Thread: Mk14 vdu
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Old 25th Oct 2020, 10:11 pm   #597
Timbucus
Octode
 
Join Date: Mar 2019
Location: Barry, Vale of Glamorgan, Wales, UK.
Posts: 1,362
Default Re: Mk14 vdu

Quote:
Originally Posted by SiriusHardware View Post
I'm going to hold off for now until Tim comes up with a definitive list of things which he has tried and wants me to try to replicate because at the moment his symptoms seem to be a bit of a moving target while he experiments.

But, I can confirm that I sometimes see that reverse 'C' shown in the image in #594. Not usually straight away, I tend to notice that it has appeared after a while.
Sensible move - glad it is the same and I am sure it is caused by the same problem I have a corrupt write. It will appear just sat in SCIOS for me (after a long time) as of course there is an NWDS assertion happening a lot around a fixed group of addresses - I am going to guess that the state of the bits on the address bus that are affected will be consistent due to that sequence which is why it is repeatable.

I was just getting it a lot more often with that dodgy resistor I think - I will put back in the 250ns memory soon to prove they only affect the PI upload rate (as the VDU is off for most of that anyway) and report back. I have slowed down the send14 script for the moment and that has stopped the static corruption on load which seems to evidence that theory.

I also took the opportunity to solder back down all the hanging around bits of wire to the 8154 and get rid of the long black clip jumper I was using to select Graphics and Text. I wondered if they were introducing noise or spurious capacitive effects.

Short runs of all the software seem fine now without spurious data but, I can get MINEFIELD to crash by the second level about 6 mines in - my next step is to put the SOC VDU back on and prove it does not do it there - which I don't think so but, best to double check.

Fixing the resistor is making finding the fault harder as it is becoming more exotic... I will obviously play with values for the NWDS pullup to see if I can get the effect to change - where is my big 10K POT ...
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