Easy to build Kempston interface expansion

[Lynxman]

I made this tonight. I followed a schematic from here and just wired it up on a prototyping board. It was fun and worked right away as Kempston in games.

I had to stack the connector because the height of the chips prevented it from going into the Spectrum. DOH!

Guess I should 3D print a case for it now.

[1024MAK]

Nice work

The circuit used is a traditional minimal address decoding design. Which is fine if you are not trying to use any other complex interfaces with it at the same time.

If however you do what to use complex interfaces, there are some other designs. Here is one from me: https://flic.kr/p/UAgQEi

You will find other schematics over in the hardware section of World of Spectrum forums.

Mark

[Joefish]

It's not too important for Kempston, but that's a nice general design for decoding if you can get the right chip. You could even add some DIP switches and make it respond to any address you like!

Official Kempston interfaces check A7, A6, A5 are low, not just A5. But they don't check the other bits are high.
Clone Kempston interfaces (that only check A5 is low) became so prevalent that nearly all other devices built since are designed to avoid clashing.

[1024MAK]

It's not too important for Kempston, but that's a nice general design for decoding if you can get the right chip. You could even add some DIP switches and make it respond to any address you like!

Thanks

Official Kempston interfaces check A7, A6, A5 are low, not just A5. But they don't check the other bits are high.
Clone Kempston interfaces (that only check A5 is low) became so prevalent that nearly all other devices built since are designed to avoid clashing.

The early joystick interfaces made by Kempston also only used address line A5. Later joystick interfaces made by Kempston used A5, A6 and A7.

There are many, many designs of Kempston compatible joystick interfaces, using various chips.

Mark

[Joefish]

I seem to remember seeing some upright ones with the Kempston brand. They also used narrow edge connectors for a time. Were they originally designed for the ZX81?

All the flat-case 'Kempston' branded ones I've looked in decode on A7 / A6 / A5. These are the ones I changed to do A7 / A6 / A3 to respond to IN 55 (to play Bomb Munchies). If you just swap a single-bit decoder type from A5 to A3 it works as IN 55, but clashes with a DivIDE.

My old DK'Tronics decodes A5 (Kempston), A0 (Keyboard polling), and A12 (Keys 6..0). Interesting because it's fairly easy to move the A12 line to mimic another row of keys. Although if you shift it to keys on the left-side of the keyboard you quickly realise they're in reverse order for Sinclair #2 or early Ultimate games!

[1024MAK]

It so happens that Kempston Joystick interfaces don’t need the signals that are present on the extra pins of the ZX Spectrum edge-connector. So why buy ZX Spectrum sized edge-connectors when cheaper ZX81 sized edge-connectors can be used.

Anyhoo, here’s some photos...









As you can see, it only uses 12 connections: +5V, 0V, the 8 bits of the data bus, one control signal and address line A5.

Note the text on the PCB. This one appears to be version 3.1 in 1983. Long after the heyday of the ZX81.

So taking it all together, I think it is safe to say, they were aimed at ZX Spectrum owners. And Kempston only started including better address decoding long after selling shed loads of their minimal address decoded versions.

Mark

[Ast A. Moore]

Wow. A gold-plated edge connector? Fancy.

[1024MAK]

Wow. A gold-plated edge connector? Fancy.

What, no mention of the gold plated D-connector pins

Mark

[Ast A. Moore]

Wow. A gold-plated edge connector? Fancy.

What, no mention of the gold plated D-connector pins

Sorry, my bad. Totally missed that. Didn’t stand out as much, what with the off-white plastic and oxidation.

[1024MAK]

Ah Ha! Found the thread I was looking for

Have a look at this thread

Mark

[Lynxman]

Awesome. I'm trying to learn more about electronics. I can put together just about anything I have a schematic over but don't understand much more than the basics. Maybe I'll try to design an SMD version.

[Lynxman]

I made a case for it: