Re: Handic VIC-Switch

Den Wed, 1 Nov 2017 09:22:52 -0500 skrev Jim Brain <brain@jbrain.com>:
> On 11/1/2017 3:24 AM, Mia Magnusson wrote:
> > Den Tue, 31 Oct 2017 23:08:14 -0500 skrev Jim Brain
> > <brain@jbrain.com>:
> >> On 10/31/2017 3:02 AM, Mia Magnusson wrote:
> >>> Sorry if this is an old topic.
> >>>
> >>> Are there any schematics available for Handic VIC-Switch, or some
> >>> similar device?
> >> http://jbrain.com/pub/cbm/hw/VICSwitch/VICSwitch.sch
> > Thank you very much!
> >
> > So what it basically does is that it uses a 3 bit counter that
> > selects each computer connector and as soon as it finds a computer
> > pulling ATN it holds the counter until ATN is released, while the
> > 74138 and 7405 holds DAT low on each not selected computer. Quite
> > clever!
> I think there is a timer that holds the line low for a bit after the
> ATN goes high., as ATN is only low while the IEC "command" is sent,
> not the data.

Yes, I see that now, a 74123.

> > I'm not that familiar with burst mode. Would an additional (or
> > actually two in parallell) 4051 for SRQ be enough?
> As previously response notes, no, it is not that easy (and, you are 
> correct, it's a pair of 4051s to add, not a '151 as I stated in my
> last email).
> >
> > How does this play with disk turbos? AFAIK they use CLK and DAT as
> > two data lines so would probably work through this device.
> 2 lines would be OK, as the system makes very few assumptions on the
> two lines (though, the reason 2 line fastloaders are 2 line
> fastloaders is that they obey the first part of the IEC protocol, and
> don't use ATN line so they can work  on the shared bus.  As a result,
> the command usage ona  2 line fastloader should obey all of the same
> rules the VIC-Switch obeys (DAT line low on ATN low, etc.)

How common are 3 line fast loaders?


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