dennis@rlgvax.UUCP (Dennis Bednar) (01/08/86)
Kermit XON/XOFF Deadlock Bug Caused by Hayes Command Echo Enabled
I recently ran into a problem of kermit "locking" up
sometimes when using a Hayes Smartmodem, and I thought I would share
it with the net.
Well, I looked into this problem and found that it was
caused by me connecting to the Hayes modem directly, and
and accidently typing control-s (XOFF). The ^S was passed through as
data to the modem. At this time, I had set the Hayes dip switch to
enable echoing during Hayes command mode, merely for convenience
to me. This led to an XON/XOFF deadlock problem between the vt100
and the Hayes Smartmodem.
Repeat by:
# this is a comment
# enter C-Kermit command mode
set modem dir # kermit command to say direct hardwired line
# I know, I know, C-kermit's dial command will interact
# with the Hayes, and this problem will not occur if I
# use the kermit "dial" command, but I was just experimenting.
set esc 1
connect
# kermit connects your terminal to the modem now
# Now type AT several times, and verify that the OK prompt appears.
# Now type control-s.
# Now type AT, but you don't see OK.
# In fact nothing you type is sent. This problem assumes that your
# terminal understands that ^S inhibits the terminal kb's from
# sending on the tty line.
You MAY NOT BELIEVE THIS, BUT THE THE HAYES ALSO ECHOED the control-s!!!!!!!
Therefore the ^S typed on my keyboard was passed through kermit,
echoed by the modem, and passed back through kermit, and sent to
my terminal screen. This caused my vt100 terminal's KBD LOCKED
(Keyboard Locked) LED to turn on. If you try typing ