Harrison Cooper wrote:

> I'd be interested in at least the ISA interface, figure its just
> basically a parallel dump of data and control for reads and writes.

  That's about it.  I was about to interject that there is an IRQ system
too, but I daresay you don't have to use it, especially if the thing
*thinks* it's a 16550.

Jerry Meng wrote:

> If you connect PIC direct to a ISA based modem, at least 8bit port
> (data) should be used, more you also have to supply the address line,
> W/R, etc, like controlling a intel 8250, it's too complex, I think.

  Not *too* complex overall, one should hope.  How much simpler could
you really expect?  A serial interface is hardly simpler when you waste
an awful lot of instruction cycles in the serial routines.  OTOH, if you
used some sort of serialiser, it would probably help to use the IRQ as
well.

> I guess the ISA modem also has a UART, if you can find it, can still
> use a SCI port to driven it.

  Ahh!  There's the trick!  It does *not*.  It imitates one to a trivial
extent.  You've never wondered why an internal modem can be set for
*any* baudrate and still works at top speed?  It just ignores the
baudrate registers and latches the parallel data into/ out of the local
processor.  Makes it very easy to emulate a 16550 or 16560 buffer set.

> I don't know if the ISA modem is supplied via +12 or -12 from ISA bus,
> if so, your PIC power supply must output those voltage.

  Dunno.  I suspect *not*, particularly the modern ones.  Look at the
board for the thicker traces.  Gnd and 5V are two at each end and should
have a tantalum between them at each end.  If there are more than one
thick one in between, or more tantalums, then yes, it does require the
+/- 12V.

  Cheers,
        Paul B.