25 Apr
2000
25 Apr
'00
10:44 a.m.
On Tue, 25 Apr 2000, Tony Duell wrote:
It seems like it must be a switcher, Tony, as the 851s do not use -5v power.
- don
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...). 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