DB9 Connector Pin 3 (TXD)
FSK to Radio
Ground is on the
DB 9 Connector Pin 5
FSK Circuit
PTT Circuit
CW Circuit
Simple Interfaces for FSK, PTT & CW Keying
Updated 12/14/2013
DB9 Connector Pin 7 (RTS)
CW to Radio
Ground is on the
DB 9  Connector Pin 5
DB9 Connector Pin 4 (DTR)
Ground is on the
DB 9 Connector Pin 5
PTT to Radio
These are the exact circuits I use for FSK, PTT and CW keying with WriteLog, but they should work for any other program as well. The PTT & FSK circuits work great with MMTTY.

These circuits are connected to the same DB 9 connector to the same serial COM port on the PC.

The transistor can be just about any general purpose NPN transistor.  I use the ECG 123 (same as NTE 123).  The 2N3904 works fine too.

The resistor value can be just about anything from 500 to 2k ohm or possibly lower or higher.  1k ohm worked great for me.

Pin 5 on the DB 9 connector is common to all circuits.  However, you may not need all three interfaces.  You can build them as single units or two in one, it doesn't matter.  For instance, for WriteLog, I don't need PTT because it is done by computer control of the radio.
Home Up
Ron, ND5S, built his interface into a very nice looking box (left photo)  with a short pigtail coming out to the DB 9 connector (which is female).

The box is Radio Shack p/n 270-1802.  He used 3 chassis mounted RCA jacks.

Says Ron "The label was made on PowerPoint on plain paper affixed with two way carpet tape and covered with clear contact paper."

Ron's PowerPoint file is available for download.
The template is pictured below. 
Download here.
Thanks Ron!
Build an FSK interface inside your RigBlaster Nomic - details from Gert, K5WW, here!

Up AA5AU Results FSK Interface RTTY Articles RTTY Radios RTTY Survey

These simple transistor interfaces will work for most radios that are capable of transmitting FSK.  If you find your transmit is inverted, you can normally change the polarity of transmit FSK with a menu option in your transceiver.  Some radios, such as the Ten-Tec Omni Series and Kenwood TS-930, do not have the capability of changing FSK polarity.  These radios require either a PNP transistor or straight through keying circuit.  These circuits are shown at the bottom of this page.

Opto-isolators can also be used instead of transistors.  Bob, W3YY, has produced a very nice high-quality circuit board using opto-isolators.  His boards are available for purchase in kit or assembled/tested form at a reasonable cost.  Check his interface page at http://w3yy.com/fsk.htm.

PNP Transistor FSK Circuit

An FSK circuit employing a PNP transistor (diagram to the left) is used when the FSK circuit using an NPN transistor causes your transmit FSK signal to be inverted and you do not have a menu option in your radio to reverse the polarity of the FSK.  The Kenwood TS-930 requires this circuit to be used.

Ten-Tec Omni Series FSK Circuit

The Ten-Tec Omni 5, Omni 6 and Omni 6+ radios can key FSK simply by coming off pin 3 of the DB 9 serial connector through a 1k ohm resistor.  If you are using a Rigblaster, try this circuit from K3DI, click here.

Modify your Signalink USB for FSK transmission as described by Frank, K7SFN. Check out this great page!