That is a fun factoid! On Sun, Dec 5, 2010 at 2:46 AM, <Ruud@baltissen.org> wrote: > Hallo allemaal, > > > I just found this email, very interesting IMHO: > > > ------- Forwarded message follows ------- > Send reply to: <email@example.com> > From: "Edward Shockley" <firstname.lastname@example.org> > 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.txtand 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 ------- > > -- > ___ > / __|__ > / / |_/ Groetjes, Ruud Baltissen > \ \__|_\ > \___| http://Ruud.C64.org > > > > > > > > Message was sent through the cbm-hackers mailing list > -- :Robert "kebernet" Cooper ::email@example.com Alice's cleartext Charlie is the attacker Bob signs and encrypts http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x9E8759F8 Message was sent through the cbm-hackers mailing listReceived on 2010-12-06 03:00:30
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