Recalbox includes the retrogame module under the name recalbox-retrogame. This module will allow to use GPIOs to reproduce the behavior of a real USB keyboard under Recalbox.
It only works if you have the line
controllers.gpio.enabled=0in recalbox.conf.
PAUSE/SELECT from an Atari 7800) :
You have to use push buttons (that go up after a press) and connect one terminal to a ground (GROUND) and the other terminal to a GPIO.
In order to launch the module automatically when Recalbox starts, it is important to add a script.
S99retrogame without file extension./usr/bin/recalbox-retrogame
exit 0
chmod 775 /etc/init.d/S99retrogame
/etc/init.d/ directory.ps aux | grep recalbox-retrogame
If the command returns an identifier for a line other than this one, you have succeeded.