Peeking inside the Altair 680

Let’s have a look inside the Altair 680 that I acquired at the hamfest a couple years ago.

Undoing four screws at the back of the unit releases the back (surprise!) and top cover to reveal the inside.

Inside the Altair 680 showing the power supply at the bottom, and circuit boards
Inside the Altair 680

The back plate has the power supply consisting of two chunky transformers, a fan, and DB25 connector. There’s also an empty spot for another DB25 connector. Thanks to the two transformers, the rear panel is fairly hefty. The DB25 connector looks like it only has 4 wires. Not sure what would be connected to it, but probably something serial terminal related. There’s not a lot of clearance between the transformers and the SRAM chips on the main board when the rear panel is in place.

Removing the expansion board (I’ll get to that in a bit) reveals the main board.

The Motorola MC6800 CPU that powers the 680 is up toward the expansion board connector. Toward the bottom rear of the main board are 8 1kbit Intel P2102 SRAM chips that provides the 680 with 1 kB of RAM. Above the RAM is an EPROM (looks like an AM 1702A EPROM). The empty sockets seem like they would provide space for 3 more EPROMs. The other notable chip on the main board is the MC6850 asynchronous communications interface adapter.

Over in the front corner of the board by the big 3.3 mF filtering capacitor, the silk screen says this 680 main board is Rev 1-6.

680 Main board Rev 1-6
680 Main board Rev 1-6

The only expansion board in this 680 appears to be a RAM expansion board. The riser card has room for 3 slots, but the two other spots are unpopulated.

Altair 680 expansion board with one connector and two unpopulated connector spaces.
Altair 680 expansion board connector

The RAM board contains an 8×4 bank of Semi 4200UCP chips. A Google search didn’t yield a whole lot of information about them, but they appear to be 4kbit SRAM chips, so this board provides the Altair with a whopping 64 kB of additional memory to play with.

Photo of the Altair 680 memory board.  A 8x4 bank of 4 kbit static RAM chips provides 64k of RAM.
RAM board

When I plugged it in and turned it on, the fan spun up and some lights came on!

Altair 680 front panel showing some data LEDs turned on.
Altair 680 front panel showing some data LEDs turned on.

No smoke released, but the data lights turning on even though most of the switches were in the down position indicates something’s not quite right. Changing the switch positions didn’t affect anything either. The Run light on even though the switch is in the HLT position also suggests something isn’t quite right. Toggling the HLT/RUN switch made the HLT LED blink on once. Toggling the RESET switch seem to affect anything. The address line switches all seem to work, but LEDs started blinking when A13 was switched on.

Looks like this unit might need some work to restore it to a functioning state. I’ll also need to do some more research to learn about how the 680 works.

Back into the closet it goes for now.

Motorola MC6809 trainer

One of the items donated to the radio club by an SK estate last year (now I’ve forgotten which one) was this microprocessor training laboratory (MTL-1) from Cleveland Institute of Electronics (CIE).

Presumably, if you had an interest in learning computer hardware and programming, you’d enroll in a class and get one of these along with class materials.  I’ve seen similar trainers in the past, but never had the opportunity to use them.  The breadboard area lets you wire up circuits that let you do things with the 6809.

This one is based on a Motorola MC6809, a contemporary of the 6502 and Z80 microprocessors.  After watching the one YouTube video I’ve found along with a bit of random button pressing, I’ve managed to figure out how to to go to different addresses and enter assembly code.  I haven’t entered anything that runs though.  I’ll have to find some time to learn 6809 assembly first.

Aside from the copyright date on the back, there’s no other indication of when this particular unit dates to.  Fun fact: 1984 was my first year of high school,

Surprisingly, I’ve found very little documentation about this particular trainer online.  I’ve found one video on YouTube where someone used it to demonstrate testing a RAM chip, and a few photos of similar units on auction sites, but nothing in the way of manuals or even course materials that might have used the trainer.

This seems like it would be a fun thing to play with, especially for anyone with a retro-computing fetish. The hunt for documentation continues. I’ve already found a few PDFs about programming the 6809, so maybe I’ll be able to figure out enough do fun things with this.

