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Re: Request for Product
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.
: 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.
/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).
: 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.
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.
-michael
8-voice music synthesizer using NadaNet networking!
Home page: http://members.aol.com/MJMahon/