Mind you don't get slipped discs
By MIKE COOK
WITH all the exercising we have been doing it is important that
you don't slip your discs! This month we shall be looking at adding
discs onto the BBC Micro and, hopefully, throwing some light on
questions such as: Can I do the upgrade myself? What discs should
I get? And what is that double row of holes at the bottom left-hand
side of the keyboard printed circuit board?
BACK in the very first lesson of the Body Building Course (The
Micro User, March 1983), I mentioned we would look at upgrading
to discs, so why has it taken this long?
Well those of you who have followed the progress of the BBC
Micro will know that when it was stated that it would be designed
by Acorn Computers, a very tight delivery date was also announced.
As we all know, production was not able to satisfy demand but
by then the computer hardware was designed and so the basic configuration
was fixed.
All things considered, Acorn did a marvellous job and produced
the very best computer available at up-to-double the price on
the market today.
Even so, like the curate's egg, there are parts of the BBC Micro
which are not as well designed as they might be.
Unfortunately the disc interface is such a one. Not that it
doesn't work well, but there have been one or two problems with
it.
The biggest snag holding up production has been a shortfall
in supplies of the P8271 disc controller chip. This is the lynch
pin of the design - you just cannot use discs without it.
The P8271 chip is one of the older types of controller. Probably
it was chosen because the designers were familiar with it and
there wasn't enough time to look at a newer model.
I heard it rumoured that the manufacturers of the device denied
it was obsolete, saying that they had not made any for a long
time, were not expecting to make any more, and did not have the
huge numbers the BBC Micro started to demand!
That chip is now becoming available again, but it would have
been pointless to describe how to do your own upgrade when the
parts were not available.
The second snag with a DIY disc upgrade has been the unique
way Acorn decided to supply the software to operate the discs
on eprom. Not in itself a snag, but Acorn refused to supply this
chip on its own.
To get one you had to buy the full upgrade from them and, as
the disc controller chip was in short supply, they were reserving
what they had for customers buying the most expensive drives at
the same time.
This software is normally known as the DOS (disc operating system)
on other computers but in BBC Micro terms it is called the DFS
(disc filing system).
As this is a very complex piece of software, it took other suppliers
some time to come up with their own versions which you could buy
separately.
Now that the history of the two major holdups has been explained
let's see what is needed to add discs to the computer.
Two units are required. The disc interface has to be fitted
inside the case of the BBC Micro and the disc drives need to be
configured and attached to the computer.
The disc interface consists of 11 ICs which are shown in Table
I.
IC 78 P8271 disc controller.
IC 79,IC 80 7438 Quad NAND buffer.
IC 82 74LS10 triple 3-input NAND gate.
IC 81,IC 86 74LS393 dual 4-bit binary counter.
IC 83,IC 84 CD4013B dual D- Flip-Flop.
IC 85 CD4020B 14 stage binary counter.
IC 87 74LS123 Dual mono-stable.
Any sideways ROM Disc Filing System eprom.
Table 1: ICs for the disc upgrade
The letter B on the end of some of the IC numbers is very important,
as devices without this letter are identical except that their
top speed is not as great and, in the disc interface circuit,
they will not be fast enough.
The chips are plugged into the appropriate sockets on the main
computer printed circuit board. The IC numbers are printed onto
the board next to the sockets.
Make sure that link S9 (located under the picture of the acorn)
is not made. If it is snip the link.
Some other links must be checked at this point and their correct
positions are shown in Table II.
S9 Open
S18 North
S19 East
S20 North
S21 2 links both horizontal
S22 North
S32 West
S33 West
On the links where there are three connections the centre and
left-hand connector may be joined with the shorting link. This
link is said to be "West".
Alternatively if the centre and right-hand connector are joined
then the link is said to be "East".
When the three connectors are in a vertical line we talk of the
links being "North" and "South".
Link S21 has four connectors in a square and so they can be joined
in horizontal or vertical pairs.
Table II: Link selections for disc operation
This assumes you already have a Model B (or have upgraded the
appropriate parts) and have operating system 1.0 or later.
For more information about how to plug ICs into sockets without
tears and which parts of the Model A to B upgrade need to be done
before the disc interface can be fitted, see the March 1983 issue
of The Micro User.
