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I am monitoring ACPI and other sensors on my mainboard with lm_sensors. there are two entries:

intrusion0:   ALARM
intrusion1:   ALARM

From what I know these are sensors to detect an open case, but there are no headers or pins to connect something like a switch on the mainboard. (AsRock Z86 Pro3 (manuals))

I wonder if I can access these sensors in other ways or if they are just not supported by the board but still active in the chipset. I know how to hide the entries in lm_sensors, that should not be the issue here.

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Does the last section in the lm_sensors FAQ/Chapter3 pertain to your question ? –  harrymc Apr 6 '12 at 20:08
    
@harrymc: partially yes, after setting the values to 0 once with echo intrusion0 is set to OK now, but intrusion1 is still set to ALARM. I still have no idea how to change those values by hand, aka opening the case. –  Baarn Apr 6 '12 at 20:22
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3 Answers

These values will show ALARM even if you do not have any headers on you motherboard. The reason is that is most likely the default setting in the BIOS. Possibly a better version of you motherboard has intrusion detection headers available and there are resettable via the BIOS only.

There is not much you can do to change the values.. as it would defy the purpose of detecting intrusions if it could be disabled by software.. for example. its easier resetting the BIOS password on the motherboard than using software to hack it.. hence.. Intrusion detection cannot be easily reset also.

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Based on your answer to my comment, I think that the setting of ALARM after boot is bogus and should be ignored. As your chassis is not open, cold reboot from power-off should have cleared up the condition of intrusion alarm. The cause could be a hardware problem with your mainboard captors or simply the way that it was designed to work.

If it bothers you, you could add to the boot the code suggested in the last section in the lm_sensors FAQ/Chapter3, to clear the alarm once during the boot :

echo 0 > /sys/class/hwmon/hwmon*/device/intrusion0_alarm

And as the above does work for you, I suppose that the second alarm can similarly be reset via :

echo 0 > /sys/class/hwmon/hwmon*/device/intrusion1_alarm

I am guessing that if your chassis/mainboard does contain valid intrusion alarm captors, they will automatically reset the condition(s) if the chassis is really opened, so that later runs of sensors will then pick it up correctly.

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It is fairly common for motherboard manufacturers to buy chips in bulk for several boards and not pin-out different functionalities per board to save cost and differentiate the models. That sounds like the case here.

The generic answer to a chipset that has features that are not enabled is to "piggy-back" a connector directly to the chip (from intrusion detection pin to either ground or one of the hot voltages for instance - depending on the implementation), though it is not a task for the novice and can lead to a board that has a lot of chips with all their functionality disabled if not careful (and sometimes lucky).

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