[RTTY] Signalink FSK Success! And a very basic question....

[Joe Subich, W4TV]

> I do have one very basic question. What exactly is this transistor
> doing? Why do we need it? Why can't I just plug the TxD lead directly
> into the Signalink?

Serial port signals are bipolar +V/-V.  The FSK input of a transceiver
expects to be connected to a switch which is either open (high
impedance) or closed (low impedance).  The NPN transistor (or a FET,
opto-isolators, etc.) creates a low impedance when the voltage on the
base (grid, LED, etc.) is above a certain level and is high impedance
when the voltage is below that threshold.

Yes, the transceiver *could have* been designed to accept a logic
level directly but FSK inputs were originally driven from the contacts
of the "polar relay" in the mechanical RTTY terminal and that tradition
has simply been carried forward for 60 years or more ...

73,

    ... Joe, W4TV


On 2014-06-14 7:49 PM, Michael Rapp wrote:
> Hi all,
>
> A few weeks ago I asked for advice about how to help me achieve FSK with my
> Signalink.  Before I had to switch between the Signalink and a W3YY
> interface, with the audio out from my FT-950 going into a poorly made sound
> card.  At the end of the day, I could decode RTTY with the Signalink but
> only transmit with the W3YY interface, which wasn't exactly optimal.
>
> I am happy -- no elated beyond measure -- to report that I now can FSK
> through my Signalink!  After a ridiculous comedy of errors with wires,
> ports, adapters, and trips to Fry's, I got it to work.  (About the only
> thing I didn't do wrong was pick up the soldering iron from the hot end!)
>
> I ended up doing a combination of K7SFN's mod (
> http://www.k7sfn.com/projects/signalink.html) and the FSK circuit on Don's
> page (http://www.aa5au.com/rttyinterface.html).
>
> Here's what I did.  I picked up a 9 pin serial port shell and solder cup.
>   I soldered some 22 awg wire to the TxD and ground pins.  I wired a 1 k ohm
> resister to the TxD lead and wired that to the base of an NTE123
> transistor.  I wired the ground from the serial port to the emitter of the
> transistor.  I wired the collector to the FSK "pass through" on the
> Signalink.
>
> You must understand....I have no electronics background.  At all.  Prior to
> starting in the ham radio hobby, the only thing I knew about a transistor
> was how to spell it.  This is also the first time I've had to look at a
> schematic (Don's) and really figure something out with it.  From my license
> studies, I remembered what the various parts of the NPN symbol meant and
> figured out what wires to connect to what pins.
>
> And to my utter elation....it works!   It actually works!   I have
> diddles!!  :D
>
> I do have one very basic question.  What exactly is this transistor doing?
>   Why do we need it?  Why can't I just plug the TxD lead directly into the
> Signalink?
>
> 73,
>