Sinclair ZX80 / ZX81 / Z88 Forums

Thanks Mark, I have totally overseen this possibility to disable internal keyboard. The phase that you are looking for is “bus contention”. Both devices contain chips that drive the data bus high as well as low. So if one is driving high and the other is driving low... As the relevant chips for the ...

Thanks a lot Mark for pointing out the reverse current. I had a look into the datasheets of 1N5817 and BAT48. With 2V difference voltage and 25°C 1N5817 has 70µA BAT48 has 2µA reverse current. As the Lithium cell I am planning with is about 200mAh, 3% is 66,7mAh. This will not overload the cell with...

Next I found this in the German ZX81 forum: http://forum.tlienhard.com/phpBB3/viewtopic.php?f=2&t=479&start=25#p6226 Up to many other interesting things it contain a battery backup solution with under-voltage protection. This solution need another chip to be on battery power. This chip(74HCT139) kee...

As Id like to keep Vdrive driver and ROM images in RAM I need to preserve it's content while power is of. So I'd like to add a Lithium Battery to keep RAM in standby mode. First idea as shown below is known as not working, because all of the data, address and control lines will collapse to 0V when p...

In the Minstrel design the keyboard driver is also generating NOP for CPU while screen data is shifted out. So Z80 solution will not work here.

There is an external ZX-Key Keyboard existing. https://www.sellmyretro.com/offer/details/zx-_key%3A-the-zx81-mechanical-keyboard-and-interface-card-37006?PageSpeed=noscript This keyboard can be connected with ZX81 by an ZX-Key expansion card at the ZX81 Bus connector. https://www.zx81keyboardadventu...

Hi Ien,

as I remember you have a minstrel 3 board.
So it might be intresting that ZXPand for minstrel is available at the moment
https://www.tindie.com/products/tynemou ... minstrels/

This seems to work with minstrel board only.

Hi Ien,

this may help as well(even if it is about ZX80):

viewtopic.php?f=7&t=3506

Max

Hi Ien, did you try out "grey border" jumper. Even if Processor is in reset state this should give a grey output, so shift register and things around that can be tested. If reset goes to high(check if it does) there should be some data seen in D0-D7. Next you may pull ram and eprom out of the board ...

After some experiments I am able to tell, that there are at least three ways to fix the pixel shift I noticed with 74HCT chips: - Replacing IC9 with 74HC08 See page 3 and 4 within https://d3s5r33r268y59.cloudfront.net/datasheets/18642/2019-11-27-20-17-38/Minstrel_Build_Instructions_v3.5.2.pdf - Chan...