Personal digital assistants devices come in all shapes and sizes, with software and add-ons to go with them. All they have in common is an ability to manage contact details and personal agendas, as well as issue reminders and sound alarms.

Perhaps the best known of all the palmtop purveyors is Psion, a plucky British company that has unfortunately bowed out of the consumer PDA market, leaving the way clear for the likes of Palm.

To key or not to key?

You can plump for a machine with or without a keyboard. The alternative is a pen which reads characters on a touch sensitive screen. Suppliers generally pitch keyboard-based PDAs more as portable computers than organisers, but once you remove the keyboard they are all more or less the same.

However, it's obvious that keyboard-based models such as Hewlett Packard's Jornada 720 are better suited to tasks such as word processing and data manipulation in applications such as spreadsheets. With this in mind the touch sensitive screens learn to read your handwriting making them the ultimate gadget. But it also makes them a lot slower, particularly if you need to write a document on a train.

To get around this problem some pen-based PDAs have keyboard attachments which make them a much more flexible option.

You have to remember that using a PDA is going to be a lot slower than a laptop PC. A PDA processor's headline speed is far from a clear indicator of true power or versatility. Many other factors need to be taken into account, such as available memory and the demands of the machine's operating system.

Size problems means lots of extensions

In order to make their offerings as pocketable as possible, PDA designers pare down the features. This leaves the door wide open for third-party peripheral makers to jump in with numerous ways of upping the function count once again and relieving you of some cash in the process.

Handspring Visor owners are treated to the widest range of extras, thanks to the Springboard expansion port. Among the most popular add-ons for the Visor are the SoundsGood and MyVox modules. The former is an MP3 player with a 64Mb memory, while the latter makes use of the Visor's built-in microphone to provide eight minutes worth of Dictaphone-style digital voice recording.

However, the most sought after gadget right now is the VisorPhone package that, as the name hints, turns a Visor into a mobile phone.

Options for Palm users are a tad more limited in the added functionality stakes but there's still plenty of choice. One of the most interesting is the Palm Portable Keyboard, a near-full-size QWERTY effort that folds away concertina-style into an eminently pocketable package.

In this wired world it's no surprise that modems are available for all models for basic internet/email access, and there are any number of clip-on covers and cases. With the introduction of the Universal Connector on the new m500 and m505 models, the variety should increase considerably.

Handsome devils

Palm's machine line-up is by far the most confusing of all PDA manufacturers', starting with cheap, rather ugly beasts and extending to high-cost, high-style and design icons.

What they all share, however, is the same simple interface: Palm OS. The m100 is typical of the entry-level product, with a rugged if unattractive casing and 2Mb of memory, while slightly further up the price scale the company delivered what many Palm aficionados had long craved: a colour screen coupled with beefed-up storage of 8Mb. But in Palm circles, these are both considered old-school offerings.

The m500 and m505 don't offer vast improvements in the interface department, but they are expandable in ways that no other Palm-produced PDA has been before.

For starters, the new machines sport MultiMedia Card slots that accommodate memory cards with capacities of up to 64Mb. Palm has also introduced the Universal Connector to allow hardware extras, such as digital cameras and modems, to be snapped on at will, but that's nothing new in the Palm OS arena.

Handspring's Palm OS-powered Visor PDAs have had such an expansion capability since day one, in the guise of Springboard modules. These have proved a grand success for Handspring, helping to shift the company's machines in sufficient numbers to significantly erode Palm's market share.

The interesting point here is that, while Palm is just getting started with the add-ons, there's already a vast selection of Springboard modules offering all manner of slot-in functionality from MP3 players to voice recorders.

The most recent arrival to the Palm OS party is Sony, with its Clie. It's a stylish little number, for sure, but doesn't bring much to the fold other than a marginally more useful picture viewer. Perhaps the handiest feature is the unit's jog dial, as it provides one-handed, stylus-free control for browsing data.

Pocket PCs synchronise with desktop PCs

Some PDAs' reduced dimensions make that a much trickier proposition. They are better suited to the handheld and two-thumbed tapping approach. However, this can often be improved by synching with a PC. This enables files to be backed up or transferred.

