I can’t believe it’s almost the end of May and the memory board is still not finished. LEO-1 has taken a bit of a back-burner position, it seems. This started in January when my wife and I moved to a new apartment. For several weeks before and after the move, I had precious little time for soldering, with packing and organizing taking priority. During the move I managed to hurt my back very badly by lifting my AKAI X-355 tape recorder incorrectly. The thing is built like a tank and weighs a ton. I was in terrible pain for about 3 weeks and couldn’t face any hobby stuff at all.
Somehow, I managed to not do any LEO-1 work during March either, having got out of the habit of even thinking about it. Then, towards the end of March I bought an old Conn organ from a charity shop and spent several weeks fixing it up. During April, I decided I needed a proper amplifier and speakers in the living room, and took it upon myself to build the amplifier myself. It came out beautifully and sounds great, but this project once again kept me away from the LEO-1.
I got back to it a couple of weeks ago, doing something that I should have done right at the start. I went through the schematics and labelled all the ICs with their numbers. The software I’m using, ExpressSCH, doesn’t do this in a very useful way when there are gates involved, because it numbers all the gates as separate ICs, which they are not. So I had to do it manually. When I finally finished, I found that I had 282 ICs in total, including the RAM, ROM and ZIF sockets. At the start, I had estimated there would be about 200. Believe it or not, I had actually gone ahead and ordered all the chips I thought I was going to need back last autumn, without actually doing a proper count. Obviously I had counted certain chips like the bus drivers and registers, but for AND, OR, NOT gates, etc., I had not bothered; I just bought a load of them cheap on eBay. This means that I now have a large quantity of surplus chips, in particular 74HC14. I had bought 50 of them for about 6 bucks and it turns out I only need three. I laughed out loud at that mistake and I still can’t really work out how it happened.
I also found I needed to change an important design decision, the decision to not use IC sockets for the numerous small chips. I had initially decided to not use sockets because they are so expensive and I need a couple of hundred of them. But while working on the memory board, I saw the glaring truth. If one of those 74HC32s for example were to fail, either due to an error on my part or for some other reason, I would simply not be able to replace it without desoldering all the wires connected to it first, and soldering them back afterwards. What is the likelihood that a chip would fail? Well, probably not very high, but accidents do happen; it’s easy to drop a screwdriver and short something out. I decided it just wasn’t worth the risk, so I bit the bullet and ordered sockets for all the chips on the Control, ALU and Register boards. I don’t trust the really cheap sockets that use blades so I had to get machine tooled ones that have nice round holes with grips inside. I also decided to get the best quality I could afford so I got ones with gold-plated contacts. By buying Jameco ValuePro parts I managed to cut the cost in half over what Mouser wanted for the real brand-name parts. It sucks that in some cases a single socket can cost over a dollar when I was able to buy nine ICs for the same money, so I wanted to try and cut the cost without sacrificing quality too much.