Again, the section on kernel configuration provides information
about configuring your kernel. For a multiport serial card,
place an sio line for each serial port on the card in the
kernel configuration file. But place the irq and vector
specifiers on only one of the entries. All of the ports on the
card should share one irq. For consistency, use the last serial
port to specify the irq. Also, specify the
COM_MULTIPORT option.
The following example is for an AST 4-port serial card on irq 7:
options "COM_MULTIPORT"
device sio4 at isa? port 0x2a0 tty flags 0x781
device sio5 at isa? port 0x2a8 tty flags 0x781
device sio6 at isa? port 0x2b0 tty flags 0x781
device sio7 at isa? port 0x2b8 tty flags 0x781 irq 7 vector siointr
The flags indicate that the master port has minor number 7
(0x700), diagnostics enabled during probe (0x080), and
all the ports share an irq (0x001).