Novasaur Retrocomputer's TTL-Only Design Emulates the Intel 8080 Processor

After seeing TTL-based computers like the Gigatron, Alastair Hewitt, a software engineer with a hardware itch, got the inspiration to build the Novasaur Retrocomputer. Like digital pioneers in the 1970s, he designed his computer without a dedicated CPU chip. His goal is to access the Internet using only TTL chips, via RS-232.

So there's the easy problem (designing a computer out of TTL) and then there's the hard problem (make it work at 12.5 MHz).

The Novasaur project log is one of the most comprehensive we have seen. Hewitt has provided continuous updates to his on-going retro-computer for over almost a year and a half--with no signs of stopping. The Novasaur has several elements that make it both an appealing and unique project.

For example, Hewitt made an early decision to fit the 8-inch by 5-inch (200mm by 125mm) circuit board into an off-the-shelf Polycase ZN-40 enclosure. In an age where most projects opt for a 3D-printed box, it is refreshing to see an era-appropriate case.

Another feature that caught our eye was the decision to emulate a CPU instead of providing a custom instruction set. Looking back at past logs, Hewitt considered several options: Z80, 6502, and even RCA's COSMAC. In the end, Novasaur emulates the Intel 8080 with an effective clock rate of about 450 kHz.

Graphics are provided by a second processor, which Hewitt admits he is generously calling a GPU. He says the circuit is really a DMA controller running in transparent mode. Regardless, Novasaur is capable of both 80-column text modes and bitmapped graphics. The text-mode supports eight colors with support for standard, bold, italic, and underline typefaces. Currently, the two implemented graphics modes are low-resolution with 200x120 pixels supporting 256 colors and high-resolution with 400x240 pixels with eight colors and four dithers. And since Novasaur supports VGA out, Hewitt has provided a demo of a colorful parrot.

Other features of the computer, now in revision 6, include:

• 256K ROM
• 512K RAM (with 64K Banks)
• 4-voice audio with a wavetable, supporting attack-decay-sustain-release (ADSR) envelopes
• VGA output
• PS/2 keyboard support

It is interesting to read through Hewitt's comprehensive project logs. You can see the direction he headed towards, and then you get to an entry with the word "redesign" in the title. They are well worth a read if you are considering your own TTL computer design.

For more information, check out the Novasaur Retrocomputer's project page for the logs, list of components, and design files.