I started on my new ZX-81 replacement board (kit) ZX81+38

Discussions about Sinclair ZX80 and ZX81 Hardware
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mahjongg
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I started on my new ZX-81 replacement board (kit) ZX81+38

Post by mahjongg » Wed Sep 25, 2019 1:22 pm

For now I decided to name it ZX81+38. It will be a fully ZX-81 compatible alternative that fits inside the original case, or can be used with my own keyboard in a wooden or 3D printed enclosure. This time around I will only use easy to solder pin through hole components, no more SMT.

I modeled a PCB so that it had the same dimension, and mounting holes as the real ZX-81, and placed my replacement connectors at the correct locations, then added a Z80 and RAM and ROM chips, so they would fit, and did not obstruct mounting holes.

I made a schematic where I placed the PTH equivalents of the needed SMT logic of my ZX81+35 ULA replacement, and did a quick check to see if it would fit on this board, and I'm reasonably hopeful it will.

see my progress reports here: https://revspace.nl/ZX81plus38_simple_t ... X-81_clone
Last edited by mahjongg on Thu Oct 10, 2019 7:38 pm, edited 1 time in total.

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Daeve
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Re: I started on my new ZX-81 replacement board (kit)

Post by Daeve » Wed Sep 25, 2019 9:10 pm

Looking forward to reading your progress!

nollkolltroll
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Re: I started on my new ZX-81 replacement board (kit)

Post by nollkolltroll » Wed Sep 25, 2019 10:29 pm

Looking forward to the trip!
/Adam

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mahjongg
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Re: I started on my new ZX-81 replacement board (kit)

Post by mahjongg » Fri Sep 27, 2019 11:47 pm

Thanks, I will post regular updates here: :mrgreen:

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1024MAK
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Re: I started on my new ZX-81 replacement board (kit)

Post by 1024MAK » Sat Sep 28, 2019 5:09 pm

Instead of UV EPROM or OTP EPROM, you could use EEPROM chips like the Microchip AT28C64B (8k bytes) or the Microchip AT28C256 (32K bytes) see https://uk.farnell.com/w/c/semiconducto ... -surcharge

In fact, if you have enough space, it should be possible to include options links/jumper shunts so that any of the above can be used.

If you do run low on PCB space, it is possible to use the area of the case where the heatsink of the 7805 regulator would have been by using a daughter board.

Would these unbuffered inverters do Texas Instruments SN74HCU04NE4?

I have bought some 6.5MHz crystals not long ago (may have been from eBay though), so I’m surprised you could not find any.

Mark

McKlaud
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Re: I started on my new ZX-81 replacement board (kit)

Post by McKlaud » Sat Sep 28, 2019 6:17 pm

6.5MHz crystals are available at Mouser and other good electronic retailers. Also 6.5MHz ceramic resonators are still in manufacturing and available. However I would go with a 13MHz unit or LTC1799 resonators.
Claudius
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mahjongg
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Re: I started on my new ZX-81 replacement board (kit)

Post by mahjongg » Mon Sep 30, 2019 1:49 am

1024MAK wrote:
Sat Sep 28, 2019 5:09 pm
Instead of UV EPROM or OTP EPROM, you could use EEPROM chips like the Microchip AT28C64B (8k bytes) or the Microchip AT28C256 (32K bytes) see https://uk.farnell.com/w/c/semiconducto ... -surcharge

In fact, if you have enough space, it should be possible to include options links/jumper shunts so that any of the above can be used.

If you do run low on PCB space, it is possible to use the area of the case where the heatsink of the 7805 regulator would have been by using a daughter board.
I will do my best to support a variety of OTP and other EPROMS, the space gained by not needing a voltage regulator doesn't probably end up near the EEPROM, but yes, not having it, nor the RF modulator wins me some space.
Would these unbuffered inverters do Texas Instruments SN74HCU04NE4?

I have bought some 6.5MHz crystals not long ago (may have been from eBay though), so I’m surprised you could not find any.
Yes, the SN74HCU04NE4 from RS is exactly the chip i'm planning to use, although I think regular (buffered) HC chips, should also work, and have a better chance for the long time to be available , the same goes for 13MHz instead of 6.5MHz PTH crystals, they simply are easier to source.
I try, if possible to always source from ONE major supplier (Farnell).

but thanks for the support.
Last edited by mahjongg on Wed Oct 30, 2019 3:21 am, edited 1 time in total.

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mahjongg
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Re: I started on my new ZX-81 replacement board (kit)

Post by mahjongg » Mon Oct 07, 2019 2:06 pm

The schematics is almost finished, I only want to add some jumper options so you can use other kinds of ROM's, specifically the original ROM of a ZX81:
Image

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mahjongg
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Re: I started on my new ZX-81 replacement board (kit) ZX81+38

Post by mahjongg » Thu Oct 17, 2019 2:08 am

The schematic is now function complete, ready to start component placement:

Image
added jumpering for alternative ROM's, and support to emulate the canonical on-screen loading bars.
also added some texts to explain parts of the schematic, and changed some gate symbols where low level logic was used, to make the schematic more readable.
open the picture in another screen to see it full size.

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mahjongg
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Re: I started on my new ZX-81 replacement board (kit) ZX81+38

Post by mahjongg » Wed Oct 30, 2019 3:19 am

did a preliminary placement of the components, here is a preview:

Image

its a (very) tight fit, there is hardly any space four routing, and I probably have to optimise the placement (component and port swapping) a lot to get the necessary traces to fit.

Ill, just start and see if I will succeed. If It doesn't fit I have no choice but to enlarge the PCB, but Ive done miracles before....

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