If you have an old board a few modifications will have to be
carried out. To find out which issue you have, take the lid off
the computer and look next to the copyright notice.
If you have a board issue 4 or later you are all right but an
issue 2 or 3 will mean you have to cut a track.
Locate IC 27, which is on the left hand edge of the board about
threequarters of the way up.
You might think it is labelled IC 89, but in fact that refers
to the empty socket to be used for Econet.
Pin 1 is the top left hand one. We are after pin 9, so count
anti-clockwise round the chip to find it (it's the second one
up on the other side).
Then with your courage in both hands and a pair of cutters in
the other snip off the pin.
Do not cut it too close to the IC as we need to attach a wire
to the stump.
There is a track leading away from the pin to the right hand
pad of a pair labelled S9. Cut this track about half-way along
its length.
You can use a sharp hobby knife to make two small cuts across
the track about an eighth of an inch apart. Then, again using
the knife, peel away the portion of the track joining the two
cuts.
Finally solder a short piece of wire from the stump of the leg
on the IC to the right hand pad of S9.
Those of you who have been paying attention might think that
the circuit is now the same as it was before we started hacking
it about. Not so, because the track continues under IC 27 to a
place where it shouldn't.
By performing this surgery pin 9 now goes only to the right
hand pad of S9.
The price of the components you will need is constantly fluctuating,
so we have decided not to offer them as a kit because, by shopping
around you can probably get a better deal.
The disc controller chip P8271 has a list price of £49.50
in "one off" quantities but several advertisers are
offering it considerably cheaper.
The cost of the other components (with the exception of the
disc filing system eprom) are relatively negligible. Several dealers
offer a complete upgrade kit for around £90.
The disc filing system eprom is likely to cost something in
the same order as the disc controller chip.
There are several independent ones available, some of which
have been reviewed in recent months.
There has appeared lately an upgrade kit enabling you to run
true double density. All of the extra components are also supplied
and it does not use the P8271 disc controller chip.
All the alternative disc filing systems claim to be compatible
with the Acorn DFS. It must be realised that this compatibility
exists only with the disc commands and the documented machine
code calls.
If a software supplier insists on direct calls to the DFS it
probably will not work with any filing system but Acorn's - and
probably not on later versions of the Acorn DFS either.
Also by doing away with the P8271 the system will not be compatible
with software that accesses this chip directly. Some software
may do this for protection and so will not run if another chip
has been substituted.
This is not a problem at the moment, but if the development
of the BBC computer is anything like that of others it will be.
It must be remembered that copying a DFS eprom, although possible,
is illegal. A great deal of effort is involved in producing one,
and if the rewards of this effort are not realised then there
is likely to be less quality software available in the future.
Copying must be regarded as theft.
The DFS eprom can be fitted in any of the four sideways ROM
sockets located in the bottom right-hand side of the board.
With this fitted, the computer will always power up with discs
selected and to use tape you must first issue the command TAPE.
Also the variable PAGE gets set higher so less free memory is
available.
Once the disc interface is fitted you can start to consider
the drives themselves. There is a bewildering variety of makes
and specifications.
Fortunately most drives can be made to work with the BBC Micro.
Those that are not suitable are usually made to add on to the
Apple or Commodore range.
Nearly all the drives will be the 5.25in size but I am told
that the BBC will also work with the older 8in drives.
If anybody is using this size I would be pleased to hear from
them.
In addition there is the recently introduced 8in drive which
works well with the BBC Micro.
As well as the size variation, you can have 35 track (this is
the older type of drive) 40 track or 80 track drives. The latter
are sometimes called double density, but should really be called
double track density.
True double density is a function, not of the disc drive, but
of the disc controller chip, and the P8271 will only support single
density encoding.
Just when you thought that you had covered all the options,
the extra one is that the drives can be single or double headed
(sided).
The latter allows you to record on both sides of the disc, thus
doubling the amount of information it will hold. This double-sided
drive looks like two drives to the computer.
There are some double-sided drives which require the disc to
be inserted the other way round to record on the other side. Sometimes
these are known as flippy discs, as you flip them over.
