Reverse-engineered an ISA card to revive an old CD-ROM drive

Being an early adopter is great if you love showing off new gadgets to your friends. But any new technology also carries the risk of being on the wrong side of a format war: just ask anyone who got into HD-DVD fifteen years ago. If, on the other hand, you were among the few who invested in CD-ROM when it was released in the mid-1980s, you have definitely made the right choice when it comes to storage media. However, that was a bit of a different story for the interface that connects the CD drive to your computer, as [Tech Tangents] found out about it when he managed to get his hands on a first-generation CM100 drive.

This marvelous piece of technology from 1985 isn't much smaller than the IBM PC it was designed to connect to, and it originally came with its own CM153 ISA interface card. But while most eBay sellers recognized the historical value of a pioneering CD-ROM drive, the PC that came with it was usually a penny model and was thrown away with the rare interface card still in it. inside. Even after searching high and low for over a year, the only information [Tech Tangents] could find on the map was a nine-year-old YouTube video that showed what the thing looked like.

A 3D rendered image of an 8-bit ISA cardLuckily, the creator of this video was willing to take high-res photos of the card, which allowed [Tech Tangents] to figure out how it worked. The board turned out to be made entirely of standard 7400 series logic chips as well as a USART 8251, which meant that it should be possible to design a replacement just by following all the traces on the board. [Tech Tangents] got to work, and after a few weeks of reverse engineering had a complete schematic and layout ready in KiCAD.

Once the PCBs were made and filled with components, it was time to test the new board with the old drive. It wasn't a simple process either: as anyone who's tried to get obscure hardware to work under MS-DOS will tell you, it involves countless hours of trying different driver versions and setting poorly documented switches. in CONFIG.SYS. Eventually, however, the driver loaded correctly and the old CD-ROM drive duly transferred the files stored on a Wolfenstein 3D disc.

If you're lucky enough to own a CM100 or similar drive from that era, you'll be happy to know that all design files for the CM153 clone are available on GitHub. This isn't the first time someone has had to recreate an interface board purely from images: we saw a similar project involving a SCSI board for a synthesizer. Thanks for the tip, [hackbyte]!