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.



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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