Page 9 of 24

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 4:23 pm
by gozzo
whoah!!!!😊😊😊😊😊😊😊

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 5:22 pm
by 1024MAK
I don't know, you wait ages for a ULA replacement to come along, then two turn up following one another! :lol:

Well done to both of you :D

Mark

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 6:34 pm
by McKlaud
Mark,

Andy is miles ahead of me. :mrgreen: :mrgreen: His ULA is working perfectly fine :geek: My version is nearly there.

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 6:43 pm
by Andy Rea
McKlaud wrote:
Sat Mar 10, 2018 6:34 pm
Mark,

Andy is miles ahead of me. :mrgreen: :mrgreen: His ULA is working perfectly fine :geek: My version is nearly there.
Not really miles ahead... I need to sort out tape input, i guess i have some experience because I already did some ula's before. The video out has some noise in it, which needs addressing.

Anyway what problems are you having ? Pretty sure myself or mark can help out.

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 6:52 pm
by McKlaud
It is look like that I should assemble the board again, because in this one I've got some synch issues. When I press the board in the socket hard I am getting the "K" prompt on the screen. There is some noise on the video out too. I've been thinking about different type of a NPN transistor there. At the moment I use BC847B one.

What about the tape input? Maybe a bias would help.

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 7:12 pm
by Andy Rea
Ah yes i have a motherboard like that, pretty sure a trace is broken somewhere but try as I might i can't find it so it only works when pressure is applied at just the right spot...

Yes bias is required I have provisioned on the PCB a spot for 2 resistors one to rail the other to ground currently they are unpopulated as i was more interested in getting other things working first. I also use npn transistor as emmiter follower bc817 gain is a non issue in emmiter follower mode but current handling can be because of the relatively low load ( 75ohm ) input of most composite inputs, I also have a 1k load on the board so that high impedance composite input monitors can be used without additional work ( a lesson from my altera based solution ) I have a 0.1uf cap right next to the transistor between collector (+5 ) and ground. I was thinking about either a small smd inductor immediately before the collector. I also put an extra 33pf between base and ground this helps a little.

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 9:20 pm
by superfo
Nice work for both, congratulation !

Now I have to dig out my board and continue working on it :)

viewtopic.php?f=6&t=445&p=3796&hilit=superfo#p3796

Re: ZX81 ULA-in-a-CPLD

Posted: Sat Mar 10, 2018 9:27 pm
by McKlaud
:mrgreen: Superfo: welcome to the club :mrgreen:

Andy:

The small smd inductor (e.g. 1uH) and than a 0.1uF capacitor between supply rail and GND might be a good idea. I will give it a try.

Re: ZX81 ULA-in-a-CPLD

Posted: Sun Mar 11, 2018 7:22 pm
by Andy Rea
Mine has had a bit of a torture test today, had to go out to see my mum of course left the house at 10.30am and also left a test zeddy with my ula installed running maxdemo... Got back at 6.15pm ( 7hours 45minutes later ) and......

It's still running ! :mrgreen:

I'm gonna pat myself on the back and call that stable...

Now then where did I leave the trumpet?

Andy

Re: ZX81 ULA-in-a-CPLD

Posted: Sun Mar 11, 2018 7:33 pm
by McKlaud
8-) Congratulation Andy!!! 8-)

My ULA was also tortured and I did a few minor mods earlier today. Now Zeddy boots up every time to the 'K' prompt. Screen and border inversion works as it should. Anyway, still works in progress.