Lee Felsenstein at Homebrew Computer Club Reunion

Lee was the MC at the main part of the club meetings back in the day, and he reprised that role on the night of the HCC reunion. He was also the designer of the computer in that day that I most desired, the Sol 20 Computer. I loved that system--the look, the keyboard, its operation.

Image by cellanr

There were just two things you wanted to know about the Sol to make life happier: Build the fully expanded system right at the outset. Opening up the heart of the system to expand it later was a major PITA. The other? Use someone else's disk subsystem. Though with the information available today a Helios disk subsystem could probably be made to work.

I still have the sales brochures for the Sol 20. I pull them out every now and then to drool over them again. Part of it is nostalgia, but part of it is the great design itself. Actual Sol 20s sell for more than I can afford, but perhaps I'll build myself a look-alike system from sheet metal and walnut wood sometime, anyway, and print up a nice black name badge.

I still have an Osborne 1 computer. This one is one I got only relatively recently. It is pretty well maxed out on upgrades (disk upgrades, video upgrades, etc.) and is a pleasure to use. It's not as pretty as a Sol, but I enjoy showing it off in current day computer classes. The kids love it--especially the floppy disk drives and the tiny screen. But...they get hooked on Zork.

Lee Felsenstein Today

In our conversation last Monday, Lee showed me a project he's working on today as an educational tool. It's a programmable logic simulator, targeted at middle school students. What Lee showed me was a pair of printed circuit boards that have captive fasteners to clamp them together around a plastic matrix. The matrix holds surface mount diodes, which the students can place into the matrix to program it. In essence, it's a 16 by 8 programmable logic array that is programmed through physically locating the diodes.

OK, I know that sounds totally abstruse to many of you, so let me tell you what makes this a great idea, and why your middle schooler ought to know about this stuff even if you've gotten through life without having to so far (assuming you don't know already).

The core of computers are built out of logic circuits. The memories feed the logic circuits with data (in current designs--it doesn't have to be that way though it's presently the assumption), in essence, the programmable logic is the complement to the memory. This analogy of the logic and memories being complementary components of a computer holds on many levels. It's possible to build logic out of memories--I've done it--but it's not efficient.

Initial education in logic circuits can be accomplished with a simple breadboard and some logic chips. A few AND chips, OR chips, NAND chips, inverters, and so on. Add some resistors and LEDs and the kids are off and running. For a little while. Once they master this, and understand what's going on, they immediately start expanding their ideas.

Then a problem hits. More chips and more wiring between them mean more complexity, and more difficulty in realizing their ideas.

At this point, it's possible to introduce them to programmable logic devices. Teach them that the logic functions they had in the ICs live inside the PLDs, and that they can program the devices rather than run wires. The problem is that this is a big, big jump up in abstraction level, especially for a kid in the middle school age bracket (which is the perfect age to introduce this stuff, which I'll go into later.)

Whereas Lee's invention maintains a physical element. The programming is accomplished by manually placing diodes into a matrix, rather than typing characters on a screen then punching the 'program' button to dump it to a Flash PLD. This keeps it from getting too abstract, encourages experimentation, and maintains the hand-on element that's necessary for students in the 9-13 years age range.

Building Blocks of Electronics

Electronic logic is building blocks. Your kids play with building blocks, right? They start with simple structures to learn how to build more complex structures. Before long, they can use every single piece they've got building large, complex structures. Once the individual blocks and a few simple ways of interconnecting them are understood, they can take off and make great big projects that reach to the ceiling.

It's the same with electronic logic. It's a collection of simple building blocks. The problem is, the complexity of assembly is a little greater. Enough that once you get past a certain level (I'd say 20-30 ICs), it gets progressively more difficult to implement your ideas. The ideas out-race the ability to construct.

This shouldn't be an obstacle. The ideas should be allowed to continue to grow, without removing the physical aspects that make the activity interesting.

The Lee Felsenstein Magic

Lee has hit a sweet spot here. With all the excitement about the Raspberry Pi (which I will save my criticisms of as an educational tool for a future article), Lee's project should have that sort of excitement going for it. This is about students building their own processor. This knowledge is important. This is what the people who caused the microprocessor revolution used to cause the revolution in our lives. This is the knowledge that put a CPU in your telephone, your oven, and your iron. This is what tunes your radio.

Assembling a processor from random logic is a huge project. Yes, people still do that (I've even build a very, very simple one from racks of relays, myself, under cover of testing those relay racks and their support wiring after installation.) Building your own processor with a PLD is a lot easier, once you understand the building blocks.

Lee explains himself well on his project page. Have a look. I will be following the progress of the project.

And I'm really glad I got a chance to meet up with Lee again after all these years. He was one of my mentors and inspirations in my youth, just as he describes those who mentored him. It seems to be a common thread that those of us getting older want to assist the younger generation just as we were assisted when getting started in technical pursuits (as hobbies--the jobs came later.)

And if you're raising a kid--don't just foist off software on them as something to play and "learn" with. Software isn't reality. I've designed any number of computers on paper and in software, and then go on to build far fewer of them. Because software and paper aren't the real thing. The real thing has all sorts of little niggles and oddities that you'll never learn about in any way other than doing the real thing. Teach your kids to solder, use solderless breadboards, and use real components at all levels of complexity. Don't try to do too much at once, start with kits then move your way toward recreating circuits on breadboards then to soldering them on prototyping boards.

But do the real thing. Right alongside your other crafts projects. Because electronics is just as much a craft with some useful products as is crochet or embroidery (both of which I do) or quilt-making or sewing (which some of those close to me do). And most of all, have fun!