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Q30855: "Device Unavailable" Using Communications Port in Real Mode

Article: Q30855
Product(s): See article
Version(s): 6.00 6.00b 7.00 | 6.00 6.00B 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 26-FEB-1990

If a "Device Unavailable" message occurs when trying to do
communications under real mode (the DOS 3.x compatibility box in
OS/2), then setcom40 must be run. This process allows the program to
access the COM port in real mode. Setcom40 sets the serial-port
address before starting the real mode program.

Real mode applications use the BIOS COM-port base addresses, which
begin at 400H to determine the presence of serial ports. However, the
OS/2 COM.SYS device driver fills these addresses with zeros to keep
real mode programs from interfering with protected mode applications
that might try to access the COM ports.

Setcom40 sets the port base addresses to values that real mode
applications will recognize.

The following syntax writes the port address for COMx in the BIOS area
of the disk, where x is the number of the serial port:

   setcom40 comx=on

The following syntax removes the address from the BIOS area of the
disk when the real mode application no longer requires it:

   setcom40 comx=off (where x is the number of the serial port)

For more information, please refer to Page 190 in the "Microsoft
Operating System/2 Beginning User's Guide" of the Microsoft OS/2 SDK
(Software Development Kit) Version 1.00.

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