However, I would strongly recommend against this practice as,
when the disc is inserted the other way round the disc is moving
in the opposite direction relative to the sleeve.
The inside of the sleeve of a disc has a coated surface which
acts as a cleaner. Just like cleaning with velvet, when you reverse
the direction, all the dirt gets wiped off the cleaner back onto
the disc.
It is true that, once deposited, it then gets wiped off again,
but I am sure you can see this process does nothing to extend
the life of the drive head or to the disc.
Also the disc tends to lap itself into the cleaning surface
and so constant change in direction will only serve to speed the
wearing process.
All this does not apply to the new 3in drives as they are constructed
in such a way that the disc does not contact the sleeve.
So which to choose? The answer depends on a combination of what
you can afford and the use you make of your computer.
If you need a lot of large files full of data, text programs
or screen dumps then an 80 track drive is worth considering.
You will then be able to store more files on each disc or have
a really huge data file.
But if you only produce short to medium length files most of
the time you will probably find the directory fills up before
you have used half the space available on an 80 track drive.
Remember, the Acorn standard disc only allows 31 files on any
one disc regardless of how small they are.
Also, as most software is supplied on 40 track discs, you could
cut yourself off from a large part of the software market.
To counter that you can get a hardware switchable 40/80 track
disc or, using the Watford DFS, you can software switch an 80
track drive so that it reads a 40 track disc.
I have also used this switch to write successfully on a 40 track
disc with an 80 track drive, but the disc has to be formatted
on a true 40 track drive.
This software switching is all right as long as the software
does not require you to press SHIFT and BREAK to run a menu automatically,
as pressing BREAK reverts the software switch to 80 tracks.
This can usually be overcome by *EXEC !BOOT or *RUN !BOOT. You
have to experiment.
Compatibility is also a problem with the new 3in drives as very
little software is available at present in that format.
The drives are very compact and neat and the discs are enclosed
in a rigid box with covers, which are automatically pushed back
when the disc is inserted over the access slots.
This makes the discs much less prone to the damage which results
if the active surface is touched with a finger or the whole thing
is sat upon.
Nothing I am sure is proof against a really determined two-year-old
(I know to my cost) but the 3in discs stand a better chance than
most.
With a double-sided drive you halve the running costs of your
system as you only have to buy half the number of discs. However
the drives themselves are more expensive and so will not start
to pay for themselves for some time.
This will, of course, depend upon how quickly you fill discs.
Bear in mind that the smaller number of discs takes up less space.
In four years with my other computer I amassed 80 double-sided
discs.
The older 35 track drives can sometimes be picked up at bargain
prices, mainly because everyone wants the 40 track type.
Many of the 35 track drives will run to 40 tracks without any
modification, but others may need adjusting. This can sometimes
be done simply by filing off the head carriage end stop.
But it's best done by an expert otherwise you may end up with
an ex-disc drive.
I think you can see the conclusion is tending towards a 40 track
disc drive for software compatibility (double-sided if you can
afford it) unless your particular situation calls for anything
else.
Now — one drive or two? As mentioned before, the BBC computer
has its disc filing system on ROM, which has both drawbacks and
advantages.
The main drawback is that the filing system is very crude in
the way it handles the allocation of disc space to files. This
is necessitated by the limited size of the eprom — and, I suspect,
lack of adequate development time.
With other computers, most of the filing system is kept on a
"system disc" and the relevant part is loaded when required.
This is known as "overlaying", as one disc function
is overlayed in the same RAM area as the others. It allows complex
and sophisticated disc operating systems to occupy a small amount
of RAM.
The disadvantage of this method is that one drive practically
needs to be dedicated to the system disc and so two drives are
mandatory for almost any application.
The BBC computer with its eprom-based system does not tie up
a drive, and so a single drive is much more useful than it is
on most computers.
Having two drives allows you to back up discs more easily. You
will also avoid constant disc swapping, particularly when copying
files.
But for most ordinary purposes a single drive will be adequate.
I do have two, but find I seldom use the second one.
However, I have noticed a trend in some software packages, heading
back to the concept of system discs. This is especially true for
compiled languages, which tend to have a large number of system
files as well as a ROM.