Desktop synching is sometimes via a desktop docking cradle and software. When the Revo is resting here, its built-in batteries are being recharged: expect around 15 hours' use between charges. Other desktop synching systems just connect to the PC and don't allow recharging at the same time

Operating systems

Operating systems are fairly crucial for PDAs as they can often define how the hardware will actually run.

Epoc and Symbian

Epoc is the previously proprietary operating system that runs on all Psion machines and takes its name from the company's vision of 'a new epoch of personal convenience'.

Like Windows CE/Pocket PC, it's designed to support a mix of both keyboard and pen input. Even so, the application line up indicates that the emphasis is on typed entry. The mainstays are the fully featured word processor and spreadsheet packages. shored up by powerful agenda and scheduling tools.

While these can be initiated with a simple stylus sweep, all data entry is via the keyboard as Epoc does not feature any form of handwriting recognition. One often-overlooked feature of Epoc is the zoom mode. Regardless of the running application or function, it's possible to cycle through several screen magnification levels to achieve the best view of the displayed information.

Moreover, Epoc is a true 32bit multitasking environment, so the word processor could be set to spell check a long document while a vast spreadsheet is recalculating - and the user is still free to manage agenda and contacts.

Latterly, Psion has spun off the development of Epoc into the hands of new company, Symbian, which is a collaboration between the British PDA pioneer and a number of big-name mobile phone manufacturers. The aim is to produce all singing, all dancing phones-cum-PDAs, but more on that later.

Windows CE and Pocket PC

While the Palm operating system has bobbed up and down in the development waters, and Epoc floated gracefully hither and thither, Microsoft's Windows-like alternative has sunk more times than a holed boat. We've lost count of how often the software giant has reinvented its palmtop Internet Explorer operating system, but on each occasion it did so with good reason: earlier incarnations were slow, memory-hungry affairs.

Right now there are essentially two flavours of Windows CE: Pocket PC and Windows For Handheld PC 2000. Pocket PC is strictly for palmtop PDAs, and supports quarter-VGA resolution (320 x 240 pixels), while Windows For Handheld PC is designed for larger devices, with screen resolutions ranging from 640 x 240 up to 800 x 600.

With both these new editions, Microsoft has righted many of its previous wrongs. The operating system now sports a consistent interface, controlling the whole gamut of Microsoft's core applications including cut-down versions of Word, Excel and Internet Explorer.

Grand improvements aside, the highly graphical nature of the Windows interface makes much greater demands on processing power and memory than either of the alternatives. While top-end Palm devices languish in just 8Mb of memory, that's not even a realistic minimum for running Pocket PC. If you want the Windows experience in your hand. then 16Mb is just the start - and more and more Pocket PC manufacturers are moving to 32Mb as standard.

Palm

Palm has the shortest and smoothest history of all the PDA operating systems, starting life in 1996 and evolving gently to incorporate wireless connectivity and support for colour displays.

Unlike Epoc and Pocket PC/Windows CE, Palm does not ape the operation of desktop computers, presenting instead a simple front end on a few useful information management tools. The upshot of this is that Palm-based PDAs are fantastically easy to control but offer only a limited set of functions. Contact and schedule management is the Palm's raison d'être, so don't expect it to do much more.

The latest edition of Palm OS, version 4.0, enables you to plug add-on peripherals into the Universal Connector on the m500 and m505, but only time will reveal how these will evolve.

Palm OS was not designed with keyboard entry in mind, relying instead on its own Graffiti built-in handwriting recognition system. This uses simplified pen strokes to represent characters and symbols, a technique that doesn't take long to master. As an alternative, you can peck away at an on-screen keyboard, but the small display renders this an irksome chore.

Because Palm OS is so undemanding, memory is less of a concern for machine manufacturers and users. While 2Mb is certainly tight, 8Mb is more than enough to cope with all Palm functions, as well as providing sufficient storage for tens of thousands of contacts and appointments.