On 20 Sep 2007, at 22:53, Xiaofan Chen wrote:

> Yesterday an engineer showed me a strange thing. It is an isolated DC
> input I/O with relay output. The isolation specification is low. So
> every input is going through an opto-coupler and the circuits are
> quite simple. Strange thing is that there is a long trace underneath
> all the optos and it is connecting to the common shield connection.
> Apparently this is not for safety reasons as it actually reduce
> clearance/creepage distance (still enough). 

I would imagine that is a flashover guard. Spurious high voltage on 
an input pin should arc to the protective guard before reaching the 
logic side.


George Smith

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