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Spectrum 48K Versions
During
the six years of production of the ZX
Spectrum, the machine underwent a considerable
number of internal modifications to its ROM and other circuitry.
The biggest changes were, of course, occasioned by the step
up to 128K and then to the Amstrad designs. In all, there
were at least thirteen different versions of the machine.
This caused predictable problems with backwards compatibility.
The issue was much worse with the 128K machines, with many
48K games, such as Elite, being totally incompatible.
In many cases, special 128K-only updates had to be produced.
It is often possible to determine
which version of the Spectrum
16/48K one has without opening the case, as there
are a number of clues:
- If the rubber keys are a light fawn colour it's an
issue 1
board.
- If the keys are dark grey, look into the edge connector
slot to see whether an aluminium heatsink is visible -especially
at the power socket end. If you don't see the heatsink
it's an issue
2 board which has the heatsink near the forward
corner of the board, under the keyboard.
- If the heatsink is visible it's an issue
3 or 3B
board - there is very little difference between them.
Although the Spectrum+
usually has an issue 3B board, this cannot always be assumed
- the Spectrum+ keyboard upgrade kit allowed owners of older
machines to house their old circuit boards in new cases.
Issue 1 was the original version
of the Spectrum, released in 1982. The large zig-zag shaped
heat sink on the right of the circuit board is a feature shared
with the latter Issue 2, but the key difference is in the
memory. The Issue 1 is essentially a 16K Spectrum, with only
the first 16K actually mounted on the board. For the 48K Issue
1, the remaining 32K is carried on a small daughter board
which is fixed to the rear of the main board.
Some issue 1 boards also have a curious additional circuit
nicknamed "the spider" or "the dead cockroach"
- due to timing error in the ULA chip it was necessary to
fit an extra 74LS(X) circuit, mounted on its own small board
suspended by little legs above the main board.
Overall, some 60,000 Issue 1 Spectrums were sold, making
it one of the rarest versions.
Introduced late in 1982, Issue 2
was very similar to its predecessor, with the major difference
being that all of the memory was now mounted directly on the
board. This was a reaction to the success of the 48K Spectrum,
effectively establishing that model as the default Spectrum.
The chip error that had necessitated the "dead cockroach"
was also fixed. The most visible exterior change was a new
colour for the keys - now blue, rather than grey as before,
to improve legibility under electric lighting. More than 500,000
Issue 2 Spectrums were sold in 1982-83.
The Issue 3
was the most widely available of all Spectrum versions, with
over 3,000,000 sold. It represented a radical departure from
the earlier versions, with a redesigned circuit board, an
uprated buzzer added to make the BEEP louder and a new low-power
ULA introduced.
The ULA modification caused
significant compatibility problems, although to be fair this
was not Sinclair's fault. The keyboard input port also reads
in a value from the EAR (microphone in) socket and on the
issue 1 and 2 Spectrums, this value is binary 1. On issue
3 Spectrums, this value is not maintained because, to reduce
power consumption, the values of the pull-up resistors are
altered. The result is that the EAR bit now floats until the
ULA has warmed up. The unfortunate consequence is that games
and other software which check the whole byte, and not just
the keyboard bits, will not work. This was only a problem
in the first place because of sloppy programming - keyboard
routines were not suppose to check the whole keyboard input
byte, but lazy programmers did it anyway.
The Issue 3B
was a very minor redesign, built using slightly different
components and circuitry. There is very little practical difference
between it and the earlier Issue 3. The board is usually to
be found in the Spectrum+.
A 6C001-7 ULA chip is the main new feature of the Issue
4A and 4B
boards, which otherwise are virtually identical to the Issue
3B.
The Issue 5 is a major redesign,
or more precisely a tidying-up: six decoder/multiplexer chips
(IC3, IC4, IC23, IC24, IC25 and IC26) were replaced with a
Mullard ULA type ZX8401. A 74LS04 hex inverter chip (IC28)
provides the six inverters required for the new circuit. As
might be expected, these changes greatly altered the appearance
of the board; however, they did not significant change its
operation at the software level.
The final version of the Spectrum 48K, the Issue 6,
differs only in fairly minor ways from its immediate predecessor:
on some boards, the main ULA is provided by Saga rather than
Ferranti and certain components (principally capacitors and
resistors) differ.
Continue
to 128K Spectrum versions...
© Chris Owen 1994-2003
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