Re: Two projects

From: Olaf Seibert (
Date: 1998-03-20 21:45:25 (Frank Kontros) wrote:
> >The third problem was that after replacing the lines I still didn't
> >recognise the data on the bus. After some puzzling I found out that all
> >bytes were inverted. This puzzled me because this simply meant that the
> >byte was inverted on purpose because writing a byte directly to the PIA
> >appears as so on the bus. This also means that an attached device has to
> >invert it again to get the original byte. Has anybody an idea why this is
> >done?
> That is because of inverted logic of IEEE-488 bus standard. I'm sure in that,
> why recall my diploma project. (If you told me earlier... :) Most of C64 IEEE
> interfaces uses inverting by 'software'. Maybe this because of few bi-dir
> bus drivers in 74xx TTL series.

Yes, in the PETs, and basically also in the 64, most signals (but I
don't think ALL signals) go OUT though an inverter but they come IN
without an inverter. I'm sure that's needed somehow electrically. To
compensate for that, the software inverts the signals again. On top of
that, some signal names have another inversion in them: NRFD, NDAC...
(NOT ready for data, NOT data accepted) All this can be very confusing.

> >The second project, X-DMA, is a project to devellop an interface which
> >connects the expansion slot of the C64/128 directly with an ISA-slot of an
> >PC. I finnished the theoretical part and I want to let you read it and to
> >give comment on it. This project was also the reason why I asked those
> >questions about the VIC and the SID. As it is meant to be published on my 
> >site as well, it is in HTML.

> Nice dream. But if you placing it under VIC (or SID) unused locations, you never

While we're dreaming: not only PCs have ISA slots, so if possible don't
do anything that depends on PCs unnecessarily.

> Frank
___ Olaf 'Rhialto' Seibert  rhialto(at)   "I would be dead in
\X/ a week if I didn't do /something/. I'd be splattered all over the walls."

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