DIY sideways ROM board
By MIKE HARRISON
ONE of the most powerful features of the BBC Micro is its ability
to give you instant software via the sideways ROMs.
Unfortunately, you can soon run out of spare sockets for them,
so at The Micro User we've come up with our own do-it-yourself
sideways ROM board. Now you can have up to 16 sideways ROMs in
your micro, simply and cheaply.
We've also included the circuit for a RAM board which may be
used to develop ROM software or to store data or machine code
programs which can be loaded into the RAM from cassette or disc,
and called from Basic. This increasing the amount of user memory
available to up to about 44k.
The RAM board may be combined with the ROM extender board, or
used as a stand-alone unit.
Note that when the ROM extender board has been connected all
paged ROMs must be moved from the BBC board onto the ROM board.
The operating system ROM should, of course, be left where it is.
In the BBC Micro the number of the ROM currently selected is
held in a four bit latch (IC 76), bits 0 and 1 being decoded by
IC 20 to provide the four "select" signals for the onboard
ROMs.
This extender board uses a 74LS154 to decode all four latch
outputs into the 16 ROM select lines.
All the address and data lines are buffered to reduce the loading
on the computer's address and data buses.
The RAM board is very straightforward, consisting of eight 6116
2k RAM chips, with a 74LS138 being used for address decoding.
Most of the signals to drive the extender board are taken from
one of the now vacant ROM sockets on the BBC board via a 28-way
DIL header plug, the few remaining signals being taken from the
pins of IC 76,IC 77 and S21.
These connections may be made by soldering wires to the appropriate
IC pins, or alternatively the connections may be made to the pins
of wire-wrap type IC sockets, which can then be plugged into the
sockets for ICs 76 and 77, and the ICs replaced in the wire-wrap
sockets.
A connection is required to S21 (south), and this can be made
by soldering a wire to the small exposed metal part of the jumper
joining the south-east and south-west pins of S21.
The circuit diagrams of the ROM and RAM boards are shown in
Figures I and II on Page 84 and 85
Several methods may be used to construct this unit.
The masochistic reader may like to design a printed circuit
board for it or, alternatively, Veroboard or one of the various
Eurocard type boards may be used, but this would require an enormous
amount of interwiring.
The Road-Runner wiring pen system was used for the prototype,
as this is a very easy and quick way of making a large number
of connections between IC sockets.
Unfortunately it is also quite easy to make mistakes, especially
if you forget that the IC pin numbers are reversed left to right
when looking at the underside of the board.
A few minutes spent drawing the pinouts of all the ICs with
the pins reversed - that is, pin 1 becomes pin 28 etc. - can save
a lot of time and avoid mistakes messy to correct.
The prototype ROM extender and RAM was built on two boards -
a square-pad type Eurocard for the 15 ROM sockets and the 74LS154,
and a Vero Microboard for the bus buffers and RAM chips. (The
ROM card was a very tight squeeze.)
Whichever method of construction is used, it should be remembered
that the power supplies to all the chips should be well decoupled,
and this may be achieved by connecting a 0.05 or 0.1 micro farad
capacitor between the +5v and 0v pins of each chip.
If a wiring pen is used to build the ROM or RAM boards, the
wiring to the +5v and Ov pins of all the chips should be done
with thicker wire than that used in the wiring pen, or several
strands of wiring pen-type wire.
It is strongly recommended that turned-pin type IC sockets are
used for the ROMs because normal sockets tend to become unreliable
or intermittent after several insertions of the ICs and, when
they are soldered in, solder can sometimes "wick up"
into the contacts, making them less reliable.
Although turned-pin sockets are considerably more expensive
than ordinary ones, they will make the unit a lot more reliable.
Figure I: Extension ROM board
Figure II: Paged RAM board
