| Posted By
CzajNick
on 2016-01-09
13:53:08
|  TED is dead? Strange plus/4 failure...
Hi all!
I'm just trying to bring back to life my old Plus/4 - dead CPU, TED missing (long story, don't ask  ).
To repair this one, I've just purchased the second damaged one. This one is starting up to the BASIC prompt (with cursor blinking happily), but the keyboard is not working. I assumed that both CPU and TED are OK, and it's PLA or the row buffer chip next to the keyboard matrix (U27, 6529B) that's broken. I hoped to get at least one of those P/4s working. Nope.
Now the fun begins - I've tried putting TED and CPU extracted from the Second P/4 into my First P/4 - and it resulted in *exact same results*, i.e. BASIC prompt but keyboard not working. It's highly unlikely for both P/4s to have the same keyboard-related issue, so I suppose it has something to do with TED.
So I attached my oscilloscope and observed that:
- I can see a few Chip Select pulses at U27 right after reset -> I assume PLA is working
- there's some activity on CPU's R/W for some time after reset, then it stays high
- IRQ is pulled low soon after reset, it's pulled by TED (but disconnecting IRQ from TED doesn't really help)
- I see repeating patterns on the address lines
My guess: CPU is waiting for something in a loop (that's why I see patterns on address lines), waiting for some flag in some (TED's ?) register (that's why R/W remains high - CPU performs no writes, only fetching instructions and reading some registers). Keyboard doesn't work, because CPU doesn't even reach the point when it starts to poll the keyboard.
Does it ring any bells? Is it any kind of popular failure? I have no more TEDs to check - any hint really appreciated.
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Posted By
gerliczer
on 2016-01-09
14:15:07
| Re: TED is dead? Strange plus/4 failure...
This is only my wild guessing, since I'm not an expert of the subject. Based on the fact that TED pulls down the IRQ line and there's no change I suspect that the CPU either faulty or somehow runs to a 'Crash' opcode. Could you get different ROM ICs from another machine? Or at least reseat the ones in the tested plus/4?
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Posted By
CzajNick
on 2016-01-09
15:08:47
| Re: TED is dead? Strange plus/4 failure...
Not sure if it was clear enough, but I essentially tried one CPU + TED set in both Plus4s, getting exact same result. That's why I suspect CPU or TED malfunctioning.
As of IRQ, my initial thought was that CPU is stuck exactly because broken TED or CIA chip pulls it low (mind the inverted logic - low is active). So I tried to disconnect TEDs IRQ (by bending pin 8 out), and IRQ remained high (inactive) this time - that's how I know TED is responsible for signaling the interrupt. My initial idea was that CPU was stuck in the interrupt handler, but now I think it might be other way around - CPU doesn't handle interrupts at all.
I start to think that the best thing (in terms of effort) to do is to find another TED and/or CPU for replacement  Looking around for used C16
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Posted By
SVS
on 2016-01-10
07:48:50
 | Re: TED is dead? Strange plus/4 failure...
You can try with a joystick connected.
If by moving the joystick no character is output on screen, then the problem is not a physical keyboard (keys) fault, maybe the PIO 6529B.
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Posted By
Hege
on 2016-03-02
10:47:37
| Re: TED is dead? Strange plus/4 failure...
This can be a TED chip keyboard latch failure rather then IRQ or CPU.
If you ever get to the Basic screen and get a blinking cursor then it is more likely that the CPU is working (not 100% true).
If I were you, I would get the Diag264 in an Eprom and would plug in to the Kernal's socket and see what is happening. I use it often when I meet a dead plus4, it is useful.
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Posted By
CzajNick
on 2016-07-19
07:08:29
| Re: TED is dead? Strange plus/4 failure...
Just for the record again - it *was* a defective CPU, I'm 100% positive!
Diag264 EPROM is quite a nice idea, too. I need to give it a try eventually 
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Posted By
Lavina
on 2016-07-20
03:41:33
 | Re: TED is dead? Strange plus/4 failure...
Ted's dead baby, Ted's dead.
I'm Sorry, I Couldn't Resist /D. U./ ))
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Posted By
MMS
on 2016-07-20
20:55:20
 | Re: TED is dead? Strange plus/4 failure...
Yeah, a real classic (unfortunately, in Plussies too...)
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