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mahjongg
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My new ZX81 clone, ZX81+38 built from discrete pin-through hole components (gerbers also released on Github)

Wed Oct 16, 2019 12:49 pm

I finalized the schematic of my ZX81 replacement kit (clone) that (unlike my previous effort) uses only off the shelf pin-through-hole components, and will fit inside a real ZX-81 case, or can be built up as a "new" ZX-81 to be put in an enclosure of your choice.

Image

This version should be easy to build as a kit, and supports modern PAL/NTSC TV's, has easy cassette loading from a laptop sound port, and has 16/32K RAM and a joystick port.

Its ULA replacement logic is nearly perfect, and now also supports the iconic on-screen loading stripes .
zx81loading.png
zx81loading.png (29.65 KiB) Viewed 13234 times
It will work with my keyboard replacement PCB (designed for my earlier ZX81+35) which is slightly larger, but still fits on a ZX81 case, and uses real buttons, not membrane keypads, and my own keyboard overlay.

Here is the schematic (open in new window for full size):

Image

more information can be found here: https://revspace.nl/ZX81plus38_simple_t ... X-81_clone

When finished my ZX81 clone kit will be published on GitHub

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mahjongg
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Re:My new ZX81 clone, built from discrete pin-through hole components

Wed Oct 30, 2019 12:56 pm

Finished the preliminary component placement. As you can see it will be a very tight fit, not much room to add interconnects at all.

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mahjongg
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Re:My new ZX81 clone, built from discrete pin-through hole components

Sat Nov 02, 2019 8:18 pm

Made some calculations, if I use 60 mil pads (with 100 mil pitch, and 35 mil holes) I should be able to draw TWO 10 mil wide traces, with 10mil wide separations, between the pads, while keeping to the minimum PCB manufacturers specifications (typically minimum width of 0.15mm).

I will need such, and many more, tricks to pull this off.
Another trick is to use pads for DIP IC’s that are rounded rectangles instead of circles.

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Re:My new ZX81 clone, built from discrete pin-through hole components

Sat Nov 02, 2019 11:03 pm

mahjongg wrote:
Sat Nov 02, 2019 8:18 pm
Made some calculations, if I use 60 mil pads (with 100 mil pitch, and 35 mil holes) I should be able to draw TWO 10 mil wide traces, with 10mil wide separations, between the pads,
Er, two tracks between two pads requires three separations. 60 (pads) + 20 (tracks) + 30 (separations) = 110 >100.
Ouch. :(
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mahjongg
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Re:My new ZX81 clone, built from discrete pin-through hole components

Sun Nov 03, 2019 2:17 am

Well spotted, but no I did realize there would be three, not two, separations needed, but this is what you get when you tell a story without reading your notes, I meant 8 mil, or 0,2032mm, not 10mil. :? Oops.
Sorry for misremembering.

Still it will work fine with 0.2mm (8mil) traces, and separations, 0.2 is still more than 0.15 mm. :D

:geek:

P.S. nice to see someone is paying attention. :)

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Re:My new ZX81 clone, built from discrete pin-through hole components

Sun Nov 03, 2019 5:59 am

Heck 58mil pads with 6mil traces would give you three traces between pads and would probably be fine on a modern PCB process, though I wouldn't do it unless I really had to.

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Re:My new ZX81 clone, built from discrete pin-through hole components

Sun Nov 03, 2019 3:40 pm

plugwash wrote:
Sun Nov 03, 2019 5:59 am
Heck 58mil pads with 6mil traces would give you three traces between pads and would probably be fine on a modern PCB process, though I wouldn't do it unless I really had to.
Yeah, but that is really stretching it.
II have never seen a (dual layer) PCB with 3 traces between Pads, and even with just two traces pads tend to become uncomfortabliy small if you just use round ones.

In a multi layer (say 4 layer) board the pads in the inner layers have different rules, and can be much smaller, its much easier to route three traces between pads on inner layers.

Most PCB manufacturers have strict rules for the minimum amount of metal (“flesh”) in the ring of a pad, and an 58 mil pad really doesn't have enough “flesh” if you keep the hole 35 mil. And you can’t make the hole much smaller than 35 mil, or leads and pins won’t fit, not to mention if you want to de solder components soldered into such small pads it becomes very hard to do, without damaging the pads.

Its just that looking at the current rats-nest I can see many instances where having just one wire between two pads simply isn't enough, and I don't expect I can update the routing enough to solve that, So I am forced to use this solution.

