How do you make COM3 work?

[Checkpoint Technologies]

[ I'm posting this for an associate without net access... ]

    Does anyone have experience using the IBM AT standard communications
port COM3 under Interactive Unix/386 version 2.2?

   I'm running an Intel 386/33 with two standard serial ports on the mother-
board.  My goal (unsuccessful so far, obviously) is to add an internal modem
as serial port 3.  The modem is a Practical Peripherals (although I have
tried an Everex as well), that can reside at COM1, COM2, COM3, or COM4.

    Interactive's Operating System Guide states a simple procedure for using
a standard IBM COM3 port, sharing IRQ 4 with COM1, but having a base I/O
address at 0x3E8.  The procedure is to add the new asy device in the file
/etc/conf/sdevice.d/asy with a dummy, un-used IRQ (to bypass kconfig),
followed by hard-coding the address and IRQ for the additional device in the
file /etc/conf/pack.d/asy/space.c.

    The problem is, the procedure doesn't work.  I lose all communication
with both /dev/tty00 and /dev/tty02 (must be due to the interrupts banging),
while /dev/tty01 functions as it should.  Disabling the second serial port
and rejumpering the modem will allow me to access /dev/tty00 (motherboard
COM1) and /dev/tty01 (the internal modem) without any problems.

    Interactive is no help, they say it must be my hardware.  I know from
the modem documentation that it does indeed support shareable interrupts
at IRQ 4, base 0x3E8 when jumpered for COM3.

    Having only cursory Unix experience, I also know that I can boot the
machine under DOS and properly address COM1 - COM3 as follows:

                 COM1 - IRQ 4, 0x3F8 to 0x3FF
                 COM2 - IRQ 3, 0x2F8 to 0x2FF
                 COM3 - IRQ 4, 0x3E8 to 0x3EF

Ideas, anyone?

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