Stephan Paukner :: syslog

I recently managed to reactivate my dear old Epson HX-20, a retro computer released in 1983 which I used at the end of the 1980’s and early 1990’s to learn programming. Since I even could read in some BASIC programs that were still stored on the micro cassette, I wondered if I could rescue the code directly, without using OCR on printouts or even typing it off by hand. I was aware that I was very lucky that this machine still worked after all those years—the soldering seemed to have been much more rigid back then, it might cause more issues to attempt to run old PCs which are a decade younger! To be on the safe side, I invested into a new NiCd battery pack and replaced the original one.

My research first led me to the machine’s RS-232 output, internally called “COM0”. Someone had used that some years ago to directly connect an HX-20 to a PC’s serial port, using a special cable and some adapters. Sadly, it seems that this is no longer an option, since these cables disappear, and serial-to-USB adapters only seem to work with a certain chip in this case.

Then I stumbled upon the GPL software HXTape, and I was totally baffled: What, the Epson HX-20 had an external cassette interface as well? I knew that concept from our even older machine, the Texas Instruments TI-99/4A. It connected to a special music cassette recorder and encoded data into simple “magnetic bits” onto the tape:

It was quite funny to listen to the noisy sounds when playing the MCs on an ordinary player.

This bidirectional data transfer works over ordinary mono audio cables, one for each direction. And now, it turns out the HX-20 had such an interface as well, and we never used it. But the point is, one could exploit it to decode the audio signals into the original bits and bytes by connecting the HX-20’s “MIC” port to the microphone input of a PC using a simple mono audio cable with standard 3.5 mm jacks! (How tremendous the analog world was! Keep a music cassette lying around in the basement for decades and then just play it. Try this with your ¡Phone!) And that audio decoding is exactly what HXTape is doing.