The Psion Series 3 range of personal digital assistants (PDAs) came with two slots for optional memory expansion packs for backup of data (marketed as ‘Solid State Memory’) or software modules with a capacity of up to 4 MB. Software was produced by both Psion and third-party developers.
The earlier memory expansion packs used a button cell to keep memory stable, whereas later ones used flash memory so didn’t require this.
The Psion Series 3 was introduced in 1991, and the final 3mx model was discontinued in 1999. It was known for a long battery life, durability, and stable and versatile software. Around 1.5 million were sold.