| Posted By
CzajNick
on 2016-02-06
17:21:44
|  TED is (not) dead...
Hi all,
Some time ago I posted about my plus/4 which boots fine (i.e. video is OK, and it shows the Commodore BASIC greeting), but keyboard is not working. The diagnosis is quite interesting, so I decided to write this post - perhaps it could help someone?
I've initially suspected TED chip. Recently I had a chance to swap chips from a working one, only to find that it's the CPU that's faulty! I find it quite strange - dead CPU usually means a dead machine.
I suspect, that it's IRQ pin that's broken - I'm not sure about Commodore BASIC implementation, but it could perform periodic keyboard scans driven by the interrupts from TED (either timer interrupt, or perhaps a vertical blanking). Could someone elaborate on this topic?
So, if the keyboard is dead, you can't rule the CPU out, unfortunately. And now I need 1 TED and 2 CPUs, eBay prices are unreasonably high ;-(
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Posted By
SVS
on 2016-02-07
11:59:38
 | Re: TED is (not) dead...
You could check the joysticks. If the moviments produce (as they have) some alfabetic letters on the screen, then maybe you have the keyboard fault.
Another check is to reset the Plus/4 by keeping pressed the STOP key. Does it enter in Monitor environment, by showing 6502 registers?
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Posted By
CzajNick
on 2016-02-09
04:27:10
| Re: TED is (not) dead...
Hmm, perhaps I wasn't clear enough - I'm 100% positive, that faulty CPU was the reason. Replacing it solved the problem!
I just wrote about it, because I think it's quite an unusual failure mode for the CPU 
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Posted By
crock
on 2016-02-09
17:29:33
 | Re: TED is (not) dead...
Hi CrajNick, a few years ago I created the Diag264 diagnostic tool to help identify hardware faults. I'm always looking for ways to improve it's operation and I'd be very interested to see how it behaves with your CPU, if it is indeed the IRQ signal that is dead.
Assuming it's of not so much use to you, would you be interested in parting with it? I'd give you something towards your TED/CPU replacement fund in compensation.
Rob
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Posted By
CzajNick
on 2016-03-15
07:00:20
| Re: TED is (not) dead...
Hi!
Apologies for answering so late, it's been awhile since I visited the forum. Busy time...
Thanks for the link, it looks way cool and I will definitely give it a try!
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