Just found out that with a sudo screen /dev/ttyS_56_0 38400
I can open a terminal using the specified tty, so I can check if commands reach the device. It works
. So, I now have a working chain from messaging to /dev/ttyS_56_0, to the GC100, through the RS-232 cable, to the amplifier. Next step: make sure that messages get sent to the right COM from Dianemo / LMCE.