Re: Request for Product

["Bill Garber" <willy46pa@comcast.net>]

"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.

: /DEVSEL is only one signal, and is not always active when the other
: signals are active.  For example, when firmware on a card is running,
: /DEVSEL is inactive unless the $C0sx space is referenced, but /IOSEL
: is active if the $Csxx address space is being referenced (by instruction
: fetches from ROM, for example).

Yes, that I knew.

: > : BTW, as long as neither switched card is active, it should
: > : be possible to switch the select lines with the power on
: > : without any problem.
: >
: > So, you're saying the cards can be flipped, and a warm boot init'd.
: > Interesting. Wonder if the slot itself can be init'd without having
: > an effect on the other slots, possibly allowing an annunciator line
: > to select the card in use.
:
: 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.

: Of course, a card's firmware may not be able to "remember" what its
: state was if the firmware of the two cards use overlapping slot state
: space in the text page "holes", so re-starting each card's firmware
: after switching may be necessary.

Definitely.

Apple II Forever, Apple II Together
Bill @ GarberStreet Enterprizez
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