Category: Electronics

Explorations in things electronic

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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.

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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.

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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.

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Atari 2600 refurb plan

After going through a few forum posts, I think I’m going to start with replacing some of the capacitors and the 7805 5V voltage regulator on one of the Atari 2600s (the older one).

Physically the caps look fine but after who knows how long in storage, they probably need replacing anyway. There are three electrolytic capacitors, five mylar film (green chiclets) capacitors, two polystyrene capacitors and one ceramic disk capacitor I’m planning to replace.

I’ve replaced the video cord with an RCA-F adapter, so now the video signal will go through regular RG-6 coax right to the TV’s coax/cable input.

The power connector is a 3.5 mm tip-ring jack, which I’d like to replace as well with a more standard 2.1 mm barrel jack, but finding one with the right footprint is proving a bit difficult. That might be a second slightly more involved project.

Time to go shopping in my parts bins. I think I should have most of what I need already on hand.

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Project: Atari 2600 Repair/Restoration

On the bench are two Atari 2600 video game systems that were given to me by one of the members of the local Slack community I hang out in. They’d been sitting in storage for the past few decades and didn’t seem to be working so he asked if anyone wanted them before they went out to the dumpster. I, of course, said I’d take them.

They’re of two different vintages based on the serial numbers, and came with a number of controllers and cartridges. For having been in storage, they looked in decent shape, although the older one was obviously sitting out in the open more than the other newer one.

The older Atari 2600 system. SN 82227274
The older Atari 2600 system. SN 82227274

The 2600 isn’t too difficult to disassemble, and there’s already a pretty good Atari 2600 teardown on the iFixit site. This particular unit was made for Atari by Dimerco Electronic Corp in Taiwan and has serial number 82227274 according to the sticker on the back. I was able to brush off a lot of the dirt on this one, but it could still use a good cleaning.

Inside looked relatively clean with some tarnish and corrosion on the RF shield. No obvious issues with any of the components. From the silkscreen, this is a Rev 14 board.

Atari 2600 main board
Atari 2600 main board

Underneath the RF shield are the brains of the system: the MOS 6532 RAM-I/O-Timer (RIOT) chip just below the cartridge connector, the MOS 6507 CPU, and the Atari television interface adapter (TIA) chip at the bottom.

Underneath the RF shield; MOS 6532 RIOT chip (top), MOS 6507 CPU (middle), TIA chip (bottom)
Underneath the RF shield; MOS 6532 RIOT chip (top), MOS 6507 CPU (middle), TIA chip (bottom)

The RCA jack to the right is where the video signal goes out to the TV.

Initial testing of the two systems didn’t show anything on the screen. I ordered some RCA-F connector adapters (no idea these even existed until I started looking for them) to replace the RF modulator, and was able to get some images on the screen with one of the cartridges inserted. There was no response to any of the switches or controllers though, so more troubleshooting is going to be required.

Atari game on the television
It lives…sort of

I’ll need to do some more research on repairing and refurbishing these. I’ve found some potentially useful threads on the AtariAge Atari 2600 forum, so I’ll dig around there some more.