[Elecraft] Re: N6XI NAQP RTTY July, 2007 with K3

[wayne burdick]

Can't hurt, Don, thanks. Of course the base limiting resistor will 
generally protect the transistor from excessive zener current when it's 
in Vbe breakdown.

Wayne


On Jul 22, 2007, at 12:57 PM, Don Wilhelm wrote:

> Wayne,
>
> You might want to bump up that parts count to 3 - and add a diode from 
> the base to the emitter of that transistor (cathode toward the base) - 
> protects the transistor from damage on the most negative excursion of 
> the RS232 signal line which can go to 25 volts negative by the RS232 
> spec.
>
> There is not likely to be a problem when driving from a PC serial port 
> because the PC usually does not drive that far negative, but just the 
> same, I like to have things that work over the full range of the spec 
> rather than work in most situations.
>
> 73,
> Don W3FPR
>
> wayne burdick wrote:
>> Jeff,
>> The K3's I/O module already provides for the use of RTS or DTR for 
>> PTT and/or CW KEY in (two separate control lines). The exception is 
>> FSK, which is on the ACC connector. In the present incarnation, 
>> driving this line from a PC requires two external parts (one 
>> garden-variety NPN transistor and one resistor). I used MMTTY with 
>> this interface during NAQP this weekend.
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