Shugart 851 power problem

From: Richard Erlacher <edick_at_idcomm.com>
Date: Tue Apr 25 18:37:30 2000

Don's observation is an astute one, since a linear supply doesn't generally
allow both positive and negative supplies to interact. Unless this is a
MAJOR ground loop thing, we're probably looking at a switcher.

Has anybody tried powering on the drives AFTER the IMSAI box is in
steady-state?

Dick

----- Original Message -----
From: Don Maslin <donm_at_cts.com>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, April 25, 2000 4:44 PM
Subject: Re: Shugart 851 power problem


>
>
> On Tue, 25 Apr 2000, Tony Duell wrote:
>
> > >
> > > Hi,
> > >
> > > I have a pair of 851s in a nice case with power supply.
> >
> > One detail you've omitted : What sort of power supply is it (linear or
> > switcher)? The reason I ask is that with most linear supplies, a short
on
> > one output will not turn off the other outputs, while with most
switchers
> > a short on any output will shut the entire supply down (there are
> > exceptions to _both_ of those 'rules' though...).
>
> It seems like it must be a switcher, Tony, as the 851s do not use -5v
power.
>
> - don
>
> > > They are attached to a Cromemco 16FDC in an IMSAI box
> > > with a Cromemco ZPU and an Expandoram 64K board. I'm
> > > using the ROM enable on the 16FDC to drive the phantom
> > > line. There is a reason for these details...
> > >
> > > Here is the problem:
> > >
> > > #1 Power up the drives by themselves:
> > > At the drives:
> > > 5 volt line reads 4.9
> > > -5 volt line reads -4.9
> > > 24 volt line reads 23.9
> > >
> > > #2 Power up the IMSAI box:
> > > At the drives:
> > > 5 volt line reads 1.2
> > > -5 volt line reads -0.9
> > > 24 volt line reads 2.6
> > > and (needless to say) the drives don't work. The Cromemco
> > > ROM monitor gives all sorts of errors trying to access them.
> >
> > This looks like a classic case of 'latch up'. You may know about a
> > component called a thyristor/SCR, a 4 layer semiconductor device with 3
> > connections (anode, cathode, gate) that once turned on by applying a
> > current to the gate connection will then stay turned on until the
current
> > through the anode connection drops below a certain (low) value.
> >
> > Well, it turns out that it's hard to avoid making these when making
> > integrated circuits. You don't want them, but they're there. Normally
> > they do no harm, but sometimes if a pin on a chip is pulled past the
> > supply rail they turn on and short out said supply rails.
> >
> > And you could be doing just that. Signals on the controller cable from
> > the S100 box could be latching up chips in the drives. Thus shutting
down
> > the drive's PSU.
> >
> > What I don't understand, though, is why correctly-powered drives do this
> > when a powered-down controller is turned on. I could understand if
drives
> > connected to a powered-up controller would refuse to turn on. Nor can I
> > understand why turning off the controller and turning it on again makes
> > any difference.
> >
> >
> >
> > >
> > > #3 Power off the IMSAI box:
> > > (Same as #1)
> > >
> > > #4 Power on the IMSAI box again:
> > > At the drives:
> > > 5 volt line reads 5.0
> > > -5 volt line reads -5.0
> > > 24 volt line reads 24.0
> > > and the drives work fine.
> > >
> > > What gives? Does this make sense to anybody out there?
> > >
> > >
> >
> > -tony
> >
> >
>
Received on Tue Apr 25 2000 - 18:37:30 BST