The Casio SF-4000 "digital diary" is an early PDA from 1988 which included a telephone directory, memo function, calendar, schedule function, and calculator. It features a "secret function" that allowed you to store confidential information that was password-protected, and search functions.

The SF-4000 has 32K of memory which equalled to 31,902 characters. It has a six-line LCD screen which can display any full month from 1901 to 2099. It is powered by three lithium CR2032 batteries (NOT INCLUDED).