Thirty years ago in 1983 the first tablet computer was released: the Tandy / RadioShack TRS-80 Model 100. It ran for weeks on four AA batteries and gathered quite a following. Despite the $1099 ($1399 with extra 8-KB of memory) introductory price tag, features like the built in 300 baud modem with acoustic couplers made them very popular with reporters in the field, and the built-in BASIC programming language (written by Bill Gates himself!) made them easy to adapt into various custom applications. Over six million were produced and as a result, inexpensive, used Model 100s are readily available now. Amazingly many of them still work perfectly and there is a somewhat active Club100 fan club.
I bought one that was non-functional for $20 with the goal of replacing the 80C85 motherboard with a more modern AVR or ARM CPU. While this particular motherboard had failed sometime ago due to bad capacitors, the LCD and keyboard were in perfect working order. Thanks to the combination of the age of the design, the system’s low original clock speed (2.4 MHz) and its 5 V logic make it simple for modern hardware to drive. Moore’s Law also means that the entire motherboard can be shrunk into a PCB with almost zero chips other than the MCU. Read on for what is involved in building a new brain for your Model 100.