Sony has unveiled working models of its long-awaited handheld computer based on the Palm operating system.

The expandable unit is due to hit Japanese shelves in September, and details of its launch elsewhere in the world will be announced in the autumn.

Sony intends to release the device, which it calls a "personal entertainment organiser," in two versions: colour and monochrome, with otherwise identical features and an almost identical price.

The company estimates the colour version will be priced around £360, and the monochrome version, about £330.

As fans of the Japanese consumer electronics giant have come to expect, its device comes with some of the styles and features now becoming standard fare on the company's products.

But consumers may find initial versions of the device have several shortcomings.

Chief among its features is a slot for the company's proprietary Memory Stick memory card, but the first models won't support versions of the Memory Stick used with the company's digital music players.

This means it cannot double as an MP3 player. The built-in copy protection hardware required to support the music memory cards won't be available until the second-generation models debut.

Besides offering a way to add extra memory or even transfer files to the device, Sony plans to produce a range of peripheral hardware. These include a video camera with an adapter that plugs into the Memory Stick slot.

Another feature is the "jog-dial" button, becoming standard on many Sony mobile telephones and portable electronics devices.

The button lets users scroll through lists on screen and select with a push of the same button, making one-handed operations much easier.