Blinking LEDs with a Z80

In the parts bins that I acquired long ago, four of the many ICs in one of the bins were Z80 microprocessors. At the time, I thought it would be neat to try to do something with them, but had no time and no levels in low level hardware design.

Fast forward to post-PhD time and inspired by Ben Eater‘s 8-bit CPU and 6502 projects and videos, I learned enough about microprocessors and what connections to make with them to try doing something with the Z80. YouTube also presented me with a variety of Z80 related videos to watch. This one by Julian Ilett was one I found particularly informative.

Flashing LEDs – Z80 Style by Julian Ilett

The date codes on the Z80 chips indicates they’re 1987-1988 era chips, and not knowing how they were stored before I got them, I had no idea if any of them even worked. Grabbed one of the Z80 chips, plunked it into a breadboard and started wiring it up.

Off in the corner of the breadboard is a 555 set up as an astable oscillator and functions as the clock for the Z80. The clock goes runs between 6 – 475 Hz depending on the potentiometer setting.

After getting things wired up and applying power, some LEDs would light up, but then all the address bus LEDs would blink on and off but not in the expected binary counting pattern. Seemed like I had everything wired up correctly.

After studying Julian’s video and wiring some more, I tried triggering the reset pin and boom! LEDs blinked off, then came back on and the address bus LEDs started the expected binary counting pattern! Yay, it worked! Added a button for the reset pin to make resetting the Z80 easier.

Getting this going was fun, and a good learning experience. Gained a few XPs.

What next? Well, I could continue on and try to add enough to make a functional computer out of the Z80 and following the techniques in Ben’s 6502 video series. I’ll definitely need some more breadboards, and a few more components. I’ve got a few RAM chips I could try, but no idea of any of them actually work.

It’s kind of cool just turning it on and watching the LEDs blink though.

Making a breadboard based Z80 computer would be a fun project.

Atari 2600 controllers

Went through and checked out all of the controllers that came with the Atari 2600s. There were 5 Atari joysticks, 5 other joysticks and two sets of paddles.

The joysticks are pretty simple devices, consisting of a single PCB board with 5 metal dome button-type switches.

Atari joystick PCB

The joystick consists of a plastic piece with knobby bits that contact the domes. Pushing the joystick up, down, left, or right causes the knobby bit to press down on the switch, closing it and making a signal go down the corresponding wire.

One of the joysticks was broken. The other four joysticks work ok, but aren’t super responsive and the fire button on one of them doesn’t seem to be working.

The other non-Atari joysticks sort of work, but mostly don’t. Haven’t taken them apart yet to see if there’s anything that can be repaired.

The paddles are pretty simple devices, each consisting of a single 1 Mohm potentiometer and a push button.

Atari paddle controllers

After disassembling the potentiometers and cleaning them, they worked a little bit better than they did before, but there’s still a lot of jitter in the resistance measurement when the pots are turned, which translated to jittery movements in games. I’ll probably end up replacing the potentiometers if I can find any suitable ones.

Atari 2600 progress

After replacing the voltage regulators and several capacitors on the Atari 2600s, one of them is back in operation!

Pac-Man on the Atari 2600
Pac-Man on the Atari 2600

Out of the 18 cartridges I have, half of them worked (were playable) when I plugged them in. The others just gave me squiggly lines or just a black screen. Not sure if it’s an issue with the cartridges or the cartridge slot.

Still lots of work to do on the consoles. The other console only got a quick check with a couple cartridges, so I’m not sure if it’s working yet. The switches on both of them are a bit finicky so I’ll probably have to take them out for cleaning/refurb. I don’t think any of them will need to be replaced though. The cartridge slot seemed a bit touchy too, but looking into that will be a bit more work I think.

After that I’ll tackle the controllers. There are a few of them to work on. I’ve got 5 Atari and 5 third party joysticks plus a pair of paddles. The one joystick I tried mostly worked, although it wasn’t very responsive when pushing the stick in the down direction.