Allow an external event to start transmission.

[wb2osz]

Use case: Someone wants to send a particular DTMF sequence, over the radio, to cause a remote camera to capture an image then send it over the radio.

Discussion: https://groups.io/g/RaspberryPi-4-HamRadio/topic/87661253#14112

PCSI would be ideal for this, but, I did not notice any method for some external application to start transmission of an image. Only manually clicking a button on the screen.

One possible approach might go like this:
(1) The direwolf software TNC has a builtin DTMF ("touch tone") decoder. Tone sequences, terminated by # are converted to a specially formatted packet and sent to attached applications for processing.
(2) User writes a little script to watch for a packet containing the command string. It could be from DTMF or any other origin.
(3) Script instructs camera to capture an image then instructs PCSI to send it. (There are many ways to accomplish this. A simple one would be for PCSI to watch some directory for new images and transmit them when not busy then delete the image file.)

Another possible approach:
(1) TNC receives packets over the radio. In the case of direwolf, it can convert DTMF sequences to specially formatted packets.
(2) PCSI has an option to look for a specific pattern in incoming packets.
(3) When this pattern is recognized, it runs a user-supplied script which is expected to place an image file somewhere. This could be from a camera but it could be from other sources as well.
(4) That image file is transmitted and deleted.