Adding PTT Activation to the Serial and Sound-Card Interface
February 23, 2011 3 Comments
One of the things I’d like to try soon is the JT65 digital mode. Like WSPR, it works even with very weak signals, but unlike WSPR, it allows two-way communication rather than beaconing. When trying out WSTJ, the program that runs JT65, I discovered that it switches the transceiver from receive to transmit modes using the DTR or RTS signals on a serial port, not using CAT commands (multi-byte commands through the serial port). JT65-HF, another program that supports JT65, does the same.
I therefore decided to add receive-transmit switching (PTT) to the serial/sound-card interface I hacked together for the FT-857D. I checked the DTR and RTS signals on the FT232R breakout board and discovered that both were active high, meaning that they are high most of the time, but when a program like WSPR or WSJT wants to transmit, it activates DTR or RTS, which brings them low, to near 0V. The PTT connection in the transceiver should be driven by an open-collector transistor (e.g., an NPN transistor with the collector connected to the PTT line and emitter to ground), and it’s easier to drive the transistor with an active-high logic signal. Fortunately, the maker of the FT232R, FTDI, makes available a utility called MProg that can configure these signals to active-high. I ran it, configured the chip, and the signals indeed got inverted. I added the NPN transistor to the strip board, it’s base connected to RTS via a 2.2kΩ transistor (PN2222). Now these programs can switch the transceiver to transmit using the RTS signal.
I didn’t have time to try JT65 yet (there’s a learning curve there), but I’m now ready, hardware wise.