Repairing a BBC Master #2: Basic steps

First, do no harm. Second, check the voltages. That’s how the Hippocratic Oath starts when adapted for electronics. If the device under test is not getting the proper power supply, then you can’t reasonably expect it to work. But there’s something I wanted to check, even before I apply power, and this turned out to be a learning opportunity. With… Read more »

Adventures with real-time clocks – part 2

The story so far: I’d been tinkering with an MCP795W20 real-time clock (RTC) chip and having a frustrating time of it. Weird things were happening. It seemed very finicky. I’d put part of the problem down to using a breadboard (still think I’m not entirely wrong about that). It turns out, however, that the biggest problem was my own stupidity…. Read more »

Repairing a BBC Master: #1 – plan of attack

So my poor Acorn BBC Master Turbo is sick … again. And I feel a little bit responsible. Quick recap: the Master developed a problem some years ago. This was ‘fixed’ when a very generous person donated a motherboard from his machine. I never did find the cause of the problem. Since then, however, the machine has spent a lot… Read more »

Adventures with real-time clocks – part 1

So I decided that I’d like to add a real-time clock (RTC) to the Zolatron project because … why not? Should be easy, I thought. Lots of people have done it. All I need is the right chip. Maybe that’s where I made my mistake. As it turned out, for some reason, I already own three copies of the Microchip… Read more »

Now available on Mastodon

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As it happens, I set up my Mastodon account some time ago, long before Elon Musk’s ego decided it needed a new toy to destroy. I just hadn’t used it much because … well, I didn’t use Twitter much either. However, all the kerfuffle over Twitter has prompted me to look again at Mastodon. You can find/follow me at: @speculatrix@mastodon.online… Read more »

Zolatron 64 6502-based homebrew computer: mission accomplished

Okay, so I’m going to call it. This project is finished. Well, not finished exactly. No project is ever finished. You can always do more. And so it is with the Zolatron – I have ideas and plans that will keep me happily tinkering away for years. But I’ve met my main objective – which is to better understand how… Read more »

Reviving a Centurion

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Something that has been providing me with plenty of inspiration for my Zolatron homebrew project lately is the Usagi Electric YouTube channel, particularly the series of videos on restoring a Centurion mini computer. For one thing, it’s comforting to know there are people more nuts than you. I can just image the reaction I’d get from my More Significant Other… Read more »

What’s better than a 6502 homebrew computer?

Yep, you guessed it – two of them. One of the side effects of ordering PCBs from the likes of JLCPCB is that you have to order at least five copies of each board. This doesn’t quite mean that I have enough PCBs to make five Zolatron 64s. For one thing, each of the machines above uses two of the… Read more »

Zolatron 64: Writing an OS for a 6502 homebrew computer

I’ve said it before and I’ll say it again: I have no idea how you write an operating system. Even though I’ve sort-of written one. I’m calling it ZolOS. The Zolatron 64 6502 homebrew computer project started as idle curiosity that just got out of hand. I figured I’d be happy if I could get as far as writing ‘Hello… Read more »

Zolatron 64: memory expansion

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Things are continuing apace with the Zolatron 64 6502 homebrew computer. In fact, progress has been so rapid that I haven’t really had much time to talk about it. So here’s a quick refresher – and details of the new memory expansion board. Serial killer The 6551 ACIA serial board is dead. Well, it’s not dead, but I’ve stopped using… Read more »

Elliott 405 – basic principles

This selection from ‘Big Doug’ Selway’s trove of documents on the 1950s Elliott 405 mainframe computer takes us right back to the basics. First up is a ‘functional block diagram’ of the system. And what I find interesting about this is: How basic it is. Computers were really quite crude beasts back then. How it was felt you needed to… Read more »

A newbie’s introduction to CUPL and CPLDs

When you’re dealing with computer logic, things can get very complicated very fast. This is especially true when it comes to address decoding. You can easily find yourself tangled in a messy web of AND, OR, NAND and NOR gates. And there’s a problem you can run into when stringing together a bunch of logic gates – propagation delay. It… Read more »

Zolatron 64: a bug hunter’s diary

It’s always a good day when you get a package from the fab. There’s something simultaneously exciting and daunting about unwrapping a shiny new batch of PCBs. On the one hand, there are the possibilities promised by the new device. On the other, you’re wondering whether you made some kind of mistake in the design and if the thing will… Read more »

Zolatron 64 roundup

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This is the 300th blog post on this site, and I thought it would be appropriate to mark the occasion with a summary of what’s happening with my Zolatron 64 6502-based homebrew computer project. You see, the thing is, I’m feel like I’m about to announce ‘mission accomplished’ for this project. And that’s not something I get to say very… Read more »