Note: neither 8BS nor the authors can accept any responsibility for any
changes you make to your machine. Hugh Williams has suggested that
piggy-backing two 16K roms is slightly risky, but whether you should take
his opinion over anyone else's is up to you...
ARTICLE FROM M.G. NEEDHAM 6EE
Piggy backing two DFS ROMs
Shortly after fitting a disc interface to my BBC computer, I discovered
the various DFS ROMs that could replace the original Acorn ROM. All had
useful utilities that the Acorn ROM did not possess, but all lacked
complete compatibility, most programs would work, but some would not.
This gave me the idea of fitting two DFS ROMs in the machine; the problem
was that this took two ROM slots, and at the time I did not have a ROM
board fitted so could ill afford to lose the spare ROM socket. So I hit
upon the idea of piggy-backing two DFS ROMs and switching between the two.
This has been a great success and has been in use for some time.
The procedure is straightforward provided you can use a soldering iron!
Take the Acorn DFS ROM and place an ic socket over the top, so that the ic
pins are over the top and pressing against the EPROM pins, then solder the
ic socket pins to the legs of the ROM, but NOT pin 22. Insert the other
DFS ROM into the socket, take a length of thin 4-core connecting wire, and
a miniature double pole change over switch, the wire is connected to the
switch as in the diagram below.
VIEW OF THE SWITCH FROM THE UNDERSIDE
Orange |* *| Brown
| \ / | Solder link from the orange wire
| \ / | diagonally to B.
Black |* X *| Red
| / \ | Solder link from brown wire
| / \ | diagonally to A.
A |* *| B
Orange wire to pin 22 of the top ROM.
Brown wire to pin 22 of the bottom ROM.
Red wire to pin 27 both ROMS (they should have been soldered together).
Black wire should be soldered to a standard probe clip (the miniature
probe clips are not strong enough) then clipped to ic 25 pin 6 or any
EPROM pin 22.
The wire colours are quite arbitrary and are only to help the description.
Make certain that the connecting wire to all pins is insulated and cannot
touch any other pins.
The completed unit can now be inserted into an ic socket, cut off or bend
under pin 22 on the ic socket, insert the unit into a spare ROM socket in
the BBC micro and the switch to one of the holes in the back of the
casing. If all is well you should get the usual startup; when you wish to
change the DFS flip the switch over then press break, you should now have
the new DFS running. If you wish to use another DFS ROM all you have to do
is to remove the top ROM from the unit and insert the new one
WARNING - please do not attempt the above modification unless you have
some soldering experience and are sure you know what you're doing.