Hello, I made the following setup work, that is I can send break and '?' (to get the magic sysrq help) or 's' to do an Emergency sync, and the kernel logs it: laptop apu2 USB serial port ------- null modem ------- ttyS0 internal 16550A (an apu2 is an embedded amd64 computer [4]) As it works, of course MAGIC_SYSRQ is enabled, including for serial ports, and the correct value is in the /proc pseudo-file. It works with the getty enabled or disabled. However, the following does not work to support magic sysrq, although bidirectionnal communication also works with cu [5], with the correct speed set: laptop apu2 USB serial port ------- null modem ------- USB serial port First, reading documentation, I thought that this would not be possible with USB and only with the internal serial port [1], but then, reading kernel source, it looks it should work with my adapter: Oct 11 14:30:56 apu2-init7 kernel: [ 9.915105] usb 2-2: pl2303 converter now attached to ttyUSB0 since the driver [2] contains code for magic sysrq, see line 993 for sysrq mode and line 892 for break handling, with implementation in [3] (lines 589-597). I am running Debian buster kernel 4.19.0-18-amd64 on the apu2. Should I abandon all hope to make it work with USB, or should it work? Thank your for any pointers. [1] https://www.kernel.org/doc/Documentation/admin-guide/sysrq.rst "On the serial console (PC style standard serial ports only)" [2] https://github.com/jplozi/linux-4.19/blob/loadbalancing/drivers/usb/serial/p... [3] https://github.com/jplozi/linux-4.19/blob/loadbalancing/drivers/usb/serial/g... [4] https://pcengines.ch/apu2.htm [5] https://linux.die.net/man/1/cu from the days before 2003 where I was doing UUCP cu -l ttyUSB0 -s 9600