Questions related to stty(TOMITA)

[富田黎]

Hi,

I want to know the difference between stty command with "speed" and without 
"speed".

When I change the baud rate of serial connected terminal (ttyS0), I don't know 
how the presence or absence of speed affects it as shown below.

$ stty -F /dev/ttyS0 [baudrate]
$ stty -F /dev/ttyS0 speed [baudrate]

I know the official recommends the latter, but even with the former you could 
change the baud rate in some cases and I'd like to know what the difference is.

For example, I was able to resolve the garbled ttyS0 with the former, but when 
I accessed ttyS0 with gpsd, I saw no change in baudrate.

■Garbled characters
$ cat /dev/ttyS0
→garbled text
→$ stty -F /dev/ttyS0 [baudrate]
→$ cat /dev/ttyS0
→Resolution of garbled characters

■About gpsd
$ gpsmon /dev/ttyS0
→The default baudrate
→ $ stty -F /dev/ttyS0 [baudrate]
→ $ gpsmon /dev/ttyS0
→Keep the default baudrate.
→ $ stty -F /dev/ttyS0 speed [baudrate]
→$ gpsmon /dev/ttyS0
→Confirm the changes to the baurate you set

■Environment
Raspberry Pi 4
Rasbian Buster
UART RX=GPIO 15=ttyS0

Sincerely,