[vcf-midatlantic] Monitor needed for museum

David Ryskalczyk david.rysk at gmail.com
Tue Aug 22 20:21:31 EDT 2017

I'll make an EE guess here:
When you unplug the DC side, the Pi is shut off relatively quickly so there is little room for bad writes to corrupt the card, or for the voltage to drop low enough for the CPU to execute random instructions but still high enough to run at all.
When you unplug the AC side of the power adapter, the capacitance in the power supply causes a much slower drop-off which causes the CPU to execute random instructions, likely writing to the card.

So far I haven't yet corrupted a card, but I generally power Pis from USB and unplug the USB cable first.

My suggestions? Make the Linux setup as read-only as possible as that will minimize possibility of corruption, and of course, if the image contents are static, keep a copy of the image handy on a nearby PC. Constant writing will quickly kill the SD card anyway.


> On Aug 22, 2017, at 8:16 PM, Dan Roganti via vcf-midatlantic <vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> On Tue, Aug 22, 2017 at 7:36 PM, corey cohen via vcf-midatlantic <
> vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
>> FYI.  I have found that if you powerdown the PI from the AC side of the
>> power adapter it has a good chance of corrupting boot drive.  But if you
>> put the switch on the D.C. side and use that instead, I have yet to corrupt
>> an image.
> ​i seriously doubt there is some magical combination of power off sequences
> to resolve this,
> some people on the RPi forums claim this to be the different SD types,
> class 10 vs class 4, etc
> But nobody has gotten the same class to work reliably, because you can't
> when you deliberately shutoff the power
> even though this is an embedded linux computer, eg, instant on, "instant
> off"​ -- it's not >exactly< instant off
> btw, as  you may have noticed, it's not exactly "instant on" either
> in simple terms, because there's still a file system which spools the
> writes to the SD card in a background process.,
> so you're corrupting the disk before the write operations are completed by
> unplugging the power.
> On your linux desktop do you actually unplug the power cord,
> No, you click on the shutdown button to safely power off, cause you will
> get the same result, a corrupted hard disk
> what your supposed to have in lieu of a keyboard/mouse is a Shutdown button
> wired to the gpio expansion header,
> this allows you to safely shutoff the Pi, simple as that, without
> corruption to the sd card
> there's 100s of examples, like this one,
> http://www.instructables.com/id/Simple-Raspberry-Pi-Shutdown-Button/
> now there's more exotic options out there that others may chime in about,
> but to get you going, this is the most simplest option to start with.
> a tiny push button switch, resistor, pairs of wires, and a script to run in
> the background process - you're done
>> Also I don't know why the museum doesn't keep a copy of the disk image on
>> the desktop computer so that if the PI doesn't boot, they can simply
>> re-image the sdcard in 5 minutes and we can avoid this thread being
>> rehashed in 6 months.
> ​exactly
> with the shutdown button, corruption will be minimized
> but you still have storms where you lose power
> this is where you like to have battery backup
> and if you spend the extra effort,
> you can have the system detect the battery backup was activated
> and email your IT/RPi guy to warn him
> the battery will give still you hours and hours of use,
> but you still want to go in and reset the battery backup eventually
> Dan
> _  ____
> / \__/   Scotty, We Need More Power !!
> \_/ _\__ Aye, Cap'n, but we've only got 80 columns !!

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