How can I set a custom baud rate on Linux

Question

I want to communicate over my serial port on Linux to a device with a non-standard-baud rate that is not defined in termios h   I tried the  baud rate aliasing -method from this post  but when I execute my C-program  I   ve named it  testprogram    Linux says  testprogram sets custom speed on ttyS0  This is deprecated    I did some search on Google  and it seems that there is another  newer   method to change the baud rate to a non-standard-value  On http   sourceware org ml libc-help 2009-06 msg00016 html the author says that the c flag of struct termios must be OR   d with BOTHER   CBAUDEX   B0    With this method the baud rates are set directly in the c ispeed and c ospeed-members of the struct termios  However  I don   t know how I use this method in my C program  Like the author said  there is no BOTHER defined available when I include termios h  so what should be done to set the baud rate this way   How can I set the baud rate to a non-standard-value without changing the kernel

User · Answer

You can just use the normal termios header and normal termios structure (it's the same as the termios2 when using header asm/termios).

So, you open the device using open() and get a file descriptor, then use it in tcgetattr() to fill your termios structure.

Then clear CBAUD and set CBAUDEX on c_cflag. CBAUDEX has the same value as BOTHER.

After setting this, you can set a custom baud rate using normal functions, like cfsetspeed(), specifying the desired baud rate as an integer.

User · Answer

You can set a custom baud rate using the stty command on Linux  For example  to set a custom baud rate of 567890 on your serial port  dev ttyX0  use the command   stty -F  dev ttyX0 567890

User · Answer

For Mac users  possibly also for some Linux distributions   stty ospeed 999999  stty ispeed 999999

User · Answer

There is an serial I O chip on your motherboard s CPU  16650 UART   This chip uses 8-bit port as control and data bus  and thus you can issue a command to it through writing to this chip through the control and data bus   Usually  an application did the following steps on the serial port   Set baud rate  parity  encoding  flow control  and starting   ending sequence length during program start  This setup can be done via ioctl to the serial device or  stty  command  In fact  the stty command uses ioctl to that serial device  Write characters of data to the serial device and the driver will be writing data charaters to the UART chip through its 8-bit data bus    In short  you can specify the baud rate only in the STTY command  and then all other options would be kept as default  and it should enough to connect to ohter devices

User · Answer

I noticed the same thing about BOTHER not being defined  Like Jamey Sharp said  you can find it in  lt asm termios h gt   Just a forewarning  I think I ran into problems including both it and the regular  lt termios h gt  file at the same time   Aside from that  I found with the glibc I have  it still didn t work because glibc s tcsetattr was doing the ioctl for the old-style version of struct termios which doesn t pay attention to the speed setting  I was able to set a custom speed by manually doing an ioctl with the new style termios2 struct  which should also be available by including  lt asm termios h gt    struct termios2 tio   ioctl fd  TCGETS2   amp tio   tio c cflag  amp    CBAUD  tio c cflag    BOTHER  tio c ispeed   12345  tio c ospeed   12345  ioctl fd  TCSETS2   amp tio

User · Answer

BOTHER appears to be available from  lt asm termios h gt  on Linux  Pulling the definition from there is going to be wildly non-portable  but I assume this API is non-portable anyway  so it s probably no big loss

User · Answer

dougg3 has this pretty much  I can t comment there   The main additional thing you need to know is the headers which don t conflict with each other but do provide the correct prototypes  The answer is   include  lt stropts h gt   include  lt asm termios h gt    After that you can use dougg3 s code  preferably with error checking round the ioctl   calls  You will probably need to put this in a separate  c file to the rest of your serial port code which uses the normal termios to set other parameters  Doing POSIX manipulations first  then this to set the custom speed  works fine on the built-in UART of the Raspberry Pi to get a 250k baud rate

[linux] How can I set a custom baud rate on Linux?

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