Master 512
Forum by Robin Burton
Beebug Vol. 12 No. 1 May 1993
Robin Burton takes a look at what's involved in adding a winchester (hard disc) to your Beeb, a topic relevant to all BBC micro owners.
The subject of this month's Forum has produced a steady trickle of queries over a long period of time.
Naturally it's of particular interest to 512 users, but it's useful information for any BBC micro user. Oddly, and despite that, this must be the least well covered topic ever for the BBC micro.
WANT A WINCHESTER?
When a BBC micro user acquires a 512 there's delight at the huge range of software which is available, and pleasure at the freedom offered by the vast amount of memory in the machine, at least compared with the 8-bit BBC micro.
Some BBC programs are quite remarkable given the restricted environment in which they run, but no matter how capable a program is, a major irritation that virtually everyone encounters is the limit which memory size imposes on data and file handling. You only have to sort a simple database of moderate size to become all too aware of this.
Of course strategies exist to maximise flexibility, such as multiple files for unstructured data (e.g. Inter-Word) or random access for keyed records in databases. These work, but they can be very tedious, especially when you're using floppy discs. A winchester (hard disc) certainly speeds file access and provides more space, but given cost and the limitations of the micro, this has, not surprisingly, been viewed as an expensive luxury by most users.
Even an unexpanded 512 can load a program of 200K plus a file of 150K into memory simultaneously, so to the 512 user this freedom is initially wonderful. Equally the added speed and capacity of 512 800K discs is also welcome, but as time passes the limits of disc performance again impose themselves. Loading an application and a file of the sizes above is a long way short of being instantaneousness if you're using floppies.
Worse, 512 users can find their ambitions restricted or sometimes totally frustrated. Quite a few PC programs simply won't install on floppies, while others will install, but can't be used effectively. 512 users can be in a very unpleasant position. Floppy disc limitations are more acute than in the BBC micro, but the cure is no easier.
For PCs, adding a winchester is relatively simple and cheap. Fit a controller card into the main board at a cost of about £20.00, plug in the hard disc, tell the BIOS about the drive and start to use it. Of course capacities and prices vary, but £200.00 easily buys a drive of 100Mb. these days.
For the BBC micro things are neither simple nor cheap. As was explained for display adaptors two issues ago, the design concepts of the BBC differ markedly from a PC, in consequence of which adding or changing bits has always been more difficult even if it's possible.
HOW DO THEY WORK?
A BBC micro hard disc is probably the least well understood upgrade and, as I mentioned this has tended to remain so because BBC micro magazines have rarely if ever explained what's involved. It's obvious when you have a hard disc, but judging by letters I've received a fair number of users haven't but would like to know. We'll start from basics for any recent Beeb converts.
First, to use a winchester you need ADFS, no problem for 512 users who of course must have ADFS anyway. For BBC users with only DFS who are reading this and might want to add a hard disc, ADFS is the first requirement.
Note: If you have an early DFS which uses the 8271 controller, both the software and hardware must be upgraded. ADFS requires a WD1770/1772 controller, already fitted in the B+ and Master. In an 8271 based machine you need a 1770 upgrade therefore, but finding one might take persistence these days (still available through BEEBUG Ltd.). The 1770 chip can run DFS, so your existing discs aren't scrap, but remember that you must upgrade the DFS too. The way the two controllers are programmed is totally different.)
If you have ADFS, at its simplest adding a winchester means plugging a ribbon cable into the 1MHz bus, located underneath the machine along with the floppy disc port, the parallel connector, the user port and the Tube. Naturally this assumes that you have a suitable hard disc ready, complete with all its bits – so what are they?
The 1MHz bus isn't an industry standard, it's an 'Acorn special', so there's no such thing as a 1MHz bus hard disc. The Beeb hard disc interface was therefore custom designed for a nonstandard connection. In truth, although it works well enough a BBC hard disc interface is frankly a lash-up, though perhaps it's kinder to call it an afterthought. It should be remembered that few micros, even PCs, had hard discs as standard when the BBC micro appeared.
