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Subject: 2nd Try 1.57 on Win2k or WinXP. Not more than 16 com ports. Differences between //./comX and /dev/comX
Date: Mon, 19 Apr 2004 20:14:00 +0200
Message-ID: <C2F0FC2A5C4B3E44ADF4327E699486A509F529@mail2.de.thorlabs.local>
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From: =?iso-8859-1?Q?Lutz_H=F6rl?= <lhoerl AT thorlabs DOT com>
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Hello,

thank you Dave, thank you Larry.

But unfortunately your advice to use "//./devX" did NOT solve the problem.

Dave:
Cygwin uses upto 16 COM ports, not only 8. I did not compile with
open("\\.\com8",...) but with open(buf,...) so I did not get
warnings from the compiler.

I corrected the examples below and got the result described.
What makes me nervous ist the errno = 20.

I must add that I use Win32-API to set baudrate and to read the modem
lines (DSR, DCD). I use the Win32 functions
GetCommModemStatus()
EscapeCommFunction()
GetCommState()
SetCommState()
in my program. That works fine as long as I use "/dev/comX" with X=1...16.
But it does not work with "//./comX".




I want to use (Windows-) COM port numbers greater
than 16, but when I use open() to get a file
descriptor for the devices I get the behaviour:

---1.case-----------------------------------------

errno = 0;
fd = open("/dev/com8", O_RDWR | O_NONBLOCK);
-> fd = 3, errno = 0
-> everything is OK,
   I can use the opened COM port

---2.case-----------------------------------------

errno = 0;
fd = open("//./com8", O_RDWR | O_NONBLOCK);

-> fd = 3, errno = 20 = ERROR_BAD_UNIT
   = "The system cannot find the device specified"
-> I can not use the opened COM port. The port
   number 1...16 versus 17...255 does NOT matter.

---3.case-----------------------------------------

errno = 0;
fd = open("/dev/com23", O_RDWR | O_NONBLOCK);

-> fd = -1, errno = 2 = ERROR_FILE_NOT_FOUND
   = "The system cannot find the file specified"
-> I can not use the opened COM port. The port
   number 1...16 (OK) 17...255 (not OK) DOES matter.

---end cases------------------------------------

Is there a workaround?

Lutz Hoerl

Thorlabs GmbH
Gauss-Strasse 11
D-85757 Karlsfeld
Germany

Tel.    +49 (0)8131 5956 44
FAX     +49 (0)8131 5956 99
M A I L    lhoerl at thorlabs dot com

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