NABU PC Gets CPU Upgrade, Emulates A TRS-80

A NABU PC opened up and powered on

The NABU PC caused a bit of a buzz in the retrocomputing community a couple weeks back. After all, it doesn’t happen often that a huge batch of brand-new computers from the 1980s suddenly becomes available on eBay. Out of the box, the computer itself isn’t that useful: with no internal storage, or any application software whatsoever, it can really only serve as a bare-bones development platform. But since its hardware is quite similar to that of other contemporary home computers, emulating one of those shouldn’t be too difficult, which is exactly what [Ted Fried] did: he managed to turn his NABU into a TRS-80 clone by using his MCLZ8 CPU emulator.

The MCLZ8 is basically an 800 MHz Teensy CPU with an adapter board that allows it to be plugged into a Z80 socket. It emulates the Z80 CPU in real-time, but it also holds the TRS-80 ROM and performs real-time translation between peripherals. On the input side, it reads out the ASCII characters coming in from the NABU’s 8251A UART and stores them in the virtual TRS-80’s keyboard buffer. On the output side, it transfers the TRS-80’s video data to the NABU’s TMS9918 video chip.

The motherboard of a NABU PC with a Teensy-based CPU upgradeOne problem [Ted] ran into was a difference in screen resolution: the NABU has a 40×24 character display, while the TRS-80 generates a 64×16 character image. [Ted] solved the vertical difference by simply keeping the NABU logo on the screen at all times, and decided to just ignore the 24 characters that drop off the right side – it’s not a big issue for a typical BASIC program anyway.

The repurposed NABU might not be a perfect TRS-80 clone, but that’s not the point: it shows how easily the NABU’s hardware can be reprogrammed to do other things. For example, [Ted] has already started work on a new project that doesn’t emulate the Z80, but instead runs code directly on the Teensy’s ARM A9 processor. As you might imagine, this gives the NABU several orders of magnitude more processing power, although the practical use of this is limited because the CPU still has to wait for the NABU’s slow data bus and display chip. [Ted] explains the setup and runs a few impressive demos in the video embedded below.

[Ted]’s NABU experiments are a great example of the Teensy board’s flexibility: we’ve already seen how it can emulate a Z80 as well as an 8088. We’re also curious to see what others will develop with the NABU’s hardwareif they can still buy it, of course.

11 thoughts on “NABU PC Gets CPU Upgrade, Emulates A TRS-80

  1. I think it’s impressive that so many people contribute something to an 8-Bit system and that it kind of awakens the Canadian thinkerer’s/homebrew scene.

    On the other hand.. I think the Z80 was fine and I can’t get over the fact that this system promoted cable TV (I have an aversion against such a closed Pay-TV system, satellite technology was/is more elegant/free) and has to use a limited 9918 VDP.

    The TI99/4 from the late 70s was ugly enough. I really hope the community will try to replace the VDP by something better eventually. The 9938, for example. It’s backwards compatible, so there’s no loss.

    Some people did upgrade their MSX1 computer to a pseudo-MSX2 computer by replacing the VDP, adding RAM, new MSX2 ROM set etc. With the exception of a mapper, it conformed to MSX2 specs and could run many MSX2 titles, afaik.

    So I suppose the modifications can’t be so hard for the NABU PC, either. The ROMs can stay pretty much the same, since no successor system was ever made.

    This would make the NABU much more interesting as a computer and would make pixel-artists happy, too. Anyway, these are just my two cents. Feel free to just ignore them. 🙂

    1. NABU has the 9918A VDP which the TI-99/4A used. AFAIK the only microcomputer to use the 9918 was the TI-99/4.

      Since the NABU uses the same VDP and CPU as the MSX1 computers, it was apparently trivially easy to add NABU emulation to MAME.

      1. That would be cool, yes.
        Isn’t something similar possible with C64 VIC-II already?

        If so, a replacement VDP would be really useful.
        Replacing the old VDP by a drop-in replacement (FPGA/microcontroller) is a reversible modification, after all.

        So nothing would be lost. Rather the contrary. The old 9918(A) VDPs could be stored in a safe place.

  2. They are making great strides with the “new* old NABU both at the hardware and software level, my hats off to them.

    TRS-80 Level Ii BASIC could run on other machines with only a little patching of the binary. Back in the day I typed in its 12k of hex machine code from the ‘TRS-80 Basic Decoded’ book into my Oz Applied Technology S100 over a few weeks, saving to cassette each day. I set the keyboard handler to my monitor rom, also changed the screen start address. I had a programmable character generator for my 64×16 video board, and so emulated the block graphics. I used my monitor for cassette load/save rather than the BASIC tied to the original machine’s hardware

    This was all long ago but I think the screen width and height should be changeable to other aspect ratios such as the NABU with only a little patching. Having that wonderful book really helped.

  3. The online software from the original NABU network is up on There you can download free software to run on another computer to emulate the way a NABU would download from the cable TV servers. The only extra piece of hardware required is a RS485 to USB adapter.

    Or add the NABU emulation to MAME and use the server software’s TCP function. There’s a growing list of homebrew software that’s available using the server software. Instead of booting the NABU (real or emulated) and having it download the menu, you select the software you want in the server emulator and the computer will download that instead. Would be nice if they’d add all the original 1980’s programs to that so the old menu could be bypassed.

    There’s been rapid progress on cloning the NABU floppy drive controller for making new ones. Several versions of the NABU ROM have turned up, in 4K and 8K sizes, some with floppy drive support, some without.

    I expect that at some point a new NABU ROM will be pieced together with all the best bits from all the versions that have been found.

  4. Somewthing I discovered couple of days ago while watching “Oral History of Benny Lau and Lee Lau” by Computer History Museum:

    Turns out NABU terminal was designed by ATI co-founder Benny Lau

    “I went to a company called anti-com
    which at that time they were acquired by
    a bigger company called Nabu
    which they developed
    they were ahead of their time they were
    actually developed developing a terminal
    and that is a computer that hooked up to
    a the cable through a cable modem
    so the idea was that there wasn’t any
    internet at the time
    so they were they have this idea of oh
    how about like that the head end of the
    cable company they store all the
    news and stocks quotes and
    games that people if you buy this
    terminal hook up to your cable
    and you can download this game news and
    whatever and then you can then play with it
    and use it so _I developed that terminal for them_
    and like they are they basically
    almost like the shell company they
    acquire a lot like say
    they acquired a company in Ottawa that
    specializes in making cable modem
    and anti-com specialize in designing computers
    so they acquire all this company with
    expertise in different pieces
    and they put them into together
    to provide to provide such a service
    but the problem with the company at that
    time was they they spend money
    as quickly as they they
    raise the money the company the little
    company has about 40
    something vice presidents
    i don’t know why when they go to a
    convention in toronto they send
    40 vps
    so they’re burning money like crazy so
    so uh at that time
    my my like my manager and some of the
    at anticon they decided to quit”

    no longer NABU went bankrupt, 40 VPs :)

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