Membership Card

by Chuck October 27, 2010

One of the things that I like to play with are retro-computers – old hardware from back in the dark ages of computing, hardware that a 17-year-old could understand, and that a 48-year-old can still wrap his head around.

One of the most famous of the retro computers is the Popular Electronics COSMAC ELF. I, and many others like me, got our start with computers flipping switches and pressing buttons on a home-built ELF. Many, including me, still have their old ELF sitting in a box on the shelf, and have a soft spot in their hearts for the red Q LED and the 8 toggle switches on the input panel.

Back in 2004 SpareTime Gizmos created the ELF 2000, a re-interpretation of the ELF using mostly modern components. The ELF2K is an ELF full-gallon – 32 K RAM, 32 K ROM with monitor and multiple high-level programming languages, support for terminals, Compact Flash – basically everything that you could want in a small computer.

But there is another style of ELF computer out there. These are not ELFs brought into the 21st century, and they are not faithful reproductions of the 1977 COSMAC ELF. Lee Hart’s Membership Card packs the essence of the COSMAC ELF into an Altoid’s tin. Essentially, it’s a microcomputer trainer that you can put in your shirt pocket and take with you everywhere you go.

My Membership Card computer tucked into its candy tin.It’s such a cool idea that I had to have one. So when Lee announced that he was selling kits of the Rev. B version of the Membership Card I jumped on the opportunity and bought one. Lee sent the kit complete with an Altoids tin – with Altoid powder still inside (a nice touch, that).

I hesitated over building it – I’m not 100% confident in my building skills. My ELF2K has an intermittent fault that was probably caused by a soldering error. The last thing that I wanted was another ELF-type computer sitting in my den, not working. But I eventually took the plunge, and over a weekend and a couple of week nights I got the hardware put together and tucked, if not neatly, at least cozily into the candy tin.

Next began an of evenings of frustration for me. Once I had the Membership Card complete I built a power cable for it and plugged it in. I flipped in the simplest “Hello World” program that you can use on an ELF: 7B 30 00. Turn on the Q LED, then loop back and do it again forever. And of course, nothing happened.

Starting to sense a pattern here?

I spent a frustrating evening trying everything that I know to get that tiny little program to run. Nothing. I knew that the program was going in to the RAM, the LEDs were lighting up with every press of the INP switch, and the way the ELF works those LEDs wouldn’t light up if the data wasn’t being written. But I just couldn’t get it to come back out; setting the memory to read-only and pressing INP didn’t cycle through the program that I’d just loaded. Once, and I had no idea why, I put the Membership Card in RUN mode and the Q LED came on. But I couldn’t get it to work again.

I spent a pretty miserable day the next day. At one point I was ready to pack all my 1802-based computers and figuratively put them on the side of the road with a “Free” sign on them. I’ve got four of these things, and not a single one of them works. Not my proudest achievement.

But I went home and tried again. I got out the schematic. I read the documentation. And a light dawned. Switch 9 isn’t the one that you flip to reset the ELF, switch 10 is. So I toggled in the program again. I flipped switch 10 up, then I flipped switch 9 up. And the Q LED came on.

The sun burst through the clouds casting rainbows. Little blue birds flew in carrying banners that said “Joy!” or little hearts in their beaks. There was a fanfare of trumpets and the air was filled with sweet perfume.

Or not. But I was pretty happy.

So I left the Membership Card sitting on my desk, flashing its Q LED like a little happy firework. And I went to bed, happily plotting new and exciting ways to drive myself crazy with this little tin box.

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Electronics

Solder smoke

by Chuck September 29, 2010

Occasionally, when I’m buying electronics gizmos, I’ll pad the purchase with interesting little bits of hardware that the supplier has on sale. This summer, when I was buying switches for Katie’s educational display, All Electronics  had some 8x2 LCD displays on sale for $5.50 each. Of course I bought two.

With the two displays in hand I went through my back issues of Nuts and Volts magazine looking for a “Picaxe Primer” column that I remembered, where Ron Hackett built up an LCD driver based on the Picaxe 14M chip. Look in the June 2009 issue for the column, or you can visit his Web site and buy all the parts that you need to build a serial LCD backpack. That’s what I did.

