Date : Tue, 26 Jan 1988 15:36:00 GMT
From : ssc-vax!shuksan!mikey@beaver.cs.washington.edu (Mike Fields)
Subject: Need help with floppy drive powerdo
In article <18700011@clio>, berger@clio.las.uiuc.edu writes:
>
> Are you sure this is an artifact of the Shugart 851 and not your
> controller? I have never had any problems with disks when shutting
> off power to the 851, even with the heads engaged.
>
> Mike Berger
The actual problem is a matter of timing - If the 5 volt logic supply
drops before the 24 volts for the stepping motor/head load solenoid
falls, you can get garbage. Exactly what signals (head load etc) you
will get from TTL logic when the supply falls to less than 5 volts is
very hard to predict. If it generates a head load signal and there
is still 24 volts available to the head load solenoid, you can get
a momentary load (and garbage gets written etc). Some drives are
jumpered so that the stepping motor is always on when the drive is
selected - this usually (but it depends on the filter caps) causes the
24volts to drop quickly. The correct solution is to set up the supply so
that the 24volts is cut off as soon as a power-fail condition is detected.
This would eliminate any chance of a stray write occuring. Temporary
measures could consist of increasing the filtering on the 5 volt buss to
keep the 5 volt logic alive longer, decreasing the filtering on the
24 volt buss to allow it to drop faster and/or increasing the load
on the 24 volt buss with a load resistor to get it to drop when the
power is turned off (if there is no bleader resistor and no drain on
the 24 volts, it could stay up for minutes!). This problem can occur
with all the drives I am familiar with - its just a matter of timing!
--
Mikey (yes "he likes it!")
============================================== Mike Fields
ssc-vax!shuksan!mikey (206) 251-4638 [work] 12022 NE 138th Pl.
(alt) ssc-vax!mikey (206) 821-3492 [home] Kirkland, Wa. 98034
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