Accessing serial ports under Windows

[Odud]

Has anyone experience of using the serial ports under Windows? I'm trying to send commands to the modem and read the response. I thought that I'd read all the relevant bits in the open tutorial and I have the following code

use warnings;
use Fcntl;

my $tty = "COM3";
my $text = shift || "ATH0";

sysopen(COM,$tty,O_RDWR) || die "Can't open $tty $^E";

transmit("$text");
receive();

sub transmit {
    my $data = shift;
    $data .= "\r";
    @ascii = unpack("C*",$data);
    print "transmit: [@ascii]\n";
    syswrite(COM,$data);
}

sub receive {
    sysread(COM,$buf,4);
    @ascii = unpack("C*",$buf);
    print "receive: [@ascii]\n";
    return $buf;
}

However when I run the script it hangs on the sysread instead of returning O.K. as expected. I think the port is opening correctly, because when something else is using the port the open fails with an "Access denied" message.

[chromatic]

You probably ought to check the return values of sysread and syswrite. They both return undef on error:

$result = syswrite(COM, $data);
if (defined $result) {
    print "$result bytes written\n";
} else {
    die "Couldn't write to port.\n";
}

Of ourse, there's always the CPAN module Win32::SerialPort.

[Odud]

I supposedly searched CPAN this afternoon and found nothing! - in my defence my twin daughters were climbing on me at the time. But you are quite correct I should not assume that the syswrite and sysread are working.

[jcwren]

As chromatic mentioned, there is Win32::SerialPort. I've used this rather extensively with some Perl modules I wrote to control several amateur radio transceivers, my weather station, and some equipment I designed to interface general purpose I/O to a serial port. It works quite well, and better yet, you can port it to Linux using Device::SerialPort.

You get a lot of control over the port, and unless you don't get too bizarre, all you need is a BEGIN code snipped to select Win32::SerialPort or Device::SerialPort based on the OS reported by $^O.

--Chris (who wonders why the first link isn't right, and second is)

[Anonymous Monk]

I took your code, replaced $^E with $!, and it ran fine... maybe some strange quirk on your modem? Maybe for some reason there aren't 4 bytes from the modem for the script to read, so the call to sysread is blocking?

[Odud]

If only I could remember what I was trying to do - but it was eight years ago...

[Odud]

See previous excuse for not searching CPAN properly! I want to do very simple stuff so hopefully this will do the job. The end result is hopefully going to be a script that will put call waiting back on the phone line after I have finished using it.