Winchesters for the BBC micro are therefore 'standard' drives with extra circuitry to allow connection through the 1Mhz bus. To explain the various items and their role we'll follow the route from the hard disc back towards the micro so that you can see what's involved, with a bit of history for interest.
THE HARDWARE
The drive cartridge is a standard unit (of five to ten years ago remember) of any make, though Rodime is probably most common. The drive mechanism has its own control board (for motor and head stepping) physically attached to it and both together are regarded as one unit. This unit is the disc cartridge, which could be fitted to almost any micro of the time. Two types of data recording were common, MFM and RLL, though you don't need to understand these so long as the next link in the chain is the correct type.
Moving towards the micro, the next item is a circuit which translates logical disc commands issued by a micro into commands understood by the cartridge's control circuits. In the early '80s the only suitable standard interface was SCSI (Small Computer System Interface, pronounced 'scuzzi') which was directly descended from SASI (Shugart Associates System Interface). SASI was conceived as a device independent interface which could handle virtually any peripheral, hard discs were simply its most frequent application. Although Shugart designed the interface, for general use a company name wasn't acceptable, hence the change.
Probably the most common SCSI card for discs was the ST506 and that's what BBC winchesters use. Of course in a PC the hard disc controller speaks directly to the SCSI board in its own language and that's all there is to it – not in the BBC. Since the BBC micro has no hard disc controller (and can't have one) something more is needed. This is the lash-up.
An extra circuit, variously called an Acorn adaptor or an Adaptec board (after the company that manufactured them for Acorn) is required to translate IO commands issued by ADFS through the 1MHz bus into SCSI commands that the ST506 can understand. The Acorn adaptor therefore connects to the ST506 on the hard disc side and to the 1MHz bus on the micro side.
You might wonder why ADFS doesn't 'speak' SCSI itself. There are two reasons. First, the 1MHz bus was inherited from the model B and was never originally intended for a winchester, so its physical properties and signals are quite incompatible with SCSI. Second, even if the physical connections were correct there isn't enough room in ADFS's 16K for the necessary code plus floppy disc control.
The Adaptec board therefore translates elementary requests from ADFS into more complex instructions for the SCSI board, as well as providing the correct physical connections. If you look at an Adaptec board you'll see that it contains a couple of EPROMS, a processor of its own and a large number of support chips – this is quite a complicated task. Of course that's not all it has to do, it also provides handshaking and data buffering in both directions, for which the micro also has no provision.
As a result of this considerable complexity and the fact that Acorn adaptors were a low volume item (compared with micros) they were fairly expensive. I can't remember prices now, but any extra complexity obviously adds to cost. Coupled to that, SCSI is expensive, in the context something of a sledge-hammer to crack a walnut as it were, for which reason PC hard discs inevitably soon adopted a different approach.
Hard drives themselves have improved in performance over the years of course, but the costs of SCSI are now avoided. These days the standard for PC hard discs is IDE (Integrated Drive Electronics). All control circuitry is built into the single board mounted on the disc cartridge and, since hard disc control is a simple and highly defined task, much of the complexity and cost of a general purpose interface like SCSI is avoided.
The best current IDE drives operate at data rates of up to about 1.5Mb/second, but this isn't enough for some jobs. For this reason SCSI is still employed in PCs, but generally only when large drives (typically half a gigabyte upwards) and very high (circa 6Mb/sec) data throughput is required, a common need for main storage for network file-servers. Naturally large drives are still expensive and, since they form only a small section of the market, SCSI interfaces are still relatively costly too.
Unfortunately (that word again) the fact that a BBC winchester uses SCSI doesn't mean it will be fast by today's standards. Apart from the age, hence performance of the drive, the BBC micro's 1MHz bus has a maximum instantaneous throughput of only 125Kb/sec, which in practice probably means an effective rate of half this figure. Even so this is equivalent to reading an entire 640K disc in about ten seconds, so it's a huge improvement on floppies and is still a worthwhile step.
STUMBLING BLOCKS
OK, so Beeb hard discs use a bit of special hardware, but why aren't they easier to get hold of? The answer is paradoxical, the cause is historical. (That's this month's intellectual bit over with!).
