Re: VIA shift register

On Thu, Jan 10, 2019 at 8:51 PM Pasi 'Albert' Ojala <a1bert@iki.fi> wrote:
>
> On 01/10/19 18:52, Francesco Messineo wrote:
> > On Thu, Jan 10, 2019 at 5:33 PM Pasi 'A1bert' Ojala <a1bert@iki.fi> wrote:
> >> On 10.01.2019 17:22, smf wrote:
> >>> On 10/01/2019 08:11, André Fachat wrote:
> >>>> I know this solution, but IIRC this prevents bidirectional operation
> >>>> without extra direction control.
> >>> Don't the c128 & 1571 have extra hardware for direction control on the
> >>> 6526 ?
> >> http://www.iki.fi/a1bert/Dev/burst (already mentioned earlier, but this
> >> is the "forever link"):
> >>
> >> "The C128 hardware includes a buffer driver between SRQ and the cassette
> >> read line so that cassette activity or cassette drive presence will not
> >> disturb the fast serial port. It also has a two-directional buffer that
> >> connects SRQ and DATA to the CIA1 synchronous serial port. The direction
> >> is controlled by the MMU chip. These buffers are required to hide the
> >> fast serial connection is C64 mode."
> > I can't find this bidirectional buffer in the C128 schematic
> > (zimmers). Wrong schematic?
> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-3-left.gif
>
> See CIA1 SP and CNT, U58 and U60.

yes I see that, they didn't use a bidirectional buffer anyway :) They
gated the signals through open collectors NANDs.
Ok, result is very similar, in C64 mode, they prevent the signals
reaching the serial shifter of CIA#1.

>  From my article: "Theoretically you should be able to make the
> modification work with C128 in both modes by first connecting the wires
> and then cutting/bending up U58 (74LS03) pins 3 and 8, and U60 (7407)
> pins 6 and 8. This disables the C128 fast serial hardware and the added
> wires will perform their function in C128 and C64 modes. I have not
> tested this, so if you try it, I would be very interested in your results."

I would not modify a C128 as you suggest. I'd try lifting U8P9 and
just use the same routines used in C128 mode (of course they would
need to be copied somewhere in the C64 RAM first).

Frank