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.

                        No, I did 8250 interfacing with a Z80 in
assembly.  Just load the chip with the init data, I hope we all
                        Are capable of doing that.

                                  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.


                I think the +/- 12 is only when driving out for RS-232.
Could be wrong but looking at modem card, what is there
                That would need this? Its all TTL or CMOS parts and
ASIC's.  Most likey would be running on 3.3V anyway

                          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.

                        If I remember, I'll talk to a friend who used to
be at 3COM (US Robotics....was Megahertz) who was working
                        On modem cards, see if he has any specs or such.
He isn't there anymore, but probably remembers.  A lot of what
                        Is done on the cards is addressing space and IRQ
setups, which wouldn't be needed in a standalone hardwired application.
                        The advantage of using an off the shelf card
remains that you don't have to redesign the telco interface stuff and
worry about
                        Approval.  And designing at higher speeds is
also fun.  Doing a 125 MHz circuit as we speak...write...read...