OK, so I didn’t order all the parts. I forgot to buy the 3-pin right angle male header that you need for the speed jumper. That just gives me an excuse to create an order for some other place – and pick up a few new gizmos to play with.

Building the LCD board was a simple and enjoyable job. The only problem I had was with R8. The assembly instructions say “See text” but the don’t say that the text in question is at the bottom of the page describing the LCD board. I eventually dug up the old Nuts and Volts back issue that had the discussion of the LCD backpack to figure out what was going on. Turns out R8 is the current limiting resistor for the LED backlight. If you just leave it off, you don’t need to worry about sizing it…

Anyway. I built up the board, programmed a 14M with the demonstration program, and turned it on. Nothing. Damn. Then I remembered to adjust the contrast pot – a few turns and there was the display flashing merrily away. I downloaded the serial LCD controller, plugged the LCD module into the backpack and the backpack into my breadboard, wired an 08M to send data, and that worked on the first try.

Cool.

Finally, I ran down to Radio Shack (twice – always make sure that the package you grab out of the parts bin is actually the one that’s supposed to be in the parts bin) for some #2 nuts and bolts to attach the backpack to the LCD module.

The total cost for the two serial 8x2 LCD displays was about $40. At Digi-Key a single Matrix Orbital 8x2 serial LCD is $39.95. I’m sure that the Matrix Orbital display is faster and less “pic-y,” but I’m satisfied with the new ones that I built.

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Electronics

A working ELF 2K

by Chuck September 17, 2009

The new memory arrived in the mail right on time, and I could hardly wait to get one of the two new memory chips out and into the ELF2K to see if it would work with a new chip.

Of course, I had to wait, there are always things around the house that I need to do. But eventually I got a chance to head into the den and put the new RAM chip into the socket. Put it in, powered the ELF up, flipped the RUN switch up, and son of a gun the little computer worked on the first try.

Only I didn’t have a terminal hooked up yet. So I needed to download a terminal program (TerraTerm) and set it up (baud rate, port), hook up the serial cable (take the shield off the DB9 socket on the ELF, my serial-to-USB converter sucks), power everything up again – and it still worked.

I then spent a happy half an hour toggling programs into the ELF – starting with the one from page 66 of the March 1977 issue of Popular Electronics. That’s the one where when you flip the input switch once it turns the Q LED on, and when you flip the input switch again it turns the Q LED off. That’s the first program that I ran on my original ELF, and the one that I showed my Dad. I showed it to Katie – the nostalgia was thick in the air.

I’ll be playing with the ELF now from time to time, it’s a bit of nostalgia after all. I’m probably going to have to get the I/O expansion board so that I can hook it up to my Picaxe network, but that’s a project for next winter I think.

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Electronics

RAM test rig

by Chuck September 14, 2009

Picaxe-based RAM test rig For my birthday this year my wife got me a Spare Time Gizmos ELF 2000 kit, a re-interpretation of the original 1802-based computer that I (and many others) built from plans in Popular Electronics.

Just like the first one that I built, this one had a few problems when I first turned it on. Finding and fixing a bad connection in an IDC socket was fairly easy, and I found the cold solder joint on the switch panel after just a little more work. What was harder was the fact that some memory locations just didn't seem to be changing, and I couldn't figure out why.

So I built a test rig for the 32K static RAM on the ELF2K. I used a Picaxe 40X1 as the brains, two 74HC595 chips to latch the 16-bit address required, and a handful of LEDs to show me what's going on.

I used PortC to read and write data to the RAM chip. I had some difficulty getting the bi-directional data bus to work until I put dirs= statements in the code to explicitly change from output to input when writing then reading the test data.

Since PortC was in use, I couldn't use the hardware SPI port, so I used the simple bit-banged serial protocol from the manual.

Once I had the test rig up and running it started indicating memory errors throughout the RAM chip, but especially on the last page of memory, the page that the ELF2K uses for it's system data page, and where the OS on the ELF2K was indicating there was a problem.

I've ordered a couple more RAM chips that should be here tomorrow. Once I've got a known good RAM chip to install in the ELF2K I'll be one step closer to getting it to work.

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Electronics

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