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Re:My new ZX81 clone, built from discrete pin-through hole components

Mon Nov 04, 2019 3:42 pm

35mil hole in 58 mil pad would give 11.5mil on either side of the hole. I'd expect any decent PCB manufacturer to be able to hit that.

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Re: new ZX81 clone, built from discrete pin-through hole components

Mon Nov 04, 2019 6:33 pm

I'm planning to use 60 mil high, and probably 70 or 80 mil wide rounded rectangular pads, with 35 mil holes (after plating), with one 0.4mm or two 0.2mm wide traces between the pads, and yes, any PCB manufacturer should be able to manufacture such a PCB cheaply.

And such pads should still be reasonably easy to solder (and de-solder) for an amateur, with DIP sockets and leaded components.

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Re:My new ZX81 clone, built from discrete pin-through hole components

Thu Nov 07, 2019 8:51 pm

I have designed new DIP footprint pads with reduced vertical size, so you can route two 0.2mm traces in-between two such pads.
The next picture shows the current connections that need to be routed, (rats-nest) and the new DIP footprints.

Image

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mahjongg
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Re:My new ZX81 clone, built from discrete pin-through hole components

Wed Nov 20, 2019 4:07 pm

work in progress, started on power and ground wiring:
Image

gordon77
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Re:My new ZX81 clone, built from discrete pin-through hole components

Wed Nov 20, 2019 4:25 pm

Great work!

Is it just the image or have those pads got bigger?

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Re:My new ZX81 clone, built from discrete pin-through hole components

Thu Nov 21, 2019 9:50 am

Its just the image, the images pixel resolution and size is slightly different. open each picture in a new screen for better comparison.

The pads have not changed, although non 14-pin DIPs now also have their silk screen outline outside of the pads, but the pads themselves have not changed.

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Dec 04, 2019 5:39 pm

Progress:, one week after the last preview:

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Dec 04, 2019 6:29 pm

Looking good :)
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Re: My new ZX81 clone, built from discrete pin-through hole components

Thu Dec 05, 2019 1:15 pm

Yeah, looking good
Tony

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Re: My new ZX81 clone, built from discrete pin-through hole components

Tue Dec 10, 2019 10:49 pm

Updated above picture, now it has contact fingers.

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Dec 11, 2019 8:28 am

Looking good. I am impressed by your patience :D

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Jan 15, 2020 10:30 am

Progress is slow, but I'm on it:

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Jan 29, 2020 1:51 pm

A small update (every two weeks or so):
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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Jan 29, 2020 3:35 pm

Getting there, looking good.
How many weeks to go? ;)

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Re: My new ZX81 clone, built from discrete pin-through hole components

Wed Jan 29, 2020 6:23 pm

No idea, its finished when it is finished. I'm only working on it every Tuesday evening (and night) for a couple of hours, so its slow going.

I'm worried it will be impossible to route the expansion connector, Ill give it my best try, but maybe it is impossible, and I will have to accept defeat. But for now I'm still optimistic. A difficult layout takes time, an impossible layout takes a bit longer. :mrgreen:

I'm now in a phase that I have to do extensive layout changes to add yet another trace. You can easily see that if you look at the traces underneath the Z80, (U2) many already laid traces have changed from top to bottom and vice versa just to optimize the layout.
As you can see vertical traces are routed mainly on the solder side (red) and horizontal lines on the top side (in green).

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Re: My new ZX81 clone, built from discrete pin-through hole components

Thu Jan 30, 2020 12:44 am

I'm sure it's possible, it's just a case of what you have to set the design rules to to make it work.

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Re: My new ZX81 clone, built from discrete pin-through hole components

Thu Jan 30, 2020 10:02 am

plugwash wrote:
Thu Jan 30, 2020 12:44 am
I'm sure it's possible, it's just a case of what you have to set the design rules to to make it work.
I don't want to change the design rules, the current rules are such that the PCB falls in the cheapest category, and can be made by any PCB manufacturor (0.15mm).
Obviously it could also be solved by adding layers (going to a 4 layer design), but I want to keep the PCB as low cost as possible, and a bit authentic, meaning in about the same class, or as close to it as possible, as the original ZX81.

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Re: My new ZX81 clone, built from discrete pin-through hole components

Thu Jan 30, 2020 10:08 am

Maybe some 0Ohm resistors to jump over tracks, or even wire links?

It looks like a fun problem.
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