Transmitting binary data throught ttyACM - linux

Question

Good afternoon,
I have a peripheral device which communicates over usb over virtual serial port. Everything works well under Windows with generic ACM serial driver, for example with: https://www.kernel.org/doc/Documentation/usb/linux-cdc-acm.inf
Under Linux, it uses CDC ACM drivers. Everything in sys logs seems to work ok, but communication is behaving strangely. When I connect the device, about 10 bytes at the begining of communication are lost. Next, just each second command is received ok.
My questions are:
1) Communication protocol of this device doesn't use ASCII, it is binary (it can randomly contain control characters etc...). Should I use stty for configuring just speed, data bits, stop bits and parity, or something more is necessary to be set up for binary communication? (To ignore control bits in kernel and transmit every byte - raw data.)
2) Any idea, how to test if linux ACM drivers works properly, or which another drivers should i try for my CDC ACM device?
Thanks for any idea!

Answer 1

Linux will often mangle things like line-ending characters (0x0A and 0x0D) when you try to send them over a serial port, which can cause issues if they are actually binary data and not intended as line-ending characters.
Here is a snippet from Pololu that shows how to configure your serial port correctly and then send and receive a few bytes. Pay attention to the part that calls tcsetattr in particular.
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#ifdef _WIN32
#define O_NOCTTY 0
#else
#include <termios.h>
#endif
// Gets the position of a Maestro channel.
// See the "Serial Servo Commands" section of the user's guide.
int maestroGetPosition(int fd, unsigned char channel)
{
unsigned char command[] = {0x90, channel};
if(write(fd, command, sizeof(command)) == -1)
{
perror("error writing");
return -1;
}
unsigned char response[2];
if(read(fd,response,2) != 2)
{
perror("error reading");
return -1;
}
return response[0] + 256*response[1];
}
// Sets the target of a Maestro channel.
// See the "Serial Servo Commands" section of the user's guide.
// The units of 'target' are quarter-microseconds.
int maestroSetTarget(int fd, unsigned char channel, unsigned short target)
{
unsigned char command[] = {0x84, channel, target & 0x7F, target >> 7 & 0x7F};
if (write(fd, command, sizeof(command)) == -1)
{
perror("error writing");
return -1;
}
return 0;
}
int main()
{
const char * device = "/dev/ttyACM0"; // Linux
int fd = open(device, O_RDWR | O_NOCTTY);
if (fd == -1)
{
perror(device);
return 1;
}
#ifdef _WIN32
_setmode(fd, _O_BINARY);
#else
struct termios options;
tcgetattr(fd, &options);
options.c_iflag &= ~(INLCR | IGNCR | ICRNL | IXON | IXOFF);
options.c_oflag &= ~(ONLCR | OCRNL);
options.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
tcsetattr(fd, TCSANOW, &options);
#endif
int position = maestroGetPosition(fd, 0);
printf("Current position is %d.\n", position);
int target = (position < 6000) ? 7000 : 5000;
printf("Setting target to %d (%d us).\n", target, target/4);
maestroSetTarget(fd, 0, target);
close(fd);
return 0;
}
You might be able to do the same thing with the stty command line utility.