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Late last summer, I received a very special email. Someone told me he was building a completely new computer based exclusively on TTLs: the "Isetta TTL Computer." I have absolutely no knowledge of such things, but at the time, I was fascinated by the technology. A NASA team had just managed to repair the Voyager 1 probe 24 billion kilometers away, and their computer was also still based exclusively on TTLs.

voyager-computer.jpg
(part of the TTL-based Voyager probe computer)

I've never heard of a hobbyist computer project building a pure TTL computer. There's no microprocessor, microcontroller or FPGA. The Isetta can emulate either a Z80 or a 6502 using microcode in the EPROM, using only 42 TTLs (what a cool number btw.). Another nine TTLs are required for I/O; the video processor is also microcoded and can handle up to 640x400x16 or 320x200x64.

isa-mb.png
(the Isetta TTL computer)

Now there was again the big question: "Can it run SymbOS?" I get such requests sometimes, but in this fascinating case, I was really keen to see the operating system running on such an extraordinary machine. Roelof, the builder of the Isetta TTL, is a very smart and nice guy, and so I finally made all the SymbOS sources available to him, and he made the SymbOS port himself. He quickly understood the internal structures and components of SymbOS.

Since the SymbOS kernel is based on CPC bank switching, he adjusted the Isetta memory mapping in a similiar. This made porting the kernel very easy. In September 2024, the SymbOSVM was just finished, for which Insane had designed a completely new graphics blitter. This essentially represents 100% perfection in terms of the requirements for a SymbOS display driver. Now, Roelof has simply recreated the entire blitter in microcode with its TTL processor, which is absolutely crazy.

start_button.png

Roelof worked hard, and at the beginning of this year, SymbOS started booting on the Isetta! The mouse made its first movements, the taskbar appeared, and the start menu also came alive. Now it was time to fix a lot of little things. We met at the MSX fair in Nijmegen, I helped with bug fixing and the PS/2 keyboard driver, and finally, Roelof ported the SPI-based SD driver from the Spectrum Next SymbOS version.

isa-scr2.jpg

Now came the moment of truth: The Task Manager actually started! The proof was in. SymbOS is running on the Isetta TTL; Roelof has done it. Now there are a few minor issues to resolve, and then the eighth port of SymbOS will be ready, and production of this extraordinary platform, the "Isetta TTL," can hopefully begin soon!

isa-scr3.jpg

Is SymbOS the new Doom?  ;D

Ha!!!
I think SymbOS will soon became the new reference software for small/embedded systems.
Instead of asking "Can it run Doom?" we'll be asking "Can it run SymbOS?" :D

d_kef

edit: @robcfg you beet me by 5 minutes.

Now it's time to install SymbOS on real Voyager :laugh:

Other TTL computer is Gigatron Risc without microcode.

I completely forgot to post the link to the project, I am very sorry (currently on the Mittwinter CPC Meeting at Dr.Zed):

https://hackaday.io/project/190345-isetta-ttl-computer

This is the Hackaday project page of Isetta TTL, where you get a lot of more information, schematics and documentations.

Do you know if the machine can be PALlified? I think in the EU designed system should by default run at 384 x 256 or 288, not with extreme borders like NTSC systems that are lousy converted to PAL.

Quote from: McArti0 on 13:20, 05 April 25Now it's time to install SymbOS on real Voyager :laugh:

Exactly. And the sentient beings who'd encounter Voyager would think very highly of us earthlings. All thanks to @Prodatron 🖖

Quote from: retro space on 10:53, 06 April 25Do you know if the machine can be PALlified? I think in the EU designed system should by default run at 384 x 256 or 288, not with extreme borders like NTSC systems that are lousy converted to PAL.

It has VGA video output only, so the PAL/NTSC distinction is meaningless.

Quote from: Prodatron on 12:14, 05 April 25using only 42 TTLs (what a cool number btw.)

...with Douglas Adams as the designer?  :P

Very, very impressive work by the hardware guy. He obviously "really" understood what he's doing.

And SymbOS becomes "nowadays the" standard OS for 8-bit computer.

Nice picture of the NASA computer. you have to be brave to rely on such things in the past, especially in the hostile environment (temperature, radiation, etc.).

With SymbOS onboard, the alien V-Ger wouldn't have gotten SO confused in Star Trek: The Motion Picture. I'd give it a try.

Quote from: pelrun on 12:48, 06 April 25

Quote from: retro space on 10:53, 06 April 25Do you know if the machine can be PALlified? I think in the EU designed system should by default run at 384 x 256 or 288, not with extreme borders like NTSC systems that are lousy converted to PAL.

It has VGA video output only, so the PAL/NTSC distinction is meaningless.

Can you reclock it to do PAL? Often it is just a timing thing. On my laptop I can force the VGA out to run PAL timing as well.

Uh... Why would you need to support a TV mode that is never going anywhere near a TV?

The only place PAL/NTSC has any meaning is when outputting to TVs, which have one of two different fixed timings you have to accommodate. VGA monitors do not have this problem, and if you're only outputting to VGA then you are not using PAL or NTSC timings *at all*.

This is just insane. No, really.

Just for my understanding... A group of TTLs inside the Isetta is simulating the Z80 processor only with the right combination of logic?

Haven't read the the hackaday thread so far... Will do it.
I'm also curious, but why "Isetta"? Is he a collector of old cars?

Isetta has 128k x 24bit flash with microcode. The redundancy of transistors compared to the real z80 from 1976 is enormous.
This processor can only be implemented this way currently.

Quote from: HAL6128 on 14:28, 08 April 25I'm also curious, but why "Isetta"? Is he a collector of old cars?

Roelof likes the old small car, reduced to the minimum, so it fits to his project somehow.

Quote from: McArti0 on 16:09, 08 April 25Isetta has 128k x 24bit flash with microcode. The redundancy of transistors compared to the real z80 from 1976 is enormous.

Yes, but the Microcode also does...
- the 6502
- the "insane" blitter from SymbOSVM
- the videochip
- and the Z80 itself
and probably even more, and there is still a lot of place left in the flash.

I am a layman here, so I have no idea about this. I just wonder how much microcode in (EP)ROM the 70ies TTL CPUs had?
E.g. the PDP11 or the Xerox Alto. Does someone know about this? (maybe @PulkoMandy knows?)

The Alto (which was notable for its heavy use of microcode) had 1k x 32 bits of ROM microcode and 1k x 32 bits of RAM microcode.

This project is insane. It's like a 70s-style microcode CPU scaled up to act like an FPGA implemented entirely in TTL. :o

Z80 has 8500 transistors at all.

Wang 2200 is on TTL chip only.