A TRS-80 Model I Clone Built with New Components That Are Still in Production
Glen Kleinschmidt recently finished an ambitious project to create a TRS-80 Model I clone using modern parts.
The Tandy / RadioShack TRS-80 series of microcomputers was one of the most popular of all time, and that all started with the original TRS-80 Model I—a name it received later after others were added to the line. The TRS-80 Model I was first released in 1977 and had an 8-bit Zilog Z80 processor running at 1.774MHz, 4KB of RAM as standard, and a BASIC interpreter as ROM. While other computers of the era achieved more, the TRS-80 Model I got in early and proved to be successful. Glen Kleinschmidt recently completed an ambitious project to create a TRS-80 Model I clone using new components that are still in production.
This isn’t TRS-80 hardware running on an FPGA or an emulator running on a Raspberry Pi or other single-board computer. It’s also isn’t simply a duplication of the original circuit on a new PCB. Instead, it’s more a hardware clone in the same sense as the Dragon 32 was a clone of the TRS-80 Color Computer back in the early ’80s. Kleinschmidt has actually developed an entirely new circuit that works with the TRS-80 Model I ROM and peripherals. A few improvements have been made, but nothing that sacrifices the authenticity of the build.
Importantly, all of the ICs used for this TRS-80 Model I clone are still in production today and can be purchased by anyone through regular distributors. Such a thing likely isn’t as easy to accomplish with most other microcomputers, because they used special chips like the ZX Spectrum’s ULA and the Commodore 64’s MOS Technology VIC-II. Those haven’t been in production for decades, so cloning those computers is much more difficult. The TRS-80 Model I’s monochrome video output didn’t require any unusual chips, which made it much easier to clone.
Kleinschmidt's clone contains three separate PCBs: the main motherboard, a separate video generation board, and a PS/2 keyboard interface board. That last board isn’t strictly necessary, but it makes it a lot easier to connect PS/2 keyboards instead of having to make a custom keyboard. That board will also work with any TRS-80 Model I, not just Kleinschmidt's clone. All three of those boards are housed within a sturdy metal enclosure with a tidy front panel for most of the connections, as well as switches for RAM and lower case letters. There is even a built-in speaker that receives audio through the cassette data recorder output via an LM386 amplifier — something the real TRS-80 Model I never had.
If you want to build your own TRS-80 Model I clone, Kleinschmidt has generously released all of his design files. He has also published a PDF with a thorough explanation of how everything works.