From my perspective, those keying circuits want to be inside the radio,
safe from harm. Is there any reason why they weren't made part of the
radio - or why I shouldn't put them in when I get my K3? (Assuming of
course I can get into that part to do so...)
The K3 accepts open collector(or open drain) to ground as inputs, or can
be driven with 0 <= x <= +5 volts.
Some stations use RS232 levels (which can be as high as +/- 25V) for
direct FSK. Such levels need to be converted to 5 volts for the K3
digital inputs. Further, RS232 specifies a "1" as -3 < x < -25V and a
"0" as +3 < x < +25V, so a polarity inversion (in hardware or software)
is also required.
73,
Lyle kK7P
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