FXS – Power Alarms

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On occasion, I’m seeing the following in syslog on some systems using analog cards with FXS modules:

[ 1664.861183] Power alarm on module 1, resetting!

These are typically cleared by restarting asterisk/dahdi, or power cycling the system. However, I’m wondering if anyone can explain what condition exactly causes this. My best guesses:

-Too high current draw on port (shorted pair)
-Not enough current supplying card/module (bad power supply)
-Voltage introduced to FXS port (FXS port connected to FXO or POTS line for example)