This should be added to the documentation on funet so everyone can see it. On Wed, 22 Mar 2000, Richard Atkinson wrote: > This is good stuff!!! Thanks, Bill. > > > Richard > > ---------- Forwarded message ---------- > Date: Tue, 21 Mar 2000 22:51:14 -0500 > From: Bil Herd <bherd@jersey.net> > To: 'Richard Atkinson' <rga24@hermes.cam.ac.uk> > Subject: RE: Calling Bil Herd > > > The problem with the CPM cartridge is a design flaw with the thinking behind > the cartridge where the Z80's processor gets frozen for a memory cycle and > then runs for 2 clocks and then frozen again. The problem is that if you > freeze it during a cycle (which will happen about 1/3 of the time) when the > internal bus on the Z80 is in a "float" condition, the address multiplexorss > on the CPM cart are exposed to a voltage that is neither a "1" nor a "0" > (about 1.2-1.4 volts if memory servers). This has the nasty effect of > causing the multiplexor (which cane be thought of as an amplifier for this > purpose) to oscillate since it has an "illegal" voltage on it's input. Well > if the address lines oscillate or don't stand perfectly still during certain > parts of the DRAM cycle (particuarly the Row Address Setup and Hold and the > Column Address Setup and Hold) ALL bets are off. Several rows (or columns) > of data get selected inside the DRAM all at once causing contention and > other nasty things to occur and there goes the DRAM data integrity. > > I don't know of anyway to fix it shy of changing brands of multiplexes on > the CPM cartridge or putting some strong pull-ups on the address lines to > fight the "amplified bus float" that comes out of the Z80. > > This design flaw was so nasty that there was no question in my mind about > building a Z80 into the C128 to do CPM.... Relying on the design flaws in > the cart simply wasn't an option. > > BTW, a year later I found an Apple schematic tucked in a drawer of the Z80 > cartridge for the Apple II. Turns out an early CBM engineer had copied the > design, flaw and all, from apple. > > Bil Herd > > > -----Original Message----- > From: Richard Atkinson [mailto:rga24@hermes.cam.ac.uk] > Sent: Monday, March 20, 2000 9:55 PM > To: Bil Herd > Subject: Re: Calling Bil Herd > > Bil, > > Another topic that has recently cropped up on cbm-hackers is the Commodore > CP/M cartridge. Can you remember any of the specifics of why the cartridge > wouldn't work with later C64s, and maybe even what could be done to it to > correct this? We're trying to find a hack so that CP/M cartridges can be > made to run on all machines. > > Regards, > > > Richard > > > - > This message was sent through the cbm-hackers mailing list. > To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi. > - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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