Psion Series 5

The Psion Series 5 was a PDA from Psion. It came in two main variants, the Series 5 (launched in 1997) and the Series 5mx (1999), the latter having a faster processor, clearer screen, and updated software. There was also a rare Series 5mx Pro, which differed only in having the operating system loaded into RAM and hence upgradeable. Ericsson marketed a rebadged version of the Series 5mx called the MC218.

The Psion Series 5 was a major upgrade from the Psion Series 3. A Psion Series 4 does not exist, due to Psion's concern of tetraphobia in their Asian markets. The external appearance of the Psion Series 5 and the Psion Series 5mx are broadly similar, but their mainboards and other internal components were different and not interchangeable. The screens are not interchangeable because of different screen cables.

The Series 5 was the first to feature a unique sliding-clamshell design, whereby the keyboard slides forward as the device opens to counterbalance the display, and brace it such that touchscreen actuation does not topple the device, a feature mentioned in the granted European patent EP 0766166B1. This novel design approach was the work of Martin Riddiford, an industrial designer for Therefore Design. A simplified version of this design was also used in the Psion Revo.