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1996 — Palm (Pilot) PDA
Palm started as a division of US Robotics, and with the introduction of the Palm Pilot, was able to gain its independence. The PDAs from Palm became wildly popular and soon became a household name.
That got Palm in trouble with the Pilot pen company, as the latter sued Palm for using "Pilot". Palm dropped the Pilot name, and products were simply Palm PDAs going forward.
Palm had many features that endured the PDAs to the buying public, not the least of which was the hotsync cradle. Users simply dropped the PDA into the cradle, which was connected to a computer, and all personal data would be synced to both devices.
The PDAs of Palm soon morphed into the Palm Treo, the most successful smartphone of the time. The marriage of PDA functions with the phone turned out to be a marvelous marriage, and the Palm Treo introduced millions to the concept.
1997 — Psion Series 5 handheld
Psion introduced the Series 5 PDA to replace its Series 3 model. The new Psion 5 included an innovative sliding keyboard that came forward to provide stability when the clamshell was opened.
The Series 5 used the ARM710 processor that could run 10 to 20 hours on two AA batteries. It had a touchscreen and dual serial connectors (RS-232 and IR) for connectivity to other Series 5 devices.
Most significantly, the Series 5 ran the EPOC operating system, now known as Symbian. Largely through the efforts by Nokia, Symbian came to be one of the most-used OSes in phones of all times.
1998 — Franklin REX
This PDA brought a new meaning to the word handheld as the REX was the size of a credit card. It didn't have many fancy features, Franklin was content to bring simple PIM functions to the REX.
All models of the REX were designed to sync with personal data on a computer, a good thing as early models had no way to input data on the REX itself. Later models got crude data entry capability, a vast improvement, but still not very convenient. The six month battery life made up for the lack of features on the REX.
The REX took the hardware to new heights, er, small sizes, but was pushed out of the market largely due to the popularity of the Palm PDAs.