[Usagi Electric] is known for minicomputers, but in a recent video, he shows off his TMS9900-based homebrew computer. The TMS9900 CPU was an early 16-bit CPU famously used in the old TI-99/4A computer, but as the video points out, it wasn’t put to particularly good use in the TI-99/4A because its RAM was hidden behind an inefficient interface and it didn’t leverage its 16-bit address space.

The plan is for this computer to have 2K words of ROM, 6K words of RAM, and three serial lines: one for the console terminal, another for a second user console terminal, and the third for access to a tape drive.

Note that we have two user terminals: this is a multiuser system! The computer will use the TI series 10 “Insight” data terminal.

In the video, [Usagi Electric] spends a fair bit of time making the rack-mount casing for his computer and its two power supplies. The UART for 300-baud terminal access is currently in breadboard format, but it is set up to transmit and is functional so far! Up next will be support for receiving. The UART he’s using is the TR1602B, and he spends some time reviewing its datasheet in this video.

If you’re interested in the TMS9900, you might like to check out TMS9900 Retro Build and How The TI-99/4A Home Computer Worked.