DTE to DTE Datacomm

[Rob Scott]

My IBM 286 XENIX application, which is making "write" calls to
/dev/tty00 has a little problem...

Background:

-I'm using an IBM Serial Parallel Adapter and the vanilla tty driver.
-I have initialized the driver for ~CLOCAL (to use modem control).
-I'm catching SIGHUP(generated upon loss of CD).
-I'm using CTS tied to DSR (pins 5 & 6 RS232) along with CD (pin 8)
as my externally controlled lines. CTS and DSR are toggled externally 
by the contolling device (which is not a modem).  CD is toggled upon 
loss of external device. 

My code is structured like this:
while ( not done reading in 512 byte chunks...)
	{	
	signal( SIGHUP, hangup_alarm);
	alarm( timeout, timeout_alarm);
	while ( (n = write( ... )) != num_bytes_read)
		{
		reset alarms;
		check for KILL messages;
		sleep(for_awhile);
		}
	}
		 
		
Pretty basic stuff. Right out of "Advanced UNIX Programming".

The bottom line is - I want to use CTS/DSR as flow control lines.  This 
seems to work except in the following cases:

If CTS/DSR is OFF before I attempt write to the port ( CD is ON ) it writes
anyway! The external line must be toggled once in order for the driver to 
respond.

The other case is where CD is set to OFF, then CTS/DSR is set to OFF, then 
CD is reasserted.  Upon reasserting CD even with CTS/DSR still OFF the write 
is carried out once again.

What must I do to properly monitor the state of the CTS/DSR lines prior to 
actually writing data??

I tried preceeding my writes with an initializing write call writing a null
character write(port,'',1) but that has little if any effect.

Any suggestions on what I might be missing would be appreciated !!