[asterisk-users] Digium E1 card stops working til disconnect machine power cord

A J Stiles asterisk_list at earthshod.co.uk
Thu Jul 10 07:31:31 CDT 2014

On Thursday 10 Jul 2014, Ismael Gil wrote:
> Hi there,
>  In one of my asterisk installation, there is a Digium E1 pri card
> connected. The asterisk and card are working properly.
>  The problem we have is that when a storm occurs in the area, the card
> stops working, and E1 lines connected not rise, even restart the machine.
> If however if you turn off the machine, disconnect the power cord,
> reattach it and turn on the machine again, E1 pri card correctly
> recognizes the E1 lines and the PBX starts to function properly. This only
> happens when there are storms in the area.
>  Why is it necessary to disconnect the power cord for the card to work
> after a storm? How I can identify where the problem is?
>  How I can fix it?
>  Thanks in advance,
>  Ismaeleitor

It's a phenomenon called "latch-up", which occurs when extreme voltages are 
applied to the inputs of CMOS logic ICs.

A parasitic PNPN structure -- which is unavoidably present at each pin -- 
ordinarily provides some measure of static protection, by acting as diodes to 
clamp the voltage on any pin to the power rails; but it can also act like a 
PNP and NPN transistor in a sort of deadly embrace, where each one is holding 
the other in conduction.

If enough current flows into or out of an input pin under fault conditions, one 
of these transistors can turn on, which will cause the other transistor also 
to turn on; and the input can become clamped hard against one or other power 
rail.  This situation will persist as long as any current is flowing through 
the transistors, which in practice means until the supply is disconnected  
(or, in extremis, until the device overheats and melts down).

See also Wikipedia article: http://en.wikipedia.org/wiki/Latchup


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