Re: Focus Card Problems

[mjmahon@aol.com (Michael J. Mahon)]

Sandy wrote:

>Tony said he was going to install a Transwarp in his machine and check the
>the new cards out.  On my machine I suspect it is something in the newer
>Focus, because my old one still boots from slot 7 like it always has.  Or
>from slot 6. The new one doesn't boot from 7 but works in 6. It didn't matter
>if the TWGS was on or off.  Seems fairly definitive that way.    One thing I
>didn't do was insert the SCSI card in the alternate slot when I tested the
>old one. I always have an AHSCSI card in either slot 6 or
> 7.  
> 
>As to power draw, Tony said my buffered grappler (slot 1) pulled more juice
>than the Focus.  Since it doesn't speed up printing I changed back to the
>regular grappler.  And I have that 200 watt Buggie power supply.  That
>doesn't do any more for my machine than the regular power supply.  Maybe
>jumpers would help.  

IMHO, it is not an issue of having a Transwarp in the machine, nor is it
an issue of how much power a Focus card requires.  The critical issue
is how sensitive the Focus card is to low voltage and/or extra noise on
the power.

The noise issue could be put to rest by putting some heavy bypassing
on the Focus card--say 100mf electrolytic capacitor in parallel with a
few 0.1mf monolithics.  If the problem goes away, then voltage wasn't
the problem, noise was the problem.

The voltage sensitivity issue is best addressed by powering the +5v bus
of the Apple (or just the Focus card, but that's a little harder) from an
external, adjustable supply.  If all is well, it should work reliably down
to 4.5 volts, since 10% is the nominal tolerance on most five volt
microelectronics.  If it gets flaky at, say, 4.7 volts, then there is what
is known as a "margin" problem.  This would require either some re-
design of the card, since the Apple slots should be able to supply
>4.5 volts reliably.  More likely, it would require the imposition of a
new requirement for reliable Focus operation--namely, that the Apple
have a power supply that delivers, say, 5.2 volts under load.  (That
is well within the tolerance for the Apple, and would get the Focus
card into a range of reliable operation--assuming, again, that voltage
sensitivity is the issue.

A complication in all this, is that the Focus card powers the disk
drive from +5v, and most drives exhibit wild variations in current
drain as they power up, calibrate, seek, read, write, etc.  It might
require that the card have an auxilliary regulator for the disk's +5v,
tapping a little current from the +12v to the slot.

So, to sum up--there are a number of possible problems, each with
some indicated solutions, and the additional issue of variablity from
the different drives that are used and how they behave in use.

BTW, the power jumpers Rubywand refers to are simply extra power
leads run from near the connector to the power pins on the slot with
the finicky card.  This lowers the effective series resistance of the
power supply, and marginally raises the voltage at the slot.  It also
can reduce noise somewhat.  HOWEVER, this should NEVER be
necessary if a card operates within the specs of the Apple slots,
and the power supply is operating properly--two big if's, given what
we don't know about the Focus card and the old age of the capacitors
in virtually all Apple power supplies.

The proper solution:  if the Focus is marginal when its supply
voltage(s) are within tolerance, fix the Focus card.  If the Apple
power supply cannot supply clean (enough) power within tolerance,
fix the power supply.

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/