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Commodore BX-256-80 - 8088 Co-processor

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  Commodore BX-256-80 - 8088 Co-processor by Bill Degnan - 11/10/2008 16:17
The other day I saw some rare Commodore CBM 256-80 computers for sale and despite the lack of keyboards I bought them. Turned out to be a great deal!



CBM 256-80 with 8088 co-processor


Upon inspection of the insides, I was surprised to find that both were actually the BX models. This is truly a rare find, indeed. The 256-80 BX is also known as the CBMX256-80 and the BX256-80.

In the US the CBM 256 "BX" model with the 8088 co-processor was not officially sold by Commodore. The few 8088 cards that were produced in test/prototype batches were given to the Chicago B Series User Group for free. I have a copy of the release letter to prove it. Bruce Faierson, the 2nd president of CBUG and owner of Northwest Music near Chicago, installed a few CBM 256-80's with these prototype 8088 cards and sold them in the mid 80's. I have some of his old mailers with the BX ads.


8088 co-processor board installed in CBM 256-80

I believe that the two computers I opened today are two of his modified systems. They're all stock CBM parts with the prototype 8088 cards installed. Anyway, according to most sources only two or three of the 8088 co-processor cards were known to exist. The PLA has an interesting label:

906114-05
HIGHBOY
6235

[NOTE: I know of one other PLA that will run a 8088 co-proc. It's marked 906114-04]

The Highboy is located in the upper right of
this image.

I suspect now that there are still a few BX's hiding in the bushes. Add two more to the list! If you have a CBM 256-80, open it!

Unfortunately Bruce passed away earlier this year. I spoke him a few times over the past few years and we traded a lot of emails. He helped me to build a working hi-profile B system with an 8088 card, and because of him I was able to learn a heck of a lot about the B Series and IEEE drives.

It's possible that you have the correct PLA even if you don't have the 8088 co-proc card installed. I found this to be the case with my one non-8088 CBM 256-80.

If anyone reads this post and has a CBM 256-80, please let me know what the PLA markings are, and whether it's a US or European model.

Here are some pictures, prior to cleaning, etc. Neither of the systems' monitors work, so I will need to transplant the card and the HI-BOY PLA chip to a regular 256-80 to test out the 8088 co-processor cards.

Here are some pictures of the extremely rare CBM 8088 co-processor card for the CBM PET II line. Plus some pictures of one of the 256-80's that the cards came in.

BX256-80 Pictures

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  CBM 8088 co-processor update by Bill Degnan - 12/22/2008 18:45
I took the co-processor out of the dead unit it came from and installed into a working machine.

Commodore CBM 256-80 with 8088 co-processor MS DOS 1.25 directory on display.


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  256-80 ROMS not compat with co-processor by Bill Degnan - 03/08/2009 17:40

I have strong circumstantial evidence that the program space used by the original CBM 8088 co-processor (intended for but not officially sold for the B Series) conflicts with the space that the CBM 256-80 BASIC ROMS use, and therefore a stock CBM 256-80 will not work with the 8088 co-processor. That explains why the 2 256's with the co-processors I have came with B128 BASIC ROMS in them. I will bet that no one with a 8088 co-processor and 256 BASIC ROMS will be able to get it to work properly. By swapping ROMS with another working system and/or working parts I learned of this situation. Only when I had the following ROMS in a hi-profile B Series system would the co-processor work:

901243-02 (B128 BASIC)
901242-02 (B128 BASIC)
901244-03 (probably not important KERNAL)

Both the HIBOY PLA 906114-05 and 906114-04 work with the 8088 co-proc.

To reiterate - "common knowledge" is incorrect, it's not the PLA that makes a hi-profile board compatible with the 8088 co-processor, it's probably the BASIC ROMS (you can't use the stock CBM 256-80 ROMs).

I would need to dump the program listing of the 8088 co-processor to confirm this. If there is a strong interest I will attempt to pull it down, time permitting. This is one of those things I will do eventually.

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  256-80 ROMs are fine with 8088 by Ed Shockley - 11/19/2010 19:21
Hey Bill, came across this old post while googling for something unrelated and wanted to clear this up.

The 256K ROMs work fine with the REV F 8088 coprocessor cards. But you have to have -03 or -04 of the Kernal matched with the -05 PLA. And you should have at least -03 of the BASIC ROMs as well but that's not as important. The MSDOS and CP/M-85 bootstrap loaders are the issue and why the other levels matter.

I have two and they run just fine. The 256s have a more relevant coprocessor incompatibility, at least those with CEAG sourced power supplies. Those power supplies are 1/4 inch too deep and will make contact with the boards and short them out if you don't notice it! The General Instrument made power supplies are fine. Seems like the 128s all came with GI PSs but 256s can be either.



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