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PacMan 01-23-2008 11:08 AM

Got my 8800 kit back again =)
Well I finished my kit this summer and got it working but didn't think I was going to have the time to use so I decided to sell it to a friend :rolleyes:

Turned out that it was him that didn't have the time to use it (He started working for me ;) ) so I just bought it back from him. Unfortunately it seems he has done something nasty with it. The outputs from the 8212 on the CPU card is always in a high state and the CPU is unable to write to the memory but I can still change the content from the control panel.

So I was wondering if anyone has seen this before ? Could it be that the 8212 has gone kaput or are there other issues I should look into ? I will dig out my scope this evening and check all the clock signals around the chip to see all is OK.


PacMan 02-02-2008 03:08 PM

I finally had some time to do some debugging on my unit. The 8212 seems to reset properly as all LED's goes out during the reset. But as soon as RST is released they light up again. The 8212 DS1* input has a 2MHz continuous clock and DS2 input is connected to the processor SYNC output and seem to be getting the proper signal also. As far as I can see these are the only inputs that would affect the clocking of the data so I am suspecting that the 8212 has gone kaput. I have ordered a new one so hopefully this will solve my problems.


PacMan 02-11-2008 01:08 PM

Nops, the new 8212 gives the exact same behavior so the device is not the culprit here. I will try to hook up my trusty old logic analyzer later to see if I can find anything strange.


Pops Racer 02-12-2008 01:33 AM

G'day PacMan,

I was looking over my schematics for the 8080 CPU card, the outputs of the 8212 are buffered by 2 8T97s, G and H, and those are gated by ICO. I wonder if there are any problems there? Seems strange that the lights would blank on a reset, then just stay on.


PacMan 01-07-2009 04:53 AM

So I am at it again after a year in hibernation.

Found the problem now... It turns out I missed Grants instructions on the replaced resistor on the CPU card. I'm not sure how it could have worked ,before the mod (if it ever worked), looking at the clock wave forms.

I've had it running memory tests on a few 16KByte memory cards I got from ebay some time ago and all looks good now.


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