This article is compliments of N1RCT.
New: Circuit for RITTY plus TNC simultaneously
Why bother? The basic reason to add the optional PTT/FSK circuits to RITTY or Writelog/WinRTTY is to use narrow CW filters on radios that do not permit them to be used on LSB mode, such as my Kenwood 940 or Icom 756. RITTY /WinRTTY only can operate on the final sound that comes from the radio; strong signals close by on LSB can ruin the sound of the desired signal as it can be much stronger. One way it hurts is to "capture" the AGC of the radio .. AGC is really automatic attenuator control and the strong adjacent signal causes the AGC to reduce the gain and drive the desired signal so low you will not even hear it and realize it is there. CW filters operate at one or more of the IF frequencies and can remove these strong signals before the AGC stage. Some of the new top-end models of radio do DSP at the IF frequency; it requires very fast DSP processors and even then, a fairly low IF of perhaps 75 KHz is used; a CW filter at 8 MHz or 455 KHz may well be more effective.
The first step is to obtain a RS 232 plug and shell from radio shack or cut a stock cable. You will need a female plug of the size to fit the com port you will use. For more on com ports, see:
You need to identify the following wires in
a stock cable or wire up a plug as follows:
There are only three wires needed, the DTR, TxD, and Gnd wires. This port will be used as output only; no signals go into the computer from here. This means that a unique IRQ is not required; it can "share" an IRQ; I use the same one as the mouse. For Windows, you may have to try a few to find one it will allow. However, you still need a unique com port address.
The next step is a simple circuit which converts the RS-232 signals into the levels needed by the radio.
There is very little power generated in these
circuits, so small parts are OK. I used tiny plastic transistors and 1/4
watt resistors. This circuit may give reversed tones, it did on my Knwd
940. The easy solution is to reverse the emitter and base leads of the
FSK transistor, so that the input line goes to the emitter (the pointed
end) and the base (center) goes to ground. In the same vein, the amount
of shift (170, 425, or 850) may be set by a jumper inside the radio. My
Kenwood was set to 425 Hz. Strangely, I made 10 contacts with 425 Hz shift
and reversed tones before someone told me I had a little problem! (Using
my PK 900, tho). BTW, reversed tones are really right side up! The convention
is that they sound right in LSB, which is reversed .... Which leads to
the difference between manufacturers in the polarity convention. For my
new IC-756, the circuit as shown is the correct polarity. For my IC-738
(no FSK), I use just the PTT signal to avoid using VOX.
Ready to get a little more complicated? perhaps you would like to
keep your regular TNC all hooked up ... The
next circuit permits both RITTY/WinRTTY and the TNC to receive simultaneously
and a switch that selects which (TNC or RITTY) will be allowed to transmit.
A little study will show that all combinations of FSK/AFSK/PTT are permitted
from both TNC and RITTY/WinRTTY. This complete isolation of the transmit
circuits is not required for all lines on some TNC's but it is probably
easier to do it all and eliminate all possible interactions. Also, the
center "OFF" position permits Phone operation without any parasitic effects
from the TNC's (Thanks. Claude!).
A little picture of how mine looks:
