, except
for the pin eights. The top
ERAM's pin eight should then
be soldered to a piece of wire
making sure that it does not
touch the lower ORAM'S pin
eight. These pins are the RAM's
CS pins.
Put the 2114s back into
their sockets, making sure the
ICs are the correct way round,
as shown in Fig. 3. The top and
bottom RAMs should be the
same way round. Now join the
two pin eights of the ERAMs
together, and solder the ERAMs
IC4
) 7400
Construction
EDGE CONNECTOR
together. Solder the ERAM CS
wire to one of them, making
sure it does not touch the
ORAM'S pin eight.
One 4118 as ORAM
This is a bit more difficult as the
ORAM covers up one of the
ERAM sockets. An IC socket
should be soldered into position
onthe IC4b, then a 2114 IC
placed in it with its pin eight left
outside the IC socket. The other
2114 must be mounted underneath
the IC4, This is done by
laying the top of the 2114
against the PCB over the pin
connections for IC4a, Now
solder wires from the IC4a connections
to the 21 1 4 RAM IC,
making sure it is the right way
round, so that the cutaway of
the IC is nearest the edge connector.
Do not solder a wire to
pin eight. Join the two ERAM's
pin eights together using a
piece of wire and then solder
the ERAM CS to pin eight of the
ERAM in socket IC4b.
Finishing touches
Before applying any power to
the ZX81, check all the connections
for bridges of solder bet-
T
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CONNECTIONS TO ZX81
ween tracks on the PCB and on
the tCs. Check that the connections
to the PCB are not likely to
wander about and touch anything
else. On re-assembling
the ZX81 and applying 9 volts
to it, you will find that PRINT
PEEK 16389 gives a result of
72. (f not, or the inverse 'K'
does not appear, check all your
connections again.
The reason for all this work is
that 2114s only cost 99p plus
VAT, whereas the cheapest
4816 is £7.00 plus VAT or
more. If — despite the cost —
you want to install a 4816,
place it the same way round in
the socket of the 4118 and cut
the strap from L1 to L2. A new
strap must then be inserted between
L2 and A10. This modification
will not stop you using a
16K pack, as the RAM CS connection
on the edge connector
still works.
Shopping list
There are at least three different
2114 1 K RAM chips
available. The L version takes
less current and so is useful if
you are using your machine
with extra boards. The other
two types are differentiated by
their speeds. One is 450 nS
and the other 200 nS. This is
the time (in nanoseconds)
taken to operate the chip after
the CS line is operated. The
chips needed for this project
can be up to 450 nS. The best
thing is to look at the price of
the chip, not forgetting that you
will require two of them for the
extra 1K of RAM.
The 7400 also comes in two
versions. The original, cheaper
7400 and the more expensive
74LS00. The 74LS has more to
recommend its use, however,
as it requires less power to
operate and has protection
diodes built into the input of the
gate. It is therefore best to use,
as it only costs a few pence
more. The 7400 and the
74LS00 use the same pin
numbers.
UMMER 19S ZX COMPUTING SUMMER 1982 45