| Posted By
Exin
on 2010-08-27
22:17:45
|  CPU replacement?
Hello!
I have a few plus/4 with broken CPU's.
I've heard long time ago that someone managed to make an adapter for 65xx cpu's, but you were not able to use the Serial bus.
Does someone remember it? Is it really possible to use a 2mhz version of a 8500, the 8502(Commodore 128)? Or are there really hard differences?
I searched in the forum, but didnt find much detailed info...
-Exin
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Posted By
gerliczer
on 2010-08-28
03:38:25
| Re: CPU replacement?
Hi Exin,
I think you meant this: http://www.baltissen.org/newhtm/10to816.htm. This is about replacing the 6510. If one wants to do this with the [7|8]501 it must be somewhat reworked but there are compatibility issues. This solution replaces a 65XX type processor with a 65cXX one, and since the latter fixed some quirks and introduced new instructions, it can't be considered drop-in replacment. There are software that builds on the oddities of the original part therefore it won't run correctly on the newer processor.
The [7|8]501 can't be replaced with 850[0|2] because those have different pinout and have one less processor I/O pin. To work it around like in the case of the 65cXX replacement project is not possible because one can't disable the processor I/O port so no matter what is done read/write of $00/$01 addresses will always access the integrated port.
The safest bet would be to cram a modified VHDL 6510 core into a CPLD (or a non-volatile FPGA if such thing exists) and replace the broken CPU with that.
Of course I'm not an expert so the real ones correct me if I'm wrong, please.
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Posted By
Chicken
on 2010-08-28
06:13:25
 | Re: CPU replacement?
gerliczer:
Exin refers to a project by Mr. Museum and C16DOC. They used a 6510 to "replace" a 7501/8501. I don't know much about this but as Exin stated already the serial port doesn't work. Otherwise, everything seems to be running stable but I'm not 100% if there are other issues. This was discussed in some German forum years ago.
Obviously, Ruud Baltissen used to have schematics for a 6502 to 8501 board on his website. These are long gone, though (Maybe for a reason.. it might be a good idea to ask him about this). Some years ago I found them on an Italian website and it said the source was R. Baltissen.
Fortunately, the schematics are still there  http://www.webalice.it/gratteri/65028501.html
In fact, I was so intrigued about this that I asked Solder (a hardware expert) if it's possible. He had a look at it and wondered if anybody actually tried this. Also, he said some of the surrounding logic should be placed on a CPLD to make it smaller. However, he still had spare 8501s so he didn't try this.
I meant to ask TLC about this, too but I haven't done so, yet. But I'm sure he'll jump right in when he reads this 
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Posted By
TLC
on 2010-08-28
13:00:46
| Re: CPU replacement?
Christian: I'd do that , but I'm afraid I'm not much of practical help here. Replacing the 8501 by a 2-MHz 6502 + some glue logics should be possible, but neither I have made any attempts to do that.
A 6510/8500 base should impose the problem of a missing portbit... it should be also relatively difficult to supply the missing portbit "retroactively".
The 8502 might be a better candidate for the project, as it's a 2MHz 6502 descendant (the 6510/8500 might or might not work at 2MHz at all -- at least, they're not rated for 2MHz), and as it seems to have a 7-bit onboard I/O port. Problem: it appears to be almost as rare as the 8501 is by itself (...but I might be wrong in that assumption; honestly, I've never owned a working C128 myself, only some wrecks +a partially working one that I never really used).
At the end, neither I did come up with a solution -- I used what I had (cannibalized some 8501s from wrecks, and/or some of my own 264 series computers that I rarely used, in favor of those that I used more frequently), until someone on this forum pointed me to some Hungarian supplier that still had 8501 processors (...20 years old stock) for sale.
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Posted By
Exin
on 2010-08-28
21:14:45
| Re: CPU replacement?
Yeah. I did hear about that too. But i guess a 8500/6510 will do for starts. And 1.7mhz are not so far away from 0.9mhz. Also, i have a working 1551, which i got with a working +4 for 10€ last time in the flea market. Maybe these don't depend on the IEC bus at all...
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Posted By
TLC
on 2010-08-29
07:22:03
| Re: CPU replacement?
Yes, you definitely won't have problems using a 1551 (it uses the so-called "TCBM-bus", and installs its own port chip (a 6523 TIA) on the system bus of the Plus/4 for that).
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Posted By
Pepax
on 2010-08-30
03:51:03
| Re: CPU replacement?
It was me and they still have them. http://www.hqelektronik.hu/info/en/8501.html
Exin: I still have a few spare CPUs so if I'm lucky enough to get a lift to Xzentrix I can take them with me.
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Posted By
Exin
on 2010-08-30
06:17:42
| Re: CPU replacement?
Hmm, how many, and which portbits are used in the Plus/4?
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Posted By
TLC
on 2010-08-30
12:34:18
| Re: CPU replacement?
Exin: "All" existing portbits of the 8501 are used (ie. 7).
On the schematic diag of the Plus/4: http://zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/plus4-310164-1of4.gif you can see that pins p0..p7 of U2 (excluding p5 which is not implemented) mostly connect to and handle various tasks of the serial and tape ports.
On the 6510/8500, p6 and p7 are not implemented (see http://zimmers.net/anonftp/pub/cbm/schematics/computers/c64/251469-2of2.gif). That's bad luck, this sums up to 3 missing portbits in total, in respect of the Plus/4 -- 2 unimplemented bits where they'd be needed, and 1 implemented bit where it won't be used.
P6 serves as serial CLK IN + CST WRT, and p7 as serial DATA IN (ie. their absence results in no serial bus + unability to write to tape).
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Posted By
Exin
on 2010-08-31
21:06:25
| Re: CPU replacement?
Is there anywhere a pinout with exact pin locations of the 8501 ?
(Nevermind, i found it :) )
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Posted By
Mike
on 2011-10-31
03:32:58
| Re: CPU replacement?
Dear Pepax!
I need a 7501/8501, if you have one working piece for sale and the price is good. Does anybody find a solution to replace these CPU-s with other circuits?
Thank you for the answers.
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Posted By
crock
on 2011-10-31
11:26:35
 | Re: CPU replacement?
In a moment of extreme rashness I ordered a few, sourced from China of all places!! They had no way of testing them, so they were bought in the knowledge that they were untested. Including shipping, I transferred $120 via Paypal and fully expected that was the last I would hear.
You can imagine my surprise when a long thin parcel arrived from Shenzen about 4 days later. Sadly, 4 of the 10 didn't work but I was left with 6 working cpu's, including one of the *much* better 1990 datecoded cpu's that run much cooler. So, net cost to me was about $20, which most sensible sane people would not pay.
Despite what I paid, I could not in good conscience pass that on to you so my advice would be to take a punt on a defective c16 or plus 4 on ebay, I reckon you would have a better than 50% chance of harvesting a good cpu. (a c16 went on ebay.de yesterday for 5 euro)
If not and you *really* need one, PM me - I'm always open to trade for other C= oddities or rarities.
WRT to a replacement, I asked Bil Herd about this and the very early prototypes (ie before Anders Carlsson's prototype) did use a 6510 and a bunch of external TTL logic. Before I bought my spares from China, I did poll the community for setting up a bounty to make a plug in replacement, but didn't get much support (the CommodoreBounty website has now all but closed.) I'm sure smarter people than me could do it with a 6502...
Rob
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