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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