3 days ago •
One of my side projects is to convert a board to original TTL series where possible, or at least to 74LS series with the oldest chips I can find. The breadboard prototype was fully 74LS, the kit edition is 74HCT.
At a local electronics shop I recently found quite a few ICs that were manufactured more than 43 years ago, in 1975. These are "new old stock": never used before and the pins still unbent. Some are indeed original TTL series as existed in the 1960s, not 74LS (Low-power Shottky) as introduced in 1971. They're plug-in compatible with the Gigatron design and the system works fine at full speed:
Although it is cool, it also gets hot! Original TTL consumes an order of magnitude more power. Due to this so far I only "upgraded" to original TTL the upper half the logic unit, the X register, the memory address unit and the program counter. Those 14 chips, together with a few 74LS replacements as well, already increase the current to 740 mA, or about a ten-fold from the kit version. Above that the multi-fuse and MCP start to do their thing. Remember that USB safety is the reason the kit comes with much more power-efficient 74HCT components in the first place.
The nice thing about the 7400-series is that improved series remained compatible in pin layout and voltage levels.
- 1966: introduction of the original 7400 series
- 1971: introduction of 74LS series
- 1983: introduction of 74HC(T) series
I have them all on a board now. The difference in generations is clearly visible. The old TTLs get up to 60 degree Celsius. The blue square isn't a cool oasis in the middle of a hot desert. It is a reflection on the gold plated ceramic lid of a vintage SRAM.
07/24/2018 at 20:33 •
I almost forgot about the project's original objective that I had in mind when it started one and a half year ago: to build a circuit from TTL components that can play Tic-Tac-Toe on an LED matrix. So lets correct that: today, with minor adjustments, Tom Pittman's Tic-Tac-Toe from 1977 is running on the Gigatron :-) Source code in GitHub.
Ok, the planned LED matrix has morphed into a video screen along the way. That's just feature creep.
07/16/2018 at 08:17 •
We've recently celebrated a dream milestone: there are now more Gigatrons out there than Apple-1's ever produced!
06/22/2018 at 15:38 •
06/12/2018 at 20:05 •
I've ported the Apple-1 built-in monitor program: WozMon.gcl
The original is 254 bytes, my version is $254 bytes . About half of that is for terminal output (character printing). The Apple-1 had dedicated hardware for that... With this, all original Apple-1 firmware has been ported to the Gigatron :-)
Now you can inspect memory locations, write to them and execute code.
Tutorials for the original WozMon, such as this nice one, are applicable as I tried to deviate as little as possible from the original: the differences are essentially cosmetic or slightly improved usability. The Gigatron-specific differences are:
- Welcome message is "*** WozMon" instead of error prompt ('\')
- Can use lowercase as well as uppercase for hex letters
- Delete shows effect on screen, no need for '_' (RUBOUT)
- Always print address at start of line in block examine mode
- Input buffer has the same size as screen width
- Executed code can go back to monitor with RET instruction ($ff)
- Faster screen update, and cursor symbol instead of '@'
- A bit more careful with empty lines
06/07/2018 at 21:59 •
There are several methods brewing for hooking up a keyboard. The one most in line with the system is WattSekunde's four chip design for a matrix keyboard interface. But such keyboards might be hard to find, while PS/2 keyboards are still easy to get. But their protocol is convoluted and it is unclear how to handle that with a few simple logic chips. So... we cheat!
PS/2 is simply too modern, and we need a time machine to translate its fast and complex signals to the simple signals that the Gigatron will understand. The ATtiny85 will be our Babel fish.
To get some idea: the ATtiny85 must be configured to run at 8 MHz to do its magic. 1 MHz won't be good enough to catch and transfer simple key strokes. Here is the system in action:
05/26/2018 at 15:53 •
Proud to present at the Belgrade conference today. I uploaded my slides here.
05/10/2018 at 14:32 •
Using an Arduino Uno to handle the PS/2 protocol. Computer is still running stock ROM v1. The Arduino is abusing the Loader application to inject a Terminal application. To reduce cable clutter, the computer is now also supplying power to both peripherals.
05/05/2018 at 15:18 •
05/05/2018 at 00:00 •
Dave's third Gigatron-featured video in a row. This time he is trying to slaughter it, but the Gigatron doesn't cave in. Go Gigatron Go! \o/