Back in the day, accessories for the IBM PC were dirt cheap, compared to other computer systems. Most hard drive intefaces for the CoCo were quite expensive. That's where the CoCo XT-RTC comes into play. This adapter allows you to plugin a standard Western Digital HDD controller, designed for the IBM PC, into the CoCo.
Included is the 8K boot ROM (both scrambled and unscrambled) that plugs into the HDD controller, and allows the CoCo to boot directly into OS-9. If you add +12v (7812 works) to pin 2 on the cartridge port, then you can plug the CoCo XT-RTC directly into the CoCo 3's cartridge port, without the need for a multi-pack interface.
I have no way to validate the integrity of this ROM file, but this is what I have burned into my EPROM. Please note, that this rom is "scrambled", so you can't make much sense out of it, without knowing how it is scrambled. Address lines A4, A5 and A6 are inverted, so it's trivial to unscramble. Also included is the software to scramble or unscramble the ROM image.
Text found within the ROM file, suggests that the game of life built-in to it, but I haven't investigated it, to see how to activate it.
I made the schematics real quick, and did a reasonable amount of double checking, but it is still possible I could have made a mistake. Everything looks reasonable, except for the one thing I noted on the schematics.
A closeup view of the PCB. Notice the "MADE IN USSR" 7400 ICs. The battery was never replaced, and has about 2.9 volts in 2025.
The CoCo XT-RTC with a WD1002-WX1 and OS-9 boot ROM installed.
The CoCo XT-RTC with a WD1002A-WX1 installed.
This should be accurate.
