Can a serial port be left in a dirty state? - linux

Question

I'm writing a program to communicate with an existing device via serial port, and I am noticing a weird pattern. This is being tested with both a real serial port, and a USB-to-serial adapter. I get the same results for both.
Immediately after booting up the computer or plugging in the adapter, serial port communication works fine. I can send binary data to the device, and get a response back. The program can continue to communicate with the device as long as it wants.
However, once the program ends (cleanly closing the port), running the program again results in failure to communicate. It can access the serial port just fine, but all it gets back is garbage.
(Oddly, the garbage appears to be binary data mixed with modem commands like ATE0Q0S0=0, which makes no sense. Again, I don't need to reset the device to communicate with it, just the port, so I don't know where this is coming from.)
Power-cycling or unplugging the device has no effect. It is only when I reboot the computer, or reset the USB device (via unplug, or driver reset), that I can run the program once more and get it to communicate successfully.
What would cause this? From the results, I can only assume that the serial port is not being left in a clean state after use, but I can find no documentation about properly cleaning the serial port state, other than re-applying ioctl attributes and closing the file descriptor after use, both of which I already do.
Maybe a serial port pin gets left on or something? I don't know how I would test for that, or why it would even happen.
My current "solution" is to just stick with the USB adapter, and have my program perform a USB driver reset before attempting to use the serial port, but I'm hoping there is a better solution.
Edit
As requested, here is the C program I'm using to test the serial port read/write.
include
#include <fcntl.h>
#include <inttypes.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <time.h>
#include <unistd.h>
// Saved termios that we can re-apply when we exit
struct termios savedPortAttributes;
// Set the serial port attributes so we can use it
void setPortAttributes( int fd )
{
struct termios tty;
memset( &tty, 0, sizeof(tty) );
if ( tcgetattr( fd, &tty ) != 0 ) {
printf( "tcgetaddr error: $i\n", errno );
return;
}
cfsetispeed( &tty, B9600 );
cfsetospeed( &tty, B9600 );
cfmakeraw( &tty );
tty.c_lflag = 0;
tty.c_oflag = 0;
tty.c_iflag &= ~(IXON | IXOFF | IXANY);
tty.c_cflag |= (CLOCAL | CREAD);
tty.c_cflag &= ~(PARENB | PARODD);
tty.c_cflag &= ~CSTOPB;
tty.c_cflag &= ~CRTSCTS;
tty.c_cc[VMIN] = 0;
tty.c_cc[VTIME] = 5;
if ( tcsetattr( fd, TCSANOW, &tty ) != 0 ) {
printf( "tcsetaddr error: $i\n", errno );
return;
}
if ( tcflush( fd, TCIOFLUSH ) != 0 ) {
printf( "tcflush error: $i\n", errno );
return;
}
}
void test( int fd )
{
// Send a sample MODBUS command
printf( "Writing command\n" );
char sendBuffer[] = { 0x01, 0x03, 0x00, 0x0B, 0x00, 0x02, 0xB5, 0xC9 };
int bytesWritten = write( fd, sendBuffer, sizeof(sendBuffer) );
if ( bytesWritten < 0 ) {
printf( "Error writing command.\n" );
return;
}
// We don't want to wait more than 1000ms for a response
struct timespec spec;
clock_gettime( CLOCK_MONOTONIC, &spec );
int64_t startMs = spec.tv_sec * 1000 + round( spec.tv_nsec / 1.0e6 );
// Read data back from the port
printf( "Reading from port...\n" );
unsigned char buffer[1024];
int bufferOffset = 0;
int count = 0;
while ( 1 ) {
count = read( fd, &buffer[bufferOffset], sizeof(buffer) - bufferOffset );
if ( count < 0 ) {
printf( "Error reading command.\n" );
return;
}
if ( count > 0 ) {
printf( "Bytes read: " );
for ( int i = bufferOffset; i < bufferOffset + count; i++ ) {
printf( "%02x ", buffer[i] );
}
printf( "\n" );
}
bufferOffset += count;
// Test code. If we receive part of a valid MODBUS response, grab the
// field length byte so we know if we're done reading
if ( bufferOffset >= 3 && buffer[0] == 1 && buffer[1] == 3 ) {
int messageLength = buffer[2];
if ( bufferOffset >= messageLength + 5 ) {
break;
}
}
// If it's been 1000ms, stop reading
clock_gettime( CLOCK_MONOTONIC, &spec );
int64_t timeMs = spec.tv_sec * 1000 + round( spec.tv_nsec / 1.0e6 );
//printf( "%" PRId64 " , %" PRId64 "\n", startMs, timeMs );
if ( timeMs - startMs > 1000 ) {
break;
}
}
}
void main()
{
printf( "Opening port\n" );
int fd = open( "/dev/ttyUSB0", O_RDWR|O_NOCTTY );
if ( fd == -1 ) {
printf( "Unable to open port.\n" );
return;
}
tcgetattr( fd, &savedPortAttributes );
setPortAttributes( fd );
test( fd );
test( fd );
tcsetattr( fd, TCSANOW, &savedPortAttributes );
close( fd );
}