I think I posted something similar many months ago. The I/O address seems to be partially decoded in hardware (I/O Page 'E' and the 74LS138 3 to 8 decoder) and partially by the active high and active low chip selects on the 6702. What we didn't know is whether the chip select lines are really chip select lines - or inputs to the 6702 with labels on the schematic diagram to confuse the reader! I think that you have to assume that they are really chip select lines until you find some evidence to the contrary. Based on this, it is likely that additional 6702s would require slightly different chip select wiring to make them appear at different I/O addresses. Since Commodore never released any data on these devices - all discussions are purely conjecture... Surely by now there must be some ex Commodore employees in the know willing to disclose the information? Come on guys... If I remember correctly - I think Mike has donated a 6702 to be "taken apart" to see what is actually in the package. Dave > Message Received: Sep 05 2012, 07:57 PM > From: "Gerrit Heitsch" <email@example.com> > To: firstname.lastname@example.org > Cc: > Subject: Re: 6702 chip > > On 09/05/2012 08:24 PM, Rhialto wrote: > > On Wed 05 Sep 2012 at 06:38:29 +0000, William Levak wrote: > >> > >> The original SuperPET contains sockets for four 6702 daughter boards. > > > > Perhaps that is why the output value is visible on 4 address locations. > > The addresses influenced by the circuitry around the 6702 are more than > > just those; all of "E F xxx0 xxxx" are affected, but the ones that are > > not EF00 .. EF03 all return FF (instead of the EF of the unconnected > > space). Apparently part of the address decoding is done inside the 6702. > > Another question... Was there only one 6702? Or was it like the 6530 > where you had another number on the chip to indicate what's in there? > > I tried to find a photograph of the 6702, but wasn't able to. > > Also, the schematic > > > http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/SuperPET/324055.gif > > shows a 74LS138 for decoding part of the address space and also a lot of > CS signals on the 6702 itself. This schematic decodes the chip to EF8 > 00xx though, assuming the logic of the CS-signals is as shown. > > Gerrit > > > > > Message was sent through the cbm-hackers mailing list > Message was sent through the cbm-hackers mailing listReceived on 2012-09-05 20:00:10
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