Vintage computing enthusiast Fabio Fumi recently found a long-forgotten pocket PC from the 1980s and set about building an add-on which would let the machine load software from an SD card — making it considerably more accessible for home-brew development.
"Soon after my old Sharp PC-1403 popped off a dusty box, where it laid almost forgotten for a dozen years, or two, I thought about how I could interface it with my standard laptop," Fumi explains of the project. "An ST Nucleo board (a LR053R8 I got as a giveaway from the ST booth at a Maker Faire in Rome) could to the trick, I thought."
Released in 1986 as the follow-up to Sharp's PC-1401, boasting an improved display and a generous 8kB of user memory, pushed up to 32kB on the PC-1403H variant, the Sharp PC-1403 was branded as a "pocket computer" — though, by modern standards, it was more of a calculator with ideas above its station. Despite its eight-bit CPU running at just 768kHz and one-line 24-character LCD, the device could be programmed in BASIC — but links to external devices relied on a proprietary 11-pin connector.
Rather than type programs in by hand, Fumi opted to use this proprietary interface to load them from modern storage. "While searching for info in the Sharp community," Fumi explains, "I found out that Sharp sold a floppy disk drive for the Sharp PCs, the CE-140F, which I didn't even imagine at the time. Not just that, [but] a very smart guy [Rémy Rouvin] wrote emulators for a number of similar devices including the Sharp ones, and he was so kind to share with me the results of his reverse engineering work on a real device."
That reverse engineering work was then re-implemented on an ST Nucleo development board, with a breadboard providing the necessary linkage between the two devices — with a level shifter to keep the Sharp's 5V CMOS logic happy — while also hosting an SD card reader. Hardware complete Fumi turned to the software side, recently proving the concept by having the emulated floppy correctly respond to a DSFK disk-space-free command on the Sharp.
"Next step will be to complete code for the most fundamental commands (FILES, SAVE, LOAD, at least)," Fumi writes, "using the SD card as the storage device. This is a work in progress."
The project's source code is published alongside a schematic on Fumi's GitHub repository, under an unspecified open-source license.