Problem using the same port for in/out modem

[Cherry.Pasa at XEROX.ARPA]

I am having a problem which is difficult to track down. I am using Uniplus 
Sys 5.0 Unix.

Problem:  I get into an INTERMITTANT "feedback" type situation when using the
	  same physical port for two different devices where only one device 
	  may run at a time.  When this occurs and you invoke a cu to device 
	  tty5, the getty login to tty5m gets in and causes the port to cycle
	  sending streams of characters (including some which really screw up
	  the modem setup).  What is happening is that when the cu process is
	  opened, the characters typed at the terminal are output to tty5 and 
	  are received by tty5m where they are echoed back to tty5 which then 
	  echoes them back to the port where tty5m receives them and INSTA-LOOP.
	  
	  As I understand whats supposed to be happening is that each device
	  which has a unique major-minor operates in its own memory environment.
	  If this is so, and if the file locking mechanisms, ioctls, etc. are 
	  properly operating, this phenominea should not happen.  We usually see
	  this situation if the port has not been accessed for several days and
	  then the initial command to the port is a cu command.  Once the cu is 
	  issued, the only way found to date to stop the loop is to do a rm on
	  both devices and physically remove all connections to the port.  Only
	  then can a SIG_HUP (kill -1), SIG_KILL (kill -9), or SIG_TERM (kill -15)
	  be sent to stop the character stream loop.
	  
	  Has anybody seen this before and if so, can somebody explain why this
	  doesn't happen all the time and/or what can be done to prevent it from
	  happening at all.  It only happens about 5 percent of time this port
	  is used.  It does happen to all ports configured in this manner.
	  
	  
Setup:   Where tty5  is a dial OUT ONLY port with a minor of 2 and
         where tty5m is a dial IN ONLY port with a minor of 130 (2 + 128)

     From /dev:
	 protection    owner   group   major   minor   device 
	 ------------ ------- ------- ------- ------- ---------------
          crw-rw-rw    root    root     12      2     /dev/tty5
          crw-rw-rw    uucp    uucp     12    130     /dev/tty5m	
       
     From /etc/inittab:
	t5::off:/etc/getty tty5 1200
        t5m::respawn:/etc/getty tty5m 1200


     From /etc/gettydefs:
	1200# B1200 # B1200 CS8 CREAD IGNPAR ISTRIP ICRNL IXON IXANY ISIG
	ICANON ECHO ECHOE ECHOK OPOST ONLCR TAB1 #\r\n\n\xxxxxxxx\nlogin: #300

     From /usr/lib/uucp/L-devices:
        DIR       tty1    tty1    1200
	ACUVENTEL tty1    tty1    1200
 
Hardware:
	CDC MB-1031 8-port Serial I/O Module; jumpers set for terminal set rather
	than data set.
	
	Ventel 212-3E modem with null modem interface between modem and MB-1031

Any ideas will be appreciated.
B.C. & Zot------>*

Cherry.PASA at Xerox.ARPA