[Elecraft] K3: Added protection for RS-232 port

[George & Jan]

If you are "home brewing" protection remember that RS-232 data signals go 
from minus to positive. Levels between -3 & +3 volts are undetermined. If 
you try to protect the RX & TX lines to ground then make sure that you can 
pass at least -5 volts to insure a good low.

In my 30 years experience interfacing broadcast equipment in South Florida 
(the lightning capital of the US) to tall towers & Florida Flash & Flicker:
     First is massive grounding - large conductors with many ground rods
     Second is keeping lightning away from the towers (dissipation arrays),
     Reduce all inductances in the ground system (including ground rods, 
tower bonding, primary power bonding and equipment bonding)
    Then surge protection on primary power and interconnecting lines.

Our experience found that RS-232 UARTS were were not particularly 
vulnerable. Audio distribution amplifiers failed most with power supplies 
second while RF, Video, RS 485 & Ethernet failures about equal to RS-232.

George
AI4VZ

>> I've been thinking about how one might go about adding some
>> protection for the apparently fragile (and expensive) RS-232 port on
>> the K3.  It might be possible, if the components were small enough,
>> to add them on the KIO3 I/O daughterboard, or perhaps on a little
>> external board mounted at the port.