Re: serial port question.
- From: floyd@xxxxxxxxxx (Floyd L. Davidson)
- Date: Tue, 30 May 2006 14:06:59 -0800
chris dewbery <chris.dewbery@xxxxxxxxxxxxxx> wrote:
here is a snippet of code which sets the serial port to
19200 8E1. where fd is the file descriptor for the open serial port.
int fd = open( "/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY );
For portability, O_NONBLOCK is preferred over O_NDELAY. They
are exactly identical on Linux (and most other systems), but
have subtle differences on some platforms that make O_NDELAY
non-portable.
struct termios options;
memset(&options, 0, sizeof( struct termios ) );
/* set baud rate to 19200 */
cfsetispeed(&options, B19200);
cfsetospeed(&options, B19200);
options.c_cflag |= (CLOCAL | CREAD);
Why use an OR function, when you know that the initial value
is 0?
options.c_cflag |= PARENB;
options.c_cflag &= ~PARODD; /* Even parity */
options.c_cflag &= ~CSTOPB; /* 1 Stop bit */
options.c_cflag &= ~CSIZE;
Why clear bits, when the initial value is 0?
options.c_cflag |= CS8; /* 8 data bits */
options.c_cflag &= ~CRTSCTS;
All of the above can be a one liner:
options.c_cflag = (CLOCAL | CREAD | CS8 | PARENB);
options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
options.c_iflag |= (IGNPAR); /* Ignore parity */
Why enable parity and then ignore it on input?
options.c_iflag &= ~(IXON | IXOFF | IXANY);
tcsetattr(fd, TCSANOW, &options);
This call really should be checked for an error return.
tcflush( fd, TCIOFLUSH );
This call should probably be done prior to calling tcsetattr().
you should then be able to just send and receive data by using the
commands.
Note that you have left the port in non-blocking mode. Likewise
the termios settings are for raw mode and the VTIME and VMIN
elements of the c_cc array are 0. That is usable configuration
to be sure, but the code to read and write to the port is very
dependent upon that configuration, and when used as an example
that should be explicitly pointed.
write( fd, buffer, length );
The return value of write(2) is important, and this call
necessarily must be in a loop (unless "length" is 0). If a
value of -1 is returned the value of errno has to be checked to
determine if the loop should continue or not. The write
function also may or may not write the entire block of bytes
specified by "length", and the loop must handle short writes.
Given the non-blocking and raw mode configuration, the loop
should also include a delay between non-productive calls to
avoid hogging the CPU.
bytes = read( fd, buffer, sizeof(buffer));
This also needs to be called in a loop, for the same reasons as
apply to the write(2) function.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.
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