Hi Charlie et al,
Yes, pin 5 ground is connected to the shield or ground of your radio on whatever
pin or connection that might be. On a Kenwood TS870 which as an RCA jack for
FSK input, that would the shield or outside connection on the RCA plug.
The interface in question is at http://www.aa5au.com/rttyinterface.html.
There is one important thing to know about FSK inputs. FSK inputs require either
an open or short circuit to shift keying from mark to space or vice versa.
As fact, to test your FSK radio input, you can key the radio with no
short circuit the input and the frequency should shift. In other words, if an
a mark tone, if you short the input, it will produce a space tone or vice versa.
I suppose that if there was a short in the interface transistor from base to
the voltage directly from the COM port would be applied to the FSK input and I
there could possible be some damage (I doubt it but you never know since it's
pure friggin' magic).
Mario, S56A, suggested to me recently that some protection can be afforded by
placing either a diode or 1k ohm resistor from base to ground on the interface
This would protect COM port voltage from reaching the FSK input of your radio
in case of a transistor failure (base to collector short). So it's not a bad
I've yet to incorporate this change in my setup.
73, Don AA5AU
----- Original Message -----
Sent: Friday, November 22, 2002 3:22 PM
Subject: [WriteLog] AA5AU FSK interface question
> Don et al,
> Building your interface today to tinker with this weekend, and bought the
> little project box and all. My question, that I want to
make certain before I burn something up is, the connection to the db9 pin 5,
which is ground. Is that wire also connected to the
shield of the RCA jack (ground) to the FSK jack on the back of the radio? IE:
db9 pin 7 to base, db9 pin 5 to emitter and shield of
fsk jack, and collector of transistor to center of rca jack for positive on fsk
keyer? Is this correct, or do i leave the shield
floating and pickup ground through the radio?
> Just want to make sure before I burn something up.
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