Re: Configuration of the RGA200 communication baud rate

["Dmitry Yu. Bolkhovityanov via Tech-talk" <tech-talk at aps.anl.gov>]

Hi guys!

Setting non-standard baud rates is tricky (as it is poorly documented), 
however I've managed to get though it some time ago (not for EPICS) and 
such setting should work reliably since Linux kernel 2.6.0.
I used PicoCom terminal emulator sources for reference, see 
https://urldefense.us/v3/__https://github.com/npat-efault/picocom/blob/master/termios2.c__;!!G_uCfscf7eWS!cKSRpZwLVot9cUcgFgCyKPu1XBIzaNM5bhgYzBGmbpTOcDrTLGFxoqSE60Tlfy-SfXwVAkYC3g36NVWUr-FxXyMaPWrkNyhfaJg$ 

Attached are the code to set arbitrary baud rate 
(set_tty_speed_via_termios2.c) and a sample program to test it (ttycat.c).
In my tests such non-standard setting works not only for hardware serial 
ports, but for USB-232/485 adapters too.  Looking into kernel sources 
suggests that Linux uses bitrate numbers internally, while Bnnnnn are for 
compatibility only and, AFAIR, translation between Bnnnnn and baudrates 
occur on the userspace/kernelspace border.
BTW, recent glibc 2.42 changelog (see 
https://urldefense.us/v3/__https://elixir.bootlin.com/glibc/glibc-2.42.9000/source/NEWS__;!!G_uCfscf7eWS!cKSRpZwLVot9cUcgFgCyKPu1XBIzaNM5bhgYzBGmbpTOcDrTLGFxoqSE60Tlfy-SfXwVAkYC3g36NVWUr-FxXyMaPWrkz6ln3SA$ ) contains 
following statement:
* On Linux, the <termios.h> interface now supports arbitrary baud rates;
  speed_t is redefined to simply be the baud rate specified as an
  unsigned int, which matches the kernel interface.


With best regards,
Dmitry


On Mon, 12 Jan 2026, Michael Davidsaver via Tech-talk wrote:

> On 1/12/26 11:26 AM, Johnson, Andrew N. via Tech-talk wrote:
> >  This blog post
> >  <https://urldefense.us/v3/__https://gcc02.safelinks.protection.outlook.com/?url=https*3A*2F*2Fwww.downtowndougbrown.com*2F2013*2F11*2Flinux-custom-serial-baud-rates*2F&data=05*7C02*7Ctech-talk*40aps.anl.gov*7Ca3836f92114248b786c808de52108515*7C0cfca18525f749e38ae7704d5326e285*7C0*7C0*7C639038428051304183*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=XQMOLbcC0dzIVGEFDyotqFGAC*2B65cr8Qv*2Fr3188e0io*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJSU!!G_uCfscf7eWS!bTCnxDZJfSsva3HtMnmfiFOH98GYpHn8QhyD8XV1R2Iw5jme4gFhbhJxsC1rv4xujo4gVHPVQiXMcaMT$> might
> >  be helpful on how to modify Asyn to request non-standard baud rates on
> >  some Linux systems at least.
> A quick glance at "man termios" makes mention of "ioctl(TCSETS2)" in 
> conjunction with "c_cflag |= BOTHER" (a great name!) to request arbitrary 
> baud rates.  Might make a nice codeathon project to add this as a fallback, 
> if only for Linux.
> See EXAMPLES from:
>
> https 
> ://https://urldefense.us/v3/__http://www.man7.org/linux/man-pages/man2/tcsets2.2const.html__;!!G_uCfscf7eWS!dbq7XA-2qQHA82PjWanr52eEDuu_YD7tR8vkfGlbuwQZ9IUjv5XNpMROUgbYZlcqbJPXLf0GevpB_mB0iknfIIqBQw$ 
>
> >  - Andrew
> >
> >
> >  -- 
> >
> >  Complexity comes for free, Simplicity you have to work for.
> >
> >
> >  On 1/12/26, 12:26?PM, "Tech-talk" <tech-talk-bounces at aps.anl.gov> wrote:
> >
> >  My quick google search suggests that it?s necessary to write a program to
> >  do this.
> >  For python it recommends:
> >
> >      import serial
> >
> >      ser = serial.Serial('/dev/ttyS0', 28800)
> >
> >      print(ser.baudrate)
> >
> >  I tried this on Debian and it ran with no complaints.
> >
> >      On Jan 12, 2026, at 9:53?AM, Mark Rivers <rivers at cars.uchicago.edu>
> >      wrote:
> >
> >      Hi Eric,
> >
> >      I have an RHEL 9 system.  How do I set an arbitrary baud rate using
> >      termios2 from the shell?  When I use stty I get an error:
> >
> >      [epics@13idd ~]$ stty -F /dev/ttyUSB0 28800
> >      stty: invalid argument ?28800?
> >      Try 'stty --help' for more information.
> >
> >      Thanks,
> >      Mark
> >
> >
> >  ?
> >  Eric Norum
> >  eric at norum.ca
#ifndef __SET_TTY_SPEED_VIA_TERMIOS2
#define __SET_TTY_SPEED_VIA_TERMIOS2


int set_tty_speed_via_termios2(int fd, unsigned int cflag, int speed);


#endif /* __SET_TTY_SPEED_VIA_TERMIOS2 */

#ifdef __linux__

// Check copied from https://github.com/npat-efault/picocom/blob/master/termios2.c
#include <linux/version.h>
#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,0)
  #warning "Linux kernel is too old for termios2"
#else

