Sunday, November 20, 2011


In the APRS world a new piece of software has been creating some excitement. It is a soundcard modem for packet radio by UZ7HO. It runs under Windows and emulates the AGW Packet Engine so that it can be used by any program that is designed to work with it. The reason for the excitement is that this packet modem can decode several packet signals on slightly different frequencies in parallel, resulting in many more decodes on HF where it is quite common for stations to be 50Hz or more off-frequency.

Unfortunately I haven't been able to try the new packet engine as the PTT won't key my Elecraft K2 and I don't know why. My K2 CAT cable has a transistor switch on the DTR line which goes to a 3.5mm jack plug that plugs in the K2 key socket. This was originally used for computer Morse keying using software like MixW, but it can also be used for PTT with digital mode software as the key and PTT lines are common. Using fldigi and even using a serial port test program I am able to activate the DTR line on COM2 and the K2 will respond by switching to transmit. But if I use this software modem no PTT ever occurs so although the audio is generated the packet signal is never transmitted. If I was using a SignalLink or other device such as my homebrew USBlink with fast audio derived VOX then this wouldn't matter but as I have a PTT connection on this serial port I'd really prefer to make use of it.

If I change the serial port to COM3 then the program will key my Elecraft K3, which uses a different serial cable but still uses DTR for PTT. I tried the original AGW Packet Engine, both free and paid-for Pro versions but that won't key the K2 either. So the problem must be something to do with my K2 CAT cable. But my poor old brain has been having rather a tough time recently with all the treatment making me feel like a bit of a zombie and I'm finding it rather difficult to think things through logically and find the solution which is probably staring me in the face!


Casey Bahr said...


Did you compare properties in Device Manager between the different COM ports?

Julian Moss said...

Yes, they are both the same. The problem follows the cable.