ozQRP MST3 Construction And Operation Manual page 13

The ITM firmware continuously monitors the PTT switch, Morse key and Tone pushbutton. The
inputs are held high by internal pull-up resistors and filtered against RF with 1nF capacitors.
The Morse key and PTT switch are wired in parallel but are interpreted differently depending
on whether it is in SSB or CW mode.
The ITM also controls the audio mute line and the PTT line. In normal default SSB mode, the PTT
switch is passed straight to the PTT line.
At power up the ITM emits two short beeps to signal a successful boot up.
The ITM is supplied with +5V by a 78L05 regulator.
MST3 Construction and Operation Manual – Issue 1 Page 13