#include <stdio.h>
#include <strings.h>
#include <sys/ioctl.h>
#include <asm/termbits.h>

#include "set_tty_speed_via_termios2.h"


int set_tty_speed_via_termios2(int fd, unsigned int cflag, int speed)
{
  struct termios2  t2;
  int              r;

    bzero(&t2, sizeof(t2));
    t2.c_cflag     = (cflag &~ (CBAUD | CBAUDEX)) | BOTHER;
    t2.c_iflag     = IGNPAR;
    t2.c_oflag     = 0;
    t2.c_lflag     = 0;
    t2.c_cc[VTIME] = 0;
    t2.c_cc[VMIN]  = 1;
    t2.c_ispeed    = speed;
    t2.c_ospeed    = speed;

    r = ioctl(fd, TCSETS2, &t2);
#ifdef DEBUG_TERMIOS2 // Use "-DDEBUG_TERMIOS2" switch in compiler commandline
    {
      struct termios2  t2_rb;

        ioctl(fd, TCGETS2, &t2_rb);
        fprintf(stderr, "i=%d o=%d\n", t2_rb.c_ispeed, t2_rb.c_ospeed);
    }
#endif
    return r;
}

#endif /* LINUX_VERSION_CODE */

#endif /* __linux__ */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <strings.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>

#ifdef __linux__
  #include <linux/version.h>
  #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,0)
    #warning "Linux kernel is too old for termios2"
  #else
    #include "set_tty_speed_via_termios2.h"
    #define MAY_USE_TERMIOS2 1
  #endif
#endif /* __linux__ */


int set_fd_flags  (int fd, int mask, int onoff)
{
  int  r;
  int  flags;

    /* Get current flags */
    flags = r = fcntl(fd, F_GETFL, 0);
    if (r < 0) return -1;

    /* Modify the value */
    flags &=~ mask;
    if (onoff != 0) flags |= mask;

    /* And try to set it */
    r = fcntl(fd, F_SETFL, flags);
    if (r < 0) return -1;

    return 0;
}

int main(int argc, char *argv[])
{
  enum {N_CON = 0, N_TTY = 1};

  int     fdpair[2];  // FDs for select() and read()
  int     wrpair[2];  // fds for write()
  fd_set  rfds;
  int     n;
  int     maxfd;
  int     r;
  char    buf[100];
  struct termios  newtio;

    if (argc < 3)
    {
        fprintf(stderr, "Usage: %s /dev/ttySOME_TTY_NAME SPEED_IN_BAUDS_PER_SECOND\n", argv[0]);
        return 1;
    }

    fdpair[N_CON] = 0; // stdin
    if (isatty(fdpair[N_CON])  &&  argc <= 3) // Switch to noncanonical mode (so that input is available immediately) if we are at terminal and this switch is NOT explicitly forbidden by argv[3] presence
    {
        tcgetattr(fdpair[N_CON],          &newtio);
        //cfmakeraw                    (&newtio); // Use with caution!  Ctrl+C,Ctrl+Z, etc. stop working and these changes remain in effect even after program termination
        newtio.c_lflag |=  ECHO;
        newtio.c_lflag &=~ ICANON;
        tcsetattr(fdpair[N_CON], TCSANOW, &newtio);
    }
    set_fd_flags(fdpair[N_CON], O_NONBLOCK, 1);

    fdpair[N_TTY] = open(argv[1], O_RDWR | O_NOCTTY | O_NONBLOCK);
    if (fdpair[N_TTY] < 0)
    {
        perror("open()");
        return 2;
    }
    bzero(&newtio, sizeof(newtio));
    newtio.c_cflag     = B9600 | CS8 | CLOCAL | CREAD;
    newtio.c_iflag     = IGNPAR;
    newtio.c_oflag     = 0;
    newtio.c_lflag     = 0;
    newtio.c_cc[VTIME] = 0;
    newtio.c_cc[VMIN]  = 1;
    tcflush(fdpair[N_TTY], TCIOFLUSH);
#if MAY_USE_TERMIOS2
    r = set_tty_speed_via_termios2(fdpair[N_TTY], newtio.c_cflag, atoi(argv[2]));
#else
    r = tcsetattr(fdpair[N_TTY], TCSANOW, &newtio);
#endif
    if (r != 0)
    {
        perror("tcsetattr()");
    }

    wrpair[N_CON] = 1; // stdout
    wrpair[N_TTY] = fdpair[N_TTY];

    while (1)
    {
        FD_ZERO(&rfds);
        for (maxfd = -1, n = 0;  n < 2;  n++)
            if (fdpair[n] >= 0)
            {
                FD_SET(fdpair[n], &rfds);
                if (maxfd < fdpair[n])
                    maxfd = fdpair[n];
            }
        if (maxfd < 0) return 0;

        r = select(maxfd + 1, &rfds, NULL, NULL, NULL);
        if (r < 0)
        {
            perror("select()");
            return 3;
        }

        for (n = 0;  n < 2;  n++)
            if (fdpair[n] >= 0  &&  FD_ISSET(fdpair[n], &rfds))
            {
                r = read(fdpair[n], &buf, sizeof(buf));
                if (n == N_TTY  &&  r <= 0) return 0; // Nothing more to do if serial link is inoperational (r==0 means "USB-485 adapter plugged out", so we must release the /dev/ttyUSB* file for adapter to get the same /dev/ file upon re-plugging)
                if (r < 0)
                {
                    fdpair[n] = -1;
                }
                else
                    write(wrpair[1-n], &buf, r);
            }
    }

    return 0;
}