data transfer without terminal emulation

[T J Leone]

I'm trying to write a program that transfers data between a Unix and a non-Unix
machine.  The program should be a portable as possible between SV and BSD Unix.
I've looked at the code for XMODEM (3.9?), and it looks like data is 
transferred over the tty used by the terminal emulator.  Since I won't be
using terminal emulation, I have to access a tty port directly.  It's a full
duplex serial line, directly connected to the non-Unix machine.  I've been able  to send data through the port by doing an open on the tty, but that only gives 
me one file descriptor.  The problem his, how do I make sure that I'm not  
trying to read in data that I've just written out?  XMODEM seems to have two 
different "bands" to work with (stdin and stdout), but I just have the one 
port.

Thanks in advance,
--tj