Re: Request for Product

["Michael J. Mahon" <mjmahon@aol.com>]

Bill Garber wrote:
> "Michael J. Mahon" <mjmahon@aol.com> wrote in message
> SYmdnfMb392XnE7fRVn-rQ@comcast.com">news:SYmdnfMb392XnE7fRVn-rQ@comcast.com...
> : Bill Garber wrote:
> : > "Michael J. Mahon" <mjmahon@aol.com> wrote in message
> : > JeWdnRnWgPO2eE_fRVn-3w@comcast.com">news:JeWdnRnWgPO2eE_fRVn-3w@comcast.com...
> : > : No, that's not enough, nor should it be done at all.
> : > : Power should be left on on both cards at all times so
> : > : that unpowered bus tranceivers don't load the bus.
> : >
> : > I agree. Switching power between the cards could also
> : > add undue strain on the PSU, unless you shut down each
> : > time.
> :
> : It's not the PSU that's the problem, though capacitor
> : charging could induce glitches.  It's the load that an
> : unpowered transciver would put on the address and data
> : lines.
>
> Ah, I am seeing with both eyes now. The LS245 or similar
> without power could pull down the signal on the card being
> used hence not allowing it to operate properly leaving the
> desired card on the expander rendered useless as well, or
> even stop the cards in other slots from operating fully.
>
> : > : Instead, the select lines should be switched:  I/O Select,
> : > : Device Select, I/O Strobe.  That will be enough to switch
> : > : almost all cards (execptions would be that do their own
> : > : slot decoding on board--a dubious practice).
> : >
> : > I believe only /DEVSEL is needed to be switched in.
> : > /IOSEL and /IOSTROBE are rendered inactive while /DEVSEL
> : > is High. So, therefore, with device select switched out,
> : > the card will not be accessed at all.
> :
> : /DEVSEL is a dynamic signal, not a level, so you might use an
> : annunciator to gate it, but it cannot replace it.
> :
> : And /IOSEL is also an _independent_ dynamic signal which must
> : _also_ be switched, or gated by the same annunciator signal.
> :
> : You are correct, that /IOSTROBE is common to all slots and need
> : not be switched.
>
> So the /DEVSEL and /IOSEL can be switched simultaneously from one
> card to the other using an annunciator for each card through decoding.
> A small cable to the gameport could accomplish this through software.
> This would allow the machine to remain powered and do a JMP $C700 to
> access the card which is decoded? Or PR#7 whichever is preferred.

I would expect that only one annunciator would be used, and that
signal would gate the select signals for one card, and its inverse
would gate the select signals for the other--so you flip from one
card to the other by changing the state of the annunciator.

Or, since most people would prefer not to modify their software,
just use a SPDT switch to switch the gating signals manually (when
neither card is being referenced, of course).

<snip>

> : Initializing a slot is primarily a software/firmware function,
> : assuming that the card has been reset initially.  After the select
> : lines are switched, a normal "PR#s" or whatever will initialize
> : the slot firmware if necessary.
>
> Does this also reset the annunciators? That would keep me from using
> them to decode which card on the adapter is being utilized, unless
> I use a flip-flop circuit which remains active until changed.

Nothing flips the annunciators unless they are part of its
functioning.  The Apple ROM reset code presets all annunciators
(two on and two off) and DHR will tickle AN3.

-michael

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Home page:  http://members.aol.com/MJMahon/