The BCPL language is a case in point. If you intend to use packages
like this then I would advise two drives.
Finally, some drives use the power supply in the BBC computer
which can be tapped off by the socket on the underside. This is
best avoided if possible.
If each drive has its own power supply the computer will not
get as hot.
Also many manufacturers of add-ons are utilising this socket
as a convenient source of modest current, and if you are using
the socket for disc drives it isn't available for anything else.
The BBC buggy, eprom programmers and even some future Body Building
projects will need this socket.
The cheapest way to buy disc drives is uncased and without power
supply. You will probably find you can make a box and power supply
for slightly less than the ready-built article, but you do not
save all that much.
There are many makes to choose from. I don't know of any particularly
poor ones, but they all differ, mainly in the type of noise they
make when running. On that score I have no recommendations.
No doubt I will be inundated by dealers proclaiming the virtues
of their drives or the vices of their rivals. Sometimes when a
disc is inserted into a drive it does not centre properly. The
computer then comes up with an error message.
The cure is to remove the disc and replace it while the motor
is running. I find this is more likely to happen with horizontally
mounted discs drives (the slot is horizontal) and so I prefer
the vertically mounted variety.
Unfortunately the trend seems to be horizontal, mainly, I suspect,
because they take up less space.
Remember the disc drive has to be connected to the computer
with ribbon cable. This is normally extra, so ask when you are
buying the drive.
The cable usually has a 34-way IDC (Insulation Displacement
Connector) socket to plug into the computer and an edge connector
to plug into the drive.
If you are making your own it should be as short as practical.
For maximum reliability keep it below three feet.
You should get full instructions with the drive as they are
all slightly different.
However, basically some switches or links need to be changed
inside the drive to assign it a drive number and configure it
for your system. There are seldom more than seven of these.
In addition, if you have two drives the terminating resistors
will have to be removed from the drive nearest the computer on
the cable.
These are normally all in one package that looks like an IC,
although it might be electrically removed by severing just one
link.
Before you are ready to go you need to set the links on the
bottom right-hand side of your keyboard. See Figure I.
These decide the graphics mode of the computer on switch-on
and the access time used with the discs. They are given in Table
III.
1 indicates the link is NOT made.
0 indicates the link IS made.
Links 0 & 1 - Not used
Links 2 & 3 — Determine the disc delays.
|
LINK
|
|
|
|
|
3
|
2
|
Step rate
|
Settle time
|
Head load
|
|
1
|
1
|
240 mS
|
200 mS
|
160mS
|
|
1
|
0
|
60 mS
|
500 mS
|
80 mS
|
|
0
|
1
|
60 mS
|
160mS
|
0
|
|
0
|
0
|
40 mS
|
160mS
|
0
|
Line 4 — Auto boot switch
|
LINK
|
|
|
4
|
Auto boot by pressing
|
|
1
|
Shift-Break
|
|
0
|
Break
|
Links 5,6 & 7 — Determine power-up screen mode
|
LINK
|
|
|
6
|
5
|
4
|
Screen mode
|
|
1
|
1
|
1
|
Mode 7
|
|
1
|
1
|
0
|
Mode 6
|
|
1
|
0
|
1
|
Mode 5
|
|
1
|
0
|
0
|
Mode 4
|
|
0
|
1
|
1
|
Mode 3
|
|
0
|
1
|
0
|
Mode 2
|
|
0
|
0
|
1
|
Mode 1
|
|
0
|
0
|
0
|
Mode 0
|
Table III: the keyboard link options
You do not have to have discs to use the graphics mode links.
If you are tired of Mode 7 and want a change you can select any
mode.
The other links determine the step rate (how fast pulses are
given to move the disc read/write head), the settling time (how
long to wait before using the head after movement) and the head
load time (how long to wait after selecting a drive before you
can use it).
The time is in milliseconds and should not be set faster than
the recommended speed for your drive.
Setting the speed slower than necessary will do no harm. You
can solder links across the holes or fit a DIL (Dual-In-Line)
switch. These switches are the same size as an IC.
Finally, a word to the cost-concious: Disc drives are expensive
and there is not much scope for saving money except by avoiding
buying more than you need.
Figure I
