ZX81 schematic how does it compare?

[olley]

Having a sort out and came across this leaflet for the ZX81 built by my brother many years ago, how's it compare to a modern computer, fewer wires and ic's?

 

[Lenny HB]

It would all be on one small chip now.

 

[jwells]

I have a zx81 in its original box!

 

[Gromett]

Raspberry pi schematics are available.

https://datasheets.raspberrypi.com/rpi3/raspberry-pi-3-b-plus-reduced-schematics.pdf

 

[68c]

Mine has stopped working now, or to be more exact, the modulator that converts the signal so you can see it on a normal TV.
Well remember the excitement of seeing my text displayed. Bought the extra RAM then spent hours typing in programs from magazine.
May dig it out again.

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[meanders]

Any external connections as shown in the first diagram will be on a multi-core data or power bus. Indeed looking again, even at that early stage the it looks as though the interconnects were effectively handled that way.

As Lenny HB states it would all be on (probably a small part of) a small chip albeit orders of magnitude faster and more powerful. Moores law says that processors double in size roughly every 18 months so it's been exponential growth for over 40 years!

 

[Widge]

If you want to relive the experience, but dont have a zx any more, there are loads of emulaters to run on pc's and android

 

[kevenh]

Having a sort out and came across this leaflet for the ZX81 built by my brother many years ago, how's it compare to a modern computer, fewer wires and ic's?

View attachment 718601View attachment 718603

Great. Thank for sharing.

It has all the basics blocks for showing a Computer model for educational purposes (not surprising tho ). I’m intrigued by the Sinclair Computer Logic chip. I’ll have to see what that was adding. Edit: proprietary I/O management - maybe. HO
& I wonder why the edge connector didn’t use sequential numbers for the address and data busses

Edit2: some of my Q are A here: https://retroisle.com/sinclair/zx81/techdetail.php
The tape input is managed in the SCL and as that is also the video gfx chip, that’s why we had the TV showing we were using the tape drive

 

[Gromett]

I’m intrigued by the Sinclair Computer Logic chip. I’ll have to see what that was adding.

L Break Into Program - ZX Spectrum ULA

The Spectrum ULA was a custom gate array chip that handled the screen, beeper and paging in alternate ROM chips, such as the Interface 1. ULA Pin Assignments A0-A6, A14-A15: Address lines D0-D7: Data lines U, V, Y: Video signals T0-T4: Keyboard data lines SOUND: I/O for the beeper Q: 14Mhz clock...

www.breakintoprogram.co.uk

 

[Gromett]

I wonder why the edge connector didn’t use sequential numbers for the address and data busses

That was likely for no other reason than ease of routing. Back then PCB's were designed and layed out by hand. Double sided boards were rare and even harder to design.
Ease of routing would have been a higher priority than keeping the edge connector in numerical order.

I was designing PCB's by hand in the early 80's and even single sided ones that were more than just a basic device were tough to get right first time.

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[chaser]

I have a zx81 in its original box!

Ditto.

 

[Lenny HB]

I was designing PCB's by hand in the early 80's and even single sided ones that were more than just a basic device were tough to get right first time.

That was fun I used to do the layouts for my mixing desks PCB's 2 foot long by 2 inches wide.

 

[kevenh]

That was likely for no other reason than ease of routing. Back then PCB's were designed and layed out by hand. Double sided boards were rare and even harder to design.
Ease of routing would have been a higher priority than keeping the edge connector in numerical order.

I was designing PCB's by hand in the early 80's and even single sided ones that were more than just a basic device were tough to get right first time.

Sorry, memory lane tangent.
I’ve only built small single sided PCB and just when on the 1st year of an electronics apprenticeship. Well, I don’t remember using vias to link two sides so think they were only single sided. :lol
But I did testing, fault finding & modifying on 14 layers circuit boards and multi-wire circuit boards. The latter were very expensive but products only usually needed one for the analogue signals I/O.
Towards the end of that career, the products were cloud based

 

[Widge]

That was likely for no other reason than ease of routing. Back then PCB's were designed and layed out by hand. Double sided boards were rare and even harder to design.
Ease of routing would have been a higher priority than keeping the edge connector in numerical order.

I was designing PCB's by hand in the early 80's and even single sided ones that were more than just a basic device were tough to get right first time.

I think it was double sided board, but even then it was probably down to keeping the layout as simple as possible wiithout having long winding tracks, or more likely more plated through holes which puts the cost up.
I did loads of layouts myself back then but analogue not digital, less complex in some ways with less tracks, but as we were dealing with higher currents, and some RF, that caused other problems in layout design.

 

[Gromett]

But I did testing, fault finding & modifying on 14 layers circuit boards and multi-wire circuit boards.

I bet those weren't designed by hand

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[Gromett]

I think it was double sided board

It was a double sided board. I checked before making my post

 

[stevec]

Red and blue tapes used for double sided at four times the size to be photo reduced at the pcb house. I remember it well. Boards were priced by the number of vias so it was crucial to design well!

 

[15isto2]

That was my first computer.

 

[Lenny HB]

Four times size sound like pure luxury compared to the normal 2 times.

 

[Gromett]

Four times size sound like pure luxury compared to the normal 2 times.

My first boards were drawn on by hand direct to the copper. For context I was 10 or 11 years old and I was a radio amateur who was interested in packet radio. I didn't get my license until I was 14 so built stuff for a friend of the family.
It wasn't until I went to secondary school I got access to acetate sheet and photo resist PCBs. But even then we used this horrible tape or markers directly onto the acetate. I seem to recall letraset was used as well at one point.
Not sure it saved any time to be honest. But we were learning so not an issue.

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