Putting two 16K ROMimages in one 32K EPROM
By Hugh Williams (709)
C83 asked in issue 21 if it was possible to use 32K EPROMs in the BBC to
double up the number of sideways ROM images available. The answer is yes
BUT a small hardware modification has to be made to the 32K EPROM to make
this possible. In practice this means that you can have 32K EPROMs in your
machine but you need switches on the outside of your machine to switch
between each of the two 16K images. I have several arrangements of this
type in my machine and they all work perfectly.
This small project does require soldering and some basic electronic
skills/knowledge. Neither I, nor 8-bit software will accept responsablity
for any damage that may be caused to your machine as a result of following
the instructions below.
The first step is to chose which two images you wish to have residing in
the 32K EPROM. Bear in mind that whilst both images will be stored on the
same EPROM, and share the same socket on the motherboard, ONLY ONE of the
two images may be present in the socket at a time. This is because we will
manually decode the top address line by using a switch. The above
consideration has two important bearings on choice of suitable software :
Software that needs to claim private workspace on a reset is unsuitable
since if it is not selected during a reset it will not get a chance to
reserve workspace, and will probably crash the machine if switched in
after a reset. Software of this type includes almost anything that raises
the value of page after a reset e.g. filing systems such as DFS and ADFS.
Software packages normally supplied on more than one EPROM are unsuitable
for blowing onto one large 32K EPROM. The reason is that they can never
both be present in the same socket at the same time, but for software of
this type to work both need to be able to communicate with each other to
function. The original plea to put two 16K AMX ROMS onto a 32K EPROM falls
into this category I'm afriad. (But that doesn't mean that you can't
double up some of your other ROMs to free sockets.) Toolkit and utility
type ROMs are normally the safest bet.
Try to choose your software carefully. For example, say you have two
images that you use often, and two that you use only occasionally. You
want to blow all four images onto two 32K EPROMs. If you put the two
commonly used images on the same 32K EPROM you will have to switch between
them each time you wish to use the other, and will never be able to use
them both at the same time. However if you were sensible and paired an
often used image with a rarely used image you could have both frequently
images selected normally, and use them at the same time, whilst switching
the other rarely used images in only when necessary.
Next you have to program both images into the blank 32K EPROM. Most EPROM
programmers can cope with standard 32K EPROMs. To program the 32K EPROM
load the first of the two 16K ROM images from disk into the programming
buffer, then program the LOWER half of the 32K EPROM with this image. Next
load the second 16K image from disk and program this into the TOP half of
the 32K EPROM. The above assumes that your EPROM programmer works like
mine, only programming 16K of a 32K EPROM at a time. If it can only
program the whole 32K in one go then you should join the two 16K images
together into one 32K file prior to programming.
Assuming that the programming has gone OK a hardware modification has to
be made to the EPROM for you to be able to use both 16K images in your
BEEB, so get your soldering iron out and warmed up.
CAREFULLY bend pin 27 (second pin from the top on the right hand side as
looking down onto the window side of the EPROM) until it sticks up in the
air in the opposite direction to all the other pins. This is the line
(A14) that selects the top or bottom 16K of the 32K EPROM, and we have to
decode this manually to get out whichever image we require.
Strip about 1cm of insulation from both ends of a length of thin
interconnect wire about 20cm long. Solder one of the bare ends of the wire
to the upturned leg of the EPROM. Take a 1K (1000) ohm resistor, chop both
wires coming out of it off about 1cm from the main body of the resistor.
Wrap the other bare end of the interconnect wire around the stiff metal
wire extending from either end of the resistor.
I will refer to the stiff wire coming out of the end of the resistor that
you have wrapped the interconnect wire around as the south wire from now
on, the stiff wire from the other end of the resistor will be the north
wire.
Turn your BEEB off and unplug it from the mains. Do the same with any
peripherals connected to your BEEB. Remove the fixing screws, take the top
cover off, unscrew and remove the keyboard.
Take the south wire of the resistor and solder it to the center (common)
contact of a single pole toggle switch (the simplest sort of toggle
switch). Take another length of interconnect wire and solder one end of
this to either of the two outer contacts of the toggle switch.
Connect/solder the other end of this interconnect to any negative or
ground rail inside the computer. e.g. Pin 8 (bottom left) of IC26 (just to
the left of the 6502).
Take a third and final length of interconnect wire and solder one end to
the north wire of the resistor, and the other to any positive 5 volt rail
inside the computer. e.g. Pin 16 (top right) of IC26.
Now plug the 32K EPROM onto any free ROM socket in your BEEB. You MUST now
check that everything is OK. In particular check that no bare wire ends
touch any other components in your computer. An electrical short could
damage your compuer. Fix the switch to the inside of the case in a place
where it will not touch any other componets. Better still drill a small
hole in the side of the case and fix the switch so that it protrudes
through to the outside.
After checking thoroughly fix the keyboard back in place, and fix the lid
back on plug it in and switch on. One of the two images should respond to
a *HELP, and if you flick the switch to the other position, press BREAK
once and type *HELP. Hey presto! the other image should be there.
Don't forget that you can only use one of the two 16K images at a time.
Schematic of Wiring :
______________________________ +5v
|
|
1 K ohm
resistor
|
From pin ___________|
27 of 32K |
EPROM / switch
/
|
|
0v ______________________________ Gnd.