System 80 4020 expansion unit issues

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System 80 4020 expansion unit issues

Postby nama on Sun Jan 29, 2012 5:53 pm

I pulled my System 80 out of it's box yesterday.
I wanted to test an Ohio Scientific drive with another machine and the System 80 is the only machine I have that uses a 5.25" floppy connector.
Unfortunately when the System 80 is attached to the expansion unit and the unit is turned on all I get is a screen full of seemingly random characters (although fairly consistent pattern on consecutive power ups):

Otherwise the System 80 seems to function ok without the expansion unit being attached. and with the unit attached I can 'Break' 'Reset' to the ready prompt from the corrupt screen.

So I started poking around with my logic probe in the expansion unit. I don't know if this is significant or just a symptom of some other issue, but the data out (D_out) pin 14 of the 4116 RAM (z5, z6, z7) looks weird. Basically all other RAM is pulsing high on this pin, but the these three RAM just show the pulsing orange light, but not the high/low light on the probe. I did the obvious and swapped the ram around, but that doesn't change anything.
These 3 lines connect to a single 74LS244, but so does z8 and that seems to be pulsing high like the others.

I have checked the continuity on the ribbon cable from the System 80 to the expansion unit and it all looks ok. As Tezza suggested I also changed the orientation of this cable around and that didn't help either.
I'm using a Canon 5.25" drive (not the OSI one yet) as I know when the System 80 is working this drive plays well with it.

Hoping someone here may be able to point me in the right direction for testing.
Last edited by nama on Sun Jan 29, 2012 6:45 pm, edited 1 time in total.
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Re: System 80 4020 expansion unit issues

Postby nama on Sun Jan 29, 2012 6:33 pm

Not sure if that RAM thing I mentioned above is anything to worry about. Break-resetting into basic seems to bring the RAM pin 14 to it's normal state.
Doing a "?MEM" shows 31956k RAM (not 32896k which is full 32k) without the expansion unit attached, and 48340k with the unit attached. Not sure if that is right or important at all.

Phil
Retro computing at: http://www.neoncluster.com
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Re: System 80 4020 expansion unit issues

Postby tezza on Sun Jan 29, 2012 8:01 pm

nama wrote:Not sure if that RAM thing I mentioned above is anything to worry about. Break-resetting into basic seems to bring the RAM pin 14 to it's normal state.
Doing a "?MEM" shows 31956k RAM (not 32896k which is full 32k) without the expansion unit attached, and 48340k with the unit attached. Not sure if that is right or important at all.

Phil


Those are the correct figures for RAM Phil
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Re: System 80 4020 expansion unit issues

Postby nama on Mon Jan 30, 2012 2:00 am

Thanks Terry,
I have moved this discussion to here:

http://groups.google.com/group/eaca-dis ... b05f58ac85

As you suggested the folks on the EACA forum seem knowledgable on this specific subject.

Cheers

Phil
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