PTT and band switching

In order to make the Portsdown transmitter integrated with transmit and receive systems, Pin 40 of the GPIO goes high when transmit is selected and the signals on pins 28 and 35 can be used for band switching:

Frequency Mhz	ATV Band	Pin 28	Pin 35
< 100 MHz	71 MHz	Lo	Lo
100 - 250 MHz	146 MHz	Lo	Hi
250 - 950 MHz	437 MHz	Hi	Lo
950 - 4400 MHz	23cms	Hi	Hi

Care should be taken to properly buffer these 3.3v signals from relay transients.

The band outputs can be used to "steer" the PTT line to the correct Power Amplifier using relays or AND gates - we also recommend the use of a Tx / Rx sequencer to ensure your mast head relay has changed over and your expensive pre-amp is de-powered before you start transmitting.

Many circuits and prebuilt designs are available:

Also rather than use these pins to switch an expensive RF relay on the output of Portsdown, take a look at this simple idea to use a duplexer / triplexer.