A segunda imagem está correta. (BTW, ele vem a partir daqui .)
Da resposta aceita em esta questão :
The line discipline is a software module (residing in the driver, in the kernel) pushed on top of a serial/pty device that sits between that device and the line/wire (the master side for a pty).
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You can see the effect of disabling the tty line discipline by issuing stty raw -echo (note that the bash prompt or other interactive applications like vi set the terminal in the exact mode they need, so you want to use a dumb application like cat to experience with that). Then, everything that is written to the slave terminal device makes it immediately to the master side for xterm to read, and every character written by xterm to the master side is immediately available for reading from the slave device.
Veja também este post sobre ttys.