| Posted By
Spirantho
on 2010-08-11
05:22:41
|  7501 vs 8501 - Is one more reliable?
Hi everybody,
I went looking through my 264 series machines last night. This is what I found:
5x +4 - one fully working, 4 with bad CPUs, all 8501. The working one is is a 7501.
2x C16 - one fully working, 1 with bad CPU (not sure what the working one is, probably an 8501 but not sure)
1x C116 - fully working (thank goodness!)
This got me wondering - are the 7501s as prone to failure as the 8501s? The 8501 seems to die if you just look at it in the wrong way, yet the one 7501 I have works perfectly.
As I understand it, the 7501 uses HMOS-I technology which is larger die size than the 8501's HMOS-II. I know major firms like Intel used the HMOS-II process without problem - so did MOS take shortcuts on the HMOS-II and make a ton of chips that only just work, and have a very short life? Does the 7501 have the same problem?
As an aside for anyone who likes a challenge, perhaps they can explain what's wrong with one of the 8501s (All the others do nothing at all - just black screen).
It boots into BASIC just fine, and runs commands. But if you type a line number with one number, it stores it in line 15. Two numbers becomes line 165. Three becomes 1665. Four becomes 16665. Five gives a syntax error (as it tries to store 16666 which is out of range). For instance:
1 ? "HI"
LIST
15 PRINT "HI"
10 REM
LIST
15 PRINT "HI"
165 REM
and so on.
I'm tempted to look in the BASIC ROM and see how this happens! 3-in-1 fires up the title screen, then the screen goes black instead of starting the program.
Anyone got any advice?
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Posted By
Litwr
on 2010-08-11
05:34:46
| Re: 7501 vs 8501 - Is one more reliable?
IMHO this is something wrong with ROM (90%) or RAM (9%).
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Posted By
Spirantho
on 2010-08-11
05:44:40
| Re: 7501 vs 8501 - Is one more reliable?
That's what I would usually think... but it's a consistent fault. Tried the same 8501 in two +4s and a C16, exactly the same. Definitely the CPU. Weird huh?
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Posted By
Litwr
on 2010-08-17
04:01:22
| Re: 7501 vs 8501 - Is one more reliable?
if your hardware can load something -- try
http://plus4world.powweb.com/software/Test_Suite
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Posted By
TLC
on 2010-08-18
13:56:53
| Re: 7501 vs 8501 - Is one more reliable?
Hi!
Just wanted to comment on your experiences with 8501 CPUs... I could only ever see one single 7501 CPU in my whole life :-D, and that one was broken, too  , so I can't really comment on whether 7501s are actually more stable. I'd suppose (from that, which actually matches your findings  ) that the vast majority of 264 machines came out with 8501s anyway. ...As to "why" these are that prone to faults -- that's been discussed for a couple of times as I guess... the story must go back either to badly designed layout, underestimated power dissipation (and local overheat on the chip as a consequence), or bad quality silicone die. It's an interesting matter because earlier Commodore chips (the original 1MHz 6502 and its counterparts) are usually really stable, even though some of them are the first representatives of the "cheap" manufacturing process invented by MOS Technology itself (which involves fixing and packaging a great percentage of faulty silicone chips)... The 8501 is basically a 2MHz 6502 core with some small additions... it obviously dissipates more heat than a 1MHz one. Did they underestimate that?... I don't know. The "way" these chips die is said to be due to cracks starting up from silicone die irregularities, and is triggered by heat (dilatation); some cracks would eventually cut vital traces on the silicone die, resulting in a fault. (That may be the answer to your second question: whether some crack cuts some vital trace, or just something that causes partial problems, is completely unpredictable). It's suggested (but not proven) that applying some simple cooling (like heatsinks) could prevent, or delay the, well, sad end...
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