Re: gEDA-user: rs232 ferrite size?

[John Doty <jpd@xxxxxxxxxxxxx>]

On Jan 22, 2007, at 10:49 AM, DJ Delorie wrote:

> > I can't imagine why you would want one on either TX or RX... I have
> > never had any issues with RS232 on any board I have designed.
>
> This board is going to be mounted to an ESD generator.  I'm protecting
> pretty much everything.

You worried about bad bits or overvoltage? An EMI ferrite isn't going  
to have much effect on either for a relatively high Z interface like  
RS-232.

For ESD overvoltage, there are a variety of protection devices on the  
market. I tend to just use a dual diode clipper to the rails,  
followed by 100 ohms or so series resistance to the receiver input to  
insure most of the current goes through the diodes. And RS-232  
receiver chips often have pretty good ruggedness specs.

To protect the bits, a few nF of capacitance to ground will be much  
more effective than an EMI bead. It'll soak up some ESD charge, too,  
perhaps removing the need for overvoltage protection.

If you have ringing, EMI beads at strategically chosen points in the  
wiring can help, but an input is generally not the right place.

Often the very best EMI medicine is to make sure your return lines  
are as close as possible to your signal lines. See "A TELEPHONIC  
CONTROVERSY SETTLED", Scientific American, Vol. XLIV, No. 8, page 113  
(February 19, 1881). ;-)

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxx




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