Good news for Electron users who are frustrated by the slow speed and unreliability of cassettes - the Plus 3 disc expansion unit from Acorn has arrived and brought with it new dimensions of speed and reliability in the storage of programs and data.

In appearance the Acorn Plus 3 is fairly unprepossessing. It consists of an L shaped box coloured the usual shade of Acorn cream.

The long arm of the L contains the disc interface that allows communication between the Electron and the disc drive.

This attaches to the expansion bus at the back of the Electron while the short arm (which is the drive itself) curves neatly round the right hand side of the machine.

The result is an Electron that is some three inches wider with a disc drive to the right of the keyboard. As the Electron with the Acorn Plus 3 needs so much more power than the unexpanded version it comes with its own separate power supply - the old one becomes redundant.

And Acorn Plus 1 owners needn't worry. There is an expansion bus for it on the back of the Acorn Plus 3.

Appearances can be deceptive. The Acorn Plus 3 may look fairly dull but once you get to grips with it you find a fascinating piece of equipment offering a wealth of possibilities.

The obvious attraction of using discs to store programs and data is their speed.

A program that might take minutes to save to or load from tape only takes seconds using disc storage. And the time you spent waiting for cassettes to load and save can be used for programming.

Another benefit of the increased speed is that you can take on jobs using discs that you would never dream of when you're confined to tape storage.

Loading and saving the contents of a screen display during a program is one example.

With discs it's a quick job, but with tape the program could literally take hours.

Along with the faster speed of discs comes increased reliability. With them you rarely encounter the loading and saving problems that crop up with cassettes.

And when you do it's more likely that you've mistreated the disc rather than that the system is at fault.

Not only is a disc more reliable, you can store more on it. Each of the single sided 3.5 inch, 80 track discs used by the Acorn Plus 3 can theoretically hold up to 320k.

Some versions of the Acorn Plus 3 allow both sides of the disc to be used, allowing 640k of information to be stored.

When you consider that that's 10 times the memory capacity of the Electron itself you'll see how much a disc can hold.

The Acorn Plus 3 can be looked on as a combination disc drive and interface.

The disc drive itself is fairly straightforward and can be just treated as a sort of super cassette system which uses discs instead of tapes.

Of course there's more to it than that - and you'll be able to read lots more about the subject in future issues of Electron User - but the simplicity and reliability of the discs and the drives they go into means you don't have to know much about them to use them.

You're not limited to one drive, either. It's possible to attach another drive via a standard connection and, if wanted, this can be one of the larger 5.25 inch drives.

The disc interface or the Acorn Advanced Disc Filing System (ADFS) as it is more properly known is the link between the Electron and the drive.

Once the ADFS is fitted, the Electron automatically uses the disc drive for storage.

Normal Basic commands such as LOAD, SAVE and CHAIN can still be used, but now the program will be saved to or loaded from disc instead of cassette.

Similarly, *CAT tells you the contents of the disc rather than of the cassette.

However discs are a lot more than just super cassettes, and the ADFS has a whole set of commands of its own to make use of the increased potential of discs.

Examples are *MAP, which gives information about how the disc space is used and *DELETE. There's even *TAPE which tells the Electron to use the cassette for storage.

These extra commands and what they can be used for are explained in the thorough, well written user guide that comes with the Acorn Plus 3.

Despite a few hiccups - it talks about the CSD and CSL before it explains them - it covers a fairly complex subject well, giving all the information needed for users to exploit the Acorn Plus 3's potential to the full.

To augment these extra commands the Welcome disc that comes with the Acorn Plus 3 contains a library of utilities designed to ease the life of disc users.

Easy to use and well explained on screen, these utilities are a lesson in user friendly programming.

All in all the Acorn Plus 3 is an excellent piece of work. Well made, easy to fit and well documented. Even a complete newcomer to discs will have no trouble using it for loading and saving programs.

And the expert will be delighted at the range and power of the available ADFS commands.

In fact it's a far better disc system than the official one for the BBC. The only quibble is the price, £229, which seems rather high.

Apart from this, the Acorn Plus 3 is an excellent addition to the Electron. If you can afford it, buy it.

Nigel Peters