perlmania
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Jan 11, 2010, 2:41 AM
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Accessing virtual COM port in perl
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I'm trying to write a bit of Perl code to access and read data from a serial port. The device is actually a USB GPS dongle that presents a serial port. Depending on which USB port I plug it into it presents as COM8, COM11 or COM14.
I'm using the following simple method:
# Port parameters
$Port = "COM11";
$Baud = 38400;
$Prty = "N";
$Data = 8;
$Stop = 1;
# Program
$Cmd = "mode ".$Port." baud=".$Baud." parity=".$Prty." data=".$Data." stop=".$Stop;
print "Configuring port ".$Port." operating parameters...\n";
system($Cmd);
print "Trying to open port ".$Port."...\n";
open (COM, '<', $Port) or die $!;
#while (<COM>) {
# print $_;
#}
close COM;
print "Port ".$Port." closed.\n";
print "Done.\n";
If, as per the above example, the device is on COM11 (or COM14) I get the following error:
No such file or directory
If I examine the port from a command prompt using the mode command I get:
Status for device COM11:
------------------------
Baud: 38400
Parity: None
Data Bits: 8
Stop Bits: 1
Timeout: OFF
XON/XOFF: OFF
CTS handshaking: OFF
DSR handshaking: OFF
DSR sensitivity: OFF
DTR circuit: ON
RTS circuit: ON
This proves that the port is present. I can also use mode to set the baud rate and other parameters if I choose.
I've also modified my code to use the Win32::SerialPort module but I get the same result when trying to open the port. I have not yet added the code to configure the port:
use Win32::SerialPort;
# Port parameters
$Port = "COM11";
$Baud = 38400;
$Prty = "N";
$Data = 8;
$Stop = 1;
# Program
print "Trying to open port ".$Port."...\n";
$ComPort = new Win32::SerialPort($Port, $Quiet)
|| die "Can't open port ".$Port."$^E\n";
#while (<COM>) {
# print $_;
#}
close COM;
print "Port ".$Port." closed.\n";
print "Done.\n";
The strange thing is that if I plug the dongle into the USB port that presents it as COM8 then the code seems to work.
I also get something similar if I try the following:
C:\>type com11
The system cannot find the file specified.
Whereas if I try this with com8 I do get the GPS data spewing out to the console.
I've also tried this with a PL2303 based USB to serial adapter and I get exactly the same problem - works on COM8 but not on COM11 or COM14.
The basic GUI application that came with the dongle on the other hand works on all ports. I initially thought that this might be a windows command environment issue but the mode command does work so I'm rather puzzled.
Can anyone help me understand what's going on here please?
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