[Elecraft] RTTY via FSK-D

[Richard Ferch]

Brett,

Regarding the timing problems you had with CW from a serial port: In 
this situation the timing is not done by the UART, it is done directly 
by the CPU toggling the control line (usually DTR) on and off, and is 
therefore subject to non-RTOS timing problems. If you are using a UART 
to do FSK on its standard TxD data output, the timing is controlled by 
the clock in the UART, so the timing issues that affect serial port CW 
do not affect FSK done in this way. In a sense, the UART for FSK is a 
logical equivalent to the Winkeyer chip for CW.

However, this only applies if the UART is capable of doing 5-bit codes 
at 45.45 baud. The UARTs used in PC serial ports have this capability, 
but most USB-to-serial adapters do not. One giveaway is the maximum 
speed supported by the adapter; if it is greater than 230 kb/sec, then 
the minimum speed is too high for RTTY. Currently available single 
USB-to-serial adapters are too fast for RTTY, but some 4-port or 8-port 
USB adapters can handle the slower speed.

There is an add-in for MMTTY called EXTFSK that is capable of doing 
45.45 baud RTTY with any USB-to-serial adapter. EXTFSK can key any of 
the TxD, DTR or RTS signal lines on any serial port or serial adapter; 
in fact, it can even do FSK keying via a parallel port. The downside is 
that the timing is handled by the CPU, not by the UART. However, 
apparently EXTFSK uses the same sound card timer that is used for AFSK, 
so the timing of EXTFSK-sent FSK should be the same as the timing of 
sound card AFSK, no better and no worse.

You do need a keying circuit between the serial port and the K3's FSK 
input. The same kind of one-transistor or opto-isolator circuit that is 
usually used for serial port CW or PTT will also work for FSK keying.

73,
Rich VE3KI


KC7OTG wrote:

> So I'm thinking about wiring up the FSK IN pin on the AUX connector to
> do RTTY TX "properly".  I'm curious of a few things before I take the
> time to make the cable.  
> 
> First off how do people usually drive this signal?  I assume its usually
> taken off of a serial port...  I've tried a few RTTY apps and I've
> gotten them all to work via AFSK and all seem to have their pros/cons.
> I have a feeling in the end that I'll probably most use MMTTY being that
> it has a very nice interface into N1MM (unless there is another
> application that has a nice interface into N1MM that I should be aware
> of).  
> 
> Finally my last question is that if this comes off of a UART how has the
> timing been for people?  I remember a while back I used a UART to to CW
> keying in N1MM.  I remember being embarrassed by the CW that was sent
> and I've since bought a K1EL WKUSB kit and put that together...  Very
> happy with that little box.  Would I have these same timing issues in
> this FSK_IN setup?  If there was ever a stupid thing is trying to use
> Windows as an RTOS. ;)