From Steve’s site:

“Big Mess o’ Wires 1 is an original CPU design. It does not use any commercial CPU, but instead has a custom CPU constructed from dozens of simple logic chips. Around this foundation is built a full computer with support for a keyboard, sound, video, and external peripherals.

My original goals were:

• Build the CPU from scratch, primarily using basic 7400-series logic. No 6502, Z-80, etc.
• Keep the hardware complexity to a minimum. I’m not an electrical engineer.
• Be capable of running “real” programs, not a 4-bit CPU or toy machine.
• Provide a way to interface with a PC.
• Be fast enough to run interesting programs interactively”

Interestingly I also used the 74HC595 shift registers on my LED matrix project.

Steve’s project doesn’t use soldering but rather each wire connecting the chips is wound by hand around a special gold leg on the proto board. This is an “old school” technique which takes a lot longer than soldering but if you make a mistake it’s much easier to recover from.

Things like this always fascinate me when I contrast to software development in the “real world”. Here’s a guy that had a direction, knew what he wanted to achieve and set about doing it. It’s just one person so it took a long time but he got there.

If you wanted to get 2 people to build the same thing, working as a team, my guess is the likelihood of success goes drastically down. In fact on a project like this with the addition of that one extra person you’re probably going to fail the first time.

So why is that?

Contrast this with the process of industrial ship building or air-liner development. Those projects involve literally thousands of people, each doing a small job. In the case of the air force or Department of Defense you now have thousands of vendors. And it seems from the Discovery Channel that those projects generally finish on time.

Why is that?

The common thread is hard to pin down but to me it’s people who know what they’re doing.

The ship building scenario is a very long process but infinitely broken down into minute detail. Each component, each step is itself extremely well defined and well understood. So in spite of its size, the ship building process has

• a LOW number of unknowns
• very RIGID requirements
• and a LARGE number of people and moving parts.

Now in the case of how to build a CPU from scratch, starting out I’d say Steve knew very little. I would venture (and it’s no reflection on Steve) that almost the entire process was an unknown. So how was he successful?

In contrast to the ship example he had

• a HIGH degree of unknowns
• very FLUID requirements
• a LOW number of people

Of course things are never this clear cut which is why software development is so tricky. You need to constantly manage these three variables of Unknowns vs Constraints (requirements) vs Headcount and keep them in balance if you’re gonna be successful.

And so I draw my conclusion; Any time you have a HIGH degree of unknowns with RIGID requirements and a LARGE number of people you are destined for FAILURE.

The corollary is any time you have a LOW number of unknowns with FLUID requirements and a LOW number of people, you are destined for SUCCESS.

I know which team I’d like to be on.

A resistor limits current. In the water analogy, current is the speed of the flow of water. But I never knew what a pull up/pull down resistor was. Until a few weeks ago when I started building a set of buttons for a project I’m working on…

Essentially a button is a switch, when the switch is closed the current flows, when it’s open the current doesn’t flow. Now if your switch is open, without anything driving the circuit, the value on the line is going to float all over the place, sometimes high, sometimes low. Any electrical noise will cause a change in the circuit. This causes a problem as when reading a value off the circuit you might think the switch is closed when in fact it’s just floating high.

The circuit on the left uses a pull up resistor. If Vcc is +5Volts, that goes through the 10k resistor and has nowhere to go but to pin 1. So when the switch is open, pin 1 is showing +5Volts (but very low current due to the 10k). Pin 1 will be steady, it won’t float up or down since it’s got some power to it.

When you close the switch, the current flows to ground. It’s always going to go to ground if it can so Pin 1 will show 0Volts.

The pic on the right is the pull down resistor. Same thing but the 10k resistor is between the pin and ground, causing it to “pull down” whilst the switch is open. Once it’s closed the majority of the current flows through the pin vs going to ground hence the pin reads +5V.

It’s making sense to me and now that I’ve actually built some stuff from scratch, I’ve actually experienced this voltage float and fixed it with a pull down. This blog will help me remember it in a year.

P.S. The Image is from “Resistance 2″ on the PS3. Pardon the pun.

Last night I got an email from Shawn over at [http://www.arduinofun.com/]. Shawn’s built a cool robot using various bits and pieces, including the arduino with Curious Inventor as his supplier. The coolest thing about this is he’s built it with his 11 year old son Drew. This is great on so many levels and I can’t think of anything better than mentoring a child in this manner.

I know my kids, Sydney and Ethan got a big kick out of my LED matrix/Processing project (checkout the video here [LINK]).

My next project is very ambitious;I’m thinking a CNC-style plotting/drawing thing. It’s another one the kids’d identify with. But right now parts are fairly scarce.

 Times are hard and I wonder if Curious’d sponsor me with some other free stuff? A few Stepper motors and a couple of Darlington arrays etc. would go a long way to getting me started…