Thread: Zx81
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Old 29th Apr 2020, 4:17 pm   #291
John Earland
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 442
Default Re: Zx81

[QUOTE=John Earland;1240064]
Originally Posted by SiriusHardware View Post
Well, let's go the middle way and give you a hand to do it yourself.

Here is a redrawn circuit for the ZX81.

If you have the circuit diagram, you don't need to literally follow the tracks from A to B to C because the circuit diagram tells you where they go to.

Look at the circuit diagram, find the ULA, which is IC1. Now find pin 1 (Marked 'A7'), which is on the lower edge of the drawing of the chip. Follow the black line leading away from it first down, then right, then up, to where it comes to a junction. At the junction, wires lead left and right. The left one goes to pin 1 on IC2, the 8K ROM. the right one goes to one end of resistor R21, a 1K resistor. There are no further wires going from this line to anywhere else. So now, you can check (with your meter) to see if IC1 (ULA) pin 1, IC2 (ROM) pin 1 and one end of R21 are all connected together because the circuit diagram says they should be.

Move on to the ULA pin 2, marked 'A8'. Follow the line on the circuit diagram leading away from pin 2 down, right, up and to another junction, this time going to IC2 (ROM) pin 23 and one end of R22, another 1K resistor. Check, with your meter, to see if IC1 (ULA) pin 2, IC2 (ROM) pin 23 and one end of R22 are all connected together.

And basically, repeat this process for the other 38 pins of the ULA.
I have now logged and tracked each ULA Pin in a spreadsheet, took me a little while to map it all but at least I have all 40 of the pins and their intersections and terminals! So I will now start to go through each one methodically to test connectivity. It may take me some time!
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