OS/2 FAQ List: User's Edition (4 Apr 94)

(2.4) COM3 and COM4 Support

Can I use COM3 and COM4 in OS/2?

COM3 and COM4 are supported on most PS/2s without any additional effort. 
 On ISA, EISA, PCI, and VESA LocalBus machines, some additions are 
required to CONFIG.SYS.  Using a text editor, include "(port number, base 
address, interrupt number)" parameters next to the COM.SYS and VCOM.SYS 
filenames.  One example:

DEVICE=C:\OS2\COM.SYS (3,3E8,5) (4,2E8,10)
DEVICE=C:\OS2\MDOS\VCOM.SYS (3,3E8,5) (4,2E8,10)

Parameters for COM1 and COM2 need not be included (unless they are somehow 
nonstandard).  OS/2 must end up with sequentially numbered logical COM 
ports, if possible.  For example, if (physical) port two is not installed 
but port three or port four is installed, start numbering using (2,...) in 
the DEVICE lines.  See the OS/2 2.1 Using the Operating System manual, 
starting on Page 374, for more information on COM ports under OS/2.  If 
these efforts fail, try the SIO drivers [available from (3.2) Shareware 
and Freeware Sources].  Note that IRQ 2 is actually redirected to IRQ 9 on 
the AT bus, so use (...,...,9) in the above COM.SYS and VCOM.SYS settings 
if your serial port is set to use IRQ 2.

AT bus COM ports cannot be used at the same time if they share interrupts 
because of bus design limitations (cf. "Under the Hood: How Interrupts 
Work," Byte, February, 1992).  An adapter which provides more flexibility 
in interrupt selection [e.g. the 16-bit model from STB; See (2.5) Specific 
Hardware Recommendations] may prove helpful.  Also, PolyCom, a replacement 
driver available from (3.2) Shareware and Freeware Sources, supports up to 
eight ports with the right hardware (Xenix-compatible multiport adapters).

"Smart" (coprocessor controlled) multiport communication adapters should 
be used when installing more than four ports.  Such an adapter will work 
with OS/2 if the manufacturer has written an appropriate driver.  Examples 
include:

     Company           Telephone Number 
     
     IBM (ARTIC)       (800) PS2-2227 
     Digiboard         (612) 943-9020 
     Stargate          (216) 349-1860 
     Arnet             (615) 834-8000 
     Computone         (404) 475-2725 
     Comtrol           (612) 631-7654 
     CTC Systems       (415) 966-1688 
     Equinox           (305) 255-3500 
     I-Concepts        (214) 956-7770 
     Specialix         (408) 378-7919 
     Stallion          (408) 395-5775 
     Quatech           (216) 434-3154

 Related information:

 (2.5) Specific Hardware Recommendations 
 (3.2) Shareware and Freeware Sources

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