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NMDM(4) Device Drivers Manual NMDM(4)

nmdm — nullmodem terminal driver

To compile this driver into the kernel, place the following line in your kernel configuration file:

device nmdm

Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):

nmdm_load="YES"

The nmdm driver provides two tty(4) devices connected by a virtual “null modem” cable.

If either of the two tty devices have the CDSR_OFLOW bit (“stty dsrflow”) set in their line discipline, the nmdm device will emulate the speed configured in the termios(4) settings. The speed emulation works independently in the two directions, controlled by the slower end's termios settings (c_ispeed, c_ospeed, CS5 ... CS8, CSTOPB and PARENB).

/dev/nmdmN[AB]
nullmodem device nodes. Where the A node has a matching B node.

The nmdm driver implements “on-demand device creation” so simply accessing a given instance in /dev will create it.

None.

stty(1), termios(4), tty(4)

The nmdm driver first appeared in FreeBSD 4.4.

January 23, 2018 FreeBSD-12.0