This is the web page I am writing to document my work on a homebrew computer I have decided to call the Manic-1. I will be using only logic circuits from the ubiquitous 74xx series to build it, with a few exceptions (memory, A/D conversion if there is any). Since the original 74 chips were based on bipolar transistors, I'm naming it after the manic phase of bipolar disorder. This is about the only paragraph that's going to be reliably the same if you come back more than a week from now; I scale my plans and ambition up and down constantly.
I have finally worked out a design that I'm going to stick with, mainly because I've cleared the hurdle that was sinking past ideas: the control unit. The final design is one with four registers, four stacks, and one ALU, with a 16-bit word everywhere. The key came when I was reading a wiki about computer design and it mentioned computers with completely general registers.
As such, my design allows any of the registers to be PC, a normal register, or a memory pointer. 9/16 bits in the instruction word are given to describing registers - 2 bits to select a register and 1 to determine whether or not to dereference it for three values (Ra, Rb, Rc); The only operations to use all three are math operations (Ra [operation] Rb store in Rc), others involve one or two (push, pop, move, jump, i/o).
Jumps can be accomplished in several ways; The program counter select can be overwritten, which will make a different register into the PC, although there would need to be a rigid protocol to make this work. None the less, it's speed would be unbeatable, making it ideal for fast loops.
I intend to maintain a complete set of schematics and images, open-hardware style.
An aside: How I got toner transfer to work! Things I've learned are: Use the right paper, and to apply hard even pressure for a long time when ironing.
I tried JetPrint Photo Multiproject glossy, but although others have had good results with it (and it's wonderfully easy to peel off the blank PCB when you're done), my printer, a Laserjet 4MV, hated it; Every trace that ran parallel to the direction of motion was useless because about 1/3 of the toner was missing in chunks. I believe this may be due to it being a fairly heavyweight, unflexible paper. Touching up the entire board with a sharpie would never work, so I tried the other paper that I'd heard about (Staples Basic Photo Glossy). It's not nearly as easy to peel off, but it does come through my printer correctly. I guess it depends on your printer; If one doesn't work, try another. Just as long as it's glossy, because you need that top plastic covering so that the toner will let go of the paper.
The second thing I found was that you're better off just buying a sealing iron from a hobbyist/modeling shop. It took me about 7 tries before I got a 4x3 double-sided board to work correctly (requiring no more than an inch or two of touchup with the trusty sharpie). It may have been my technique, but mom's clothes iron never worked right. I found that I would constantly either press/rub too hard and smear my traces or too lightly and have them not stick. Don't get me wrong, about 90% of 'em would be fine. But areas, mainly around the edges, always failed to take. Then I decided to try the sealing iron, and found it much easier to use. Here's the technique I've worked out.
The iron I have is from Global, with the unique name "Custom Sealing Iron," item number 360900. The hot area is about 5cm wide by 10cm long (including tip) with a maximum temp of 480* F. I turn it to near it's maximum setting (3, just before high), and after tacking a small part of the artwork down I place the flat surface down and press down fairly hard (a reasonable push from an arm held straight out), but not like I'm trying to bench-press it or something. Then I move the iron around in a circular orbit about 2cm in diameter at a rate of a little under twice per second. I count each swirl and hover over an area until I count to 100, and then move on to an adjacent spot. Above all else, keep the iron flat! You'll know if it's not, because you'll hear a scraping as you move due to the pressure on a line-like or pointlike area. If it's flat, it glides smoothly and makes gentle white noise.
After that, it's the usual; Into a bowl of hot water, let soak for 20ish minutes. Peel gently, but don't be terrified of pulling a bit. If it's JetPrint paper, the whole thing will easily come off in one piece. If it's Staples Basic, you've got a bit more work. After you get most of the paper off (you should still get almost all of it in 1 piece), gently rub the remaining blotches. At this point, finer traces (SOIC density or above) will have a layer of paper stuck over them, as will almost all pins. I found that my electric toothbrush is perfect for cleaning the board off for two reasons. First, it does a very good job of getting the crud off and exposing copper. Second, it will take off borderline toner and tell you where you need to use a sharpie for touchup. Oh yeah... touchup once, then go away and check again - you probably missed something.
Well, that's how I made it work for me. No warranty implied, YMMV ;)
Author contact is ejkeever AT nerdshack DOT com