When model B micros were current you couldn't add a winchester. That had to wait for ADFS, which although offered as an upgrade when the B+ appeared, was primarily produced for the launch of the Master.
The addition of a winchester was therefore a late option in the BBC micro's history. Unfortunately (CANCEL) – It was just bad luck that when it did become possible the interface used in PCs was changing (to ESDI, now itself superseded by IDE) so the drive type and interface was no longer in the mainstream, with attendant cost penalties.
Using an SCSI interface drives had always been pretty expensive, but by the time the BBC micro employed it, SCSI was no longer the normal approach, so prices didn't fall as they normally do after a few years for most electronics. You could pay over £400.00 for even a 10Mb BBC drive six or seven years ago, so obviously it was never the most popular add-on even though it was quite probably the most desired one.
SUPPLIERS
These days, the 8-bit BBC micro is of little interest to most Acorn suppliers and, to the best of my knowledge, none can now supply BBC winchesters. Those who specialise in hard discs (e.g. Oak, Morley, etc) might have a few spares left, but I wouldn't bank on it.
This leaves prospective purchasers with second-hand sales. Naturally, users who purchased a hard disc years ago were real enthusiasts, so it's likely a good many of them still use their Beeb with no intention of parting with it, or the hard disc, for some time. Other hard disc users, such as businesses or schools particularly, can be expected to keep machines until they fall to pieces, so they're not much help either.
A quick scan of Beebug personal ads. over a few issues will show just how rarely hard discs are offered, in fact I wouldn't be surprised if 'Wants' outnumbered 'Sales'. Apart from a good supply of patience and a lucky find in personal ads (still expect to pay about £200.00 for a working 20 or 30Mb drive) what are the other options for getting hold of a drive?
Well, you might find one for sale that doesn't work, so it will be cheaper, but then what about spares and repairs? Of course that depends on the fault, but be aware that there is no 'standard' or likely fault, the problem might be anywhere. For BBC use the components of a hard drive are a case, a PSU, an Acorn adaptor, an ST506, a drive cartridge plus connectors and cables. Depending on which parts you have and which you need, sources for spares vary from scarce to virtually nonexistent.
There are a few advertisers of PC hardware surplus in PC magazines and some offer new 'old stock' 10, 20 or 30Mb drives, with prices of well under £100.00 for 20Mb. If you buy one of these and add the other bits you might be in business. Alternatively you could canibalise an old, expired XT with a working hard drive and start from there, which might be even cheaper.
For the remaining parts, cables, plugs and sockets are no problem of course, but cost about £30.00 for a complete set. PSUs aren't totally impossible and failed units can be repaired if you know the right people. Acorn dealers no longer supply Acorn adaptors and cases have always been a problem, but it might be worth asking your local dealer about these, you never know.
HELP? – MAYBE!
All in all, acquiring a hard disc for a BBC micro isn't easy. It was never cheap and, even for used components it still isn't. Consider! A hard disc probably cost its original buyer as much if not more than the micro. Because of that there aren't many of them, so no-one needs to sell one at a bargain price. The bottom line, however you go about it, is that you'll be very lucky to get a working hard disc for a BBC for less than £200.00 all in, assuming you find one, or the appropriate parts.
I know of a limited source of Adaptec and ST506 boards and the people concerned (who do not wish to be contacted directly) occasionally come across cartridges, PSUs and cases. They can supply cables and will even build a drive for you if you have (most of) the bits. They can sometimes repair faulty drives or components, but take note that a faulty drive mechanism, if that's the diagnosis, is forever a lost cause. Even the original manufacturers either can't or won't repair them!
If anyone has a go at building a hard drive but gets stuck or has a faulty drive you can write me via Essential Software and I'll help if I can. Here are the rules.
Be very sure to explain precisely what parts you have and what parts or help you need. If you have a faulty drive supply as much info. as possible. Do include an SAE, but do not ask about a complete hard disc supply service – there isn't one!
Finally, anything you can do for yourself will be cheaper. You already know what the total cost of a drive is likely to be but, sorry to be brutal, while the help is free, phone calls, postage and the time of other people isn't, so include £2.00, payable to me, to cover initial costs.
Address: Robin Burton, *******