(Fwd) Your B Series 8088 board reversal

Hallo allemaal,


I just found this email, very interesting IMHO:


------- Forwarded message follows -------
Send reply to:	<drshock@insectria.net>
From:	"Edward Shockley" <drshock@insectria.net>
To:	<ruud@BALTISSEN.ORG>
Subject:	Your B Series 8088 board reversal
Date sent:	Sat, 4 Dec 2010 20:55:19 -0500

    Ruud,
    Came across your reversal of Bo's 8088 board at 
    http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/8088.txt and the 
    GIF of your schematic.
    
    Have been doing a project where I wanted to get the 8087 coprocessor to work on this 
    board for some MS-DOS 1.25 software I felt like porting over. It never worked even back 
    in the day but nobody bothered to figure out why. Anyway, wanted to share a fix for a PCB 
    error that causes the 8087 to hang CP/M-86 or MS-DOS on bootup.
    
    In your schematic you show pins 31 and 33 of the 8087 tied together. This is the error that 
    CBM made. The board has a silkscreen for "JP1" in the valley between the 8088 and 8087 
    but no PCB jumper was ever installed and only one solder pin hole was drilled for it. This 
    hard link between these two pins must either be cut, or cut and another hole drilled and 
    the jumper installed for the math coprocessor to work. It would be jumpered when not 
    installed, and the jumper removed if installed.
    
    The 6509, as you noted, requests the bus from the 8088 across the RQ/GT0 line. But only 
    when no 8087 is installed. When an 8087 is added to the circuit the 6509 must instead 
    (with the design point the Commodore engineer(s) decided upon) request the bus thru the 
    8087 pin 33 (RQ/GT1) and let the 8087 pin 31 be connected to the 8088 RQ/GT0. 
    Connected this way the 8087 will either request the bus for itself, or on behalf of the 6509 
    as necessary. The RQ/GT1 line of the 8088 remains non connected as original.

If the fix isn't made then when the 8087 is installed the 6509 and 
the 8087 (during initilization of either CP/M-86 or MS-DOS) will 
both try to request the bus from the 8088 on the same line and this 
will lock the bus up and hang the B.

With this change the 8087 can be installed and function as 
originally designed without problem - though with it added the 
board draws over 950mA!


FYI,

Edward Shockley
http://www.insectria.org/b128.html

------- End of forwarded message -------

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