Coal Burner | A 68000-Based Computer Project

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Welcome to the home of the Coal Burner, a homebrew 68000-based microcomputer being made by a good friend (Colantia) and I. Inspired by many a pre-'83 computer system, the idea is to interface various off-the-shelf hardware into what could have maybe been a hobbyist-oriented "home computer" around '81. As neither of us working on this project have any prior hardware development experience, this is also just a fun way to learn something new.

Our initial spec is to create a few boards which, when interfaced, should make what could be seen as an "odd take" on the OSI C4P. What do we mean by this? Well, the initial idea is to a set of boards that do the following:

To make things easier in writing, we'll just call the CPU board the "Chamber" and the disk controller the "Pan".

As-mentioned, the Chamber is going to be based around the 68000. The 8MHz IC was the fastest possible variant in '81, also the cheapest to buy these days, so it's what we'll be using. The computer will have 16K of base SRAM, 8x 6116s to be particular. ROM will be in either 2716s, 2732s, or if we're feeling like really being on the "cutting edge" for the time, 2764s. Serial will most likely be using a 6850 UART or two.

As 16K of RAM isn't really that much, just enough to "get started" in a sense, more will be available using a "Fuel" board. We'll do a simple SRAM board first, 64K of 6116s, but we may also look into doing a DRAM board with 256K or 512K of 4164s in the future. Of course, at some point, there will probably be a 1MB/2MB/4MB board available as we would like to have all 16MB of address space usable.

The Chimney is quite limited by the year which we're targeting. There are a few (limited) off-the-shelf TV interfaces like the 6847, there's the 6845 which seems quite interesting albeit complex, or there's the idea of just rolling our own video solution. We're not entirely sure which we're going to go for just yet, maybe all three will exist in some fashion?

The Pan hasn't had too much thought put into it just yet. We'll start work on it once we can get BASIC happily appearing on a TV or monitor.

We would like to run some simple OS on this hardware. Of course a BASIC interpreter will come first. After that is up and running, we may try things such as CP/M-68K.

If we ever get to the goal of putting the system in a case, we're entirely prepared to bend sheet metal and cut wooden side panels. We adore the look of the OSIs and Sol-20. We intend to emulate something similar to that case for our own purposes. Who knows, maybe we'll even make a few?

Update - April 22nd, 2023

Specs for the Chamber are finalized per-Coal. They pretty much match what was said above (68000 @ ~8MHz, 16K SRAM, 16K ROM, 2x 6850-based serial ports, and an expansion bus) with about the components we said above (6116s and 2764s). She looked into adding a circuit for dynamic memory but decided it wasn't worth it for right now. Boards are being tidied-up and should be sent for fabrication at the start of May.

Once we have a working revision of the board, a zip of the KiCad files, producton gerbers, and bill of materials will go up right here.

Update - May 12th, 2023

"I didn't order PCBs and instead went to test "by hand" at first. I got my set of chips in. I used wire wrap. It was a success. I was able to boot BASIC. Both of the serial ports seemingly worked. The "bus" pins let me hook up an external board with LEDs. I'm excited! I'll be making this into a PCB for June." -Colantia

The current "next step" for the project is:

Building a V0.1 Chamber PCB. We'll be moving onto the Chimney once we get BASIC booting on the PCB itself.