Writing binary data over serial in windows - c++

Question

I need to send binary data over a serial port, without any bytes getting reinterpreted as control characters along the way. I'm currently setting up my serial port as follows:
#include <windows.h>
// open serial port
HANDLE hSerial;
hSerial = CreateFile ("COM1", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
// get serial parameters
DCB dcbSerialParams = {0};
dcbSerialParams.DCBlength = sizeof (dcbSerialParams);
if (!GetCommState(hSerial, &dcbSerialParams)) {
cout << "error getting state\n";
exit(0);
}
// set serial params
dcbSerialParams.BaudRate = CBR_115200;
dcbSerialParams.ByteSize = 8;
dcbSerialParams.StopBits = ONESTOPBIT;
dcbSerialParams.Parity = NOPARITY;
if (!SetCommState (hSerial, &dcbSerialParams)) {
cout << "error setting parameters\n";
exit(0);
}
// set time outs
COMMTIMEOUTS timeouts = {0};
timeouts.ReadIntervalTimeout = 50;
timeouts.ReadTotalTimeoutConstant = 10;
timeouts.ReadTotalTimeoutMultiplier = 10;
timeouts.WriteTotalTimeoutConstant = 10;
timeouts.WriteTotalTimeoutMultiplier = 10;
if (!SetCommTimeouts (hSerial, &timeouts)) {
cout << "problem setting timeout values\n";
exit(0);
} else cout << "timeouts set\n";
When I issue ReadFile commands, I can get and display bytes from 0 to 255 with no problem. but I'm having no such luck with WriteFile. Is there a way to explicitly set a binary write mode?
EDIT
Ok, here's some more info. I have a windows machine and a linux single board computer hooked up through serial, the code above on the windows side is followed by:
unsigned char temp = 0;
bool keepReading = true;
while (keepReading) {
DWORD dwBytesRead = 0;
ReadFile (hSerial, &temp, 1, &dwBytesRead, NULL);
if (1 == dwBytesRead) cout << (unsigned int) temp << " ";
if (255 == temp) keepReading = false;
}
cout << endl;
bool keepWriting = true;
char send = 0;
while (keepWriting) {
DWORD dwBytesWritten = 0;
WriteFile (hSerial, &send, 1, &dwBytesWritten, NULL);
send++;
if (256 == send) keepWriting = false;
}
My code on the linux side looks like this:
int fd = open("/dev/ttymxc0", O_RDWR | O_NOCTTY);
struct termios options;
bzero (options, sizeof(options));
options.c_cflag = B115200 | CS8 | CLOCAL | CREAD;
options.c_iflat = IGNPAR;
options.c_oflag = 0;
options.c_lflag = ICANON;
options.c_cc[VMIN] = 1;
options.c_CC[VTIME] = 0;
tcflush (fd, TCIFLUSH);
tcsetattr (fd, ICSANOW, &options);
bool keepWriting = true;
char send = 0;
while (keepWriting) {
write (fd, &send, 1);
send++;
if (256 == send) keepWriting = false;
}
bool keepReading = true;
while (keepReading) {
char temp = 0;
int n = read (fd, &temp, 1);
if (-1 == n) {
perror ("Read error");
keepReading = false;
} else if (1 == n) {
cout << temp << " ";
}
if (256 == temp) keepReading = false;
}
cout << endl;
close(fd);
I start up the code on both machines, and the first set of while loops runs fine. The terminal on the windows side displays 0 through 255. Then it just sits there. If I output the number of bytes read on the linux side for the second set of while loops, it continually gives me 0 bytes. This would indicate a closed port normally, but I just sent a bunch of info through it so how could that be?

Answer 1

As Jonathan Potter mentions, most likely you don't have XON/XOFF flow control turned off. Add these lines before the call to SetCommState:
dcbSerialParams.fOutX = 0;
dcbSerialParams.fInX = 0;
Some other fields which you may need to set:
dcbSerialParams.fNull = 0;
dcbSerialParams.fDtrControl = DTR_CONTROL_DISABLE;
dcbSerialParams.fRtsControl = RTS_CONTROL_DISABLE;

Answer 2

I think what may be happening is that Linux is detecting a break, and resetting the port, or the fact that canonical mode is set is messing it up. Try these settings in addition to what you have already:
options.c_iflag |= IGNBRK;
options.c_iflag &= ~BRKINT;
options.c_iflag &= ~ICRNL;
options.c_oflag = 0;
options.c_lflag = 0;

Answer 3

Alright so I figured it out, rather, a co-worker did. On the linux side, in the file /etc/inittab I had to comment out the line:
T0:23:respawn:/sbin/getty -L ttymxc0 115200 vt100
This was grabbing the serial port in a way that made it unusable for receiving bytes. I now see the expected output.