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Sherlock Ohms

Flagrant Fan Fuels Diagnostic Failure

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Critic
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Thanks
Critic   3/4/2015 10:21:55 AM
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Thanks for an interesting story, Rich.  At first it didn't dawn on me that older computers used magnetic core memory.  Sometimes semiconductor RAM is still referred to as core, even though there are no ferrite cores in the memory.

The Space Shuttle flight control computers used magnetic core memory at first.  It wasn't until 1990 that semiconductor memory replaced the cores.  It is not uncommon for spacecraft to use older, proven technology, especially when it comes to the radiation sensitivity of semiconductors in critical applications.

patb2009
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Re: Thanks
patb2009   3/5/2015 2:13:21 AM
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The use of magnetic core for spacecraft is useful for two reasons

1) it's well known.

 

2) in the 70's it had a retain memory on power fail capacity.  

 

A forced reboot or restart, during flight ops, makes it very nice to 

have the software still in main core.  

The only thing close to that was bubble memory.

Battar
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Old timer
Battar   3/5/2015 9:27:12 AM
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Rich, 

      You must be an old timer. When I was young and wore a uniform, I was in charge of a computer room full of DEC equipment, dominated by 2 1981 model 11/70's. When they were dismantled and sold for scrap, I rescued a from panel with the blinking lights and switches for loading the boot address.  At another location, I rescued a 4K**16 bit core memory card (I think it was from an 11/10 - we never had PDP-8's - it might have been an 11/60) - I still have it to this day in our lab. Fun times.

jdesbonnet
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Iron
Similar story
jdesbonnet   3/19/2015 3:23:42 PM
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I heard rather similar story, also from a DEC field engineer: random crashes of a VAX system which persisted despite replacing almost every part in the cabinet. It turned out to be a small metal foil label which was blown about by the fans.

EricJuve
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Iron
Another DEC failure due to fans
EricJuve   3/19/2015 3:38:07 PM
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When I was working for Tektronix in the early 80s I built a system for Boeing that had 2 PDP11 70s. After we got it running we shipped it off and a couple weeks later we got a call that the system was crashing. I was sent to Seattle to find out what was wrong. I was escorted to the computer by the Boeing engineer in charge of the project and he took me to a room that was just barely big enough to hold the 2 computers. I had him boot the machines up and within just a few minutes the room became uncomfortably hot. It turns out that they hadn't taken into account the large amount of power that 2 machines that size would generate. Even before the machines crashed I explained that PDP11s needed better cooling than that. I left and I guess they got some air conditioning installed as I never heard from them again.

William K.
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Platinum
Subject to magnetic disruption?
William K.   3/19/2015 4:17:09 PM
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The whole concept of magnetic core memories being altered by an external magnetic field is a bit confusing, since I thought that memory cores were small toroid coil assemblies, and the "common knowledge" is that toroids are immune to external magnetic fields. So it must be that my undersatanding of core memories is lacking, or else, possibly, that the external AC magnetic field was simply erasing the magnetization.

bruhnstv
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Iron
Re: Subject to magnetic disruption?
bruhnstv   3/19/2015 6:54:30 PM
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William K:  Of course toroid cores are sensitive to external magnetic fields!  They are, after all, simply little pieces of magnetic material.  It is true that a toroidal inductor (especially one wound on a core of high permeability) is _relatively_ immune to coupling with external fields.  Consider, though, that if an external field is strong enough to saturate the core, the inductance of the coil will drop.  In fact, this is a method of tuning an oscillator: use a core material that saturates relatively slowly as you increase the magnetic field, and keep the oscillator current in the coil well below saturation.  Then an external "DC" magnetic field can tune the coil's inductance, and thus the oscillation frequency, over a considerable range.

Critic
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Platinum
Re: Subject to magnetic disruption?
Critic   3/20/2015 1:50:53 PM
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As bruhnstv has already said, toroids are relatively immune to external magnetic fields, but they are not shielded.

It might have been a good idea to place the memory inside a magnetic shielding enclosure, but (probably) to keep costs low, this wasn't done.  Keeping magnetic devices, like a fan motor, away from the memory would have been a good idea, too, and probably wouldn't have increased cost.

In my years of work with magnetic devices, I was always amazed that even though the devices were labeled "keep away from ferrous or magnetic materials," some customers still mounted the devices on iron angle or rails.

William K.
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Re: Subject to magnetic disruption?
William K.   3/21/2015 1:38:41 PM
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The difference between "interference RESISTANT" and "interference PROOF" is HUGE. That was brought to the for in a machine that I had to make work correctly, after it was created and built by folks who believed that a 4-20Ma analog current loop was interference proof. They neglected a whole bunch of details and as a result had a signal to noise ratio of possibly 6dB, which is poor.

As stated, avoiding the problem is much easier and cheaper than fixing it later.

Mr. Wirtel
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Gold
Fan
Mr. Wirtel   3/23/2015 9:25:08 PM
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Since apparently the device had been working before the new fan was installed, it just seems the fan installation is where the trouble shooting should have started. I was taught you make one alteration at a time so you know where to start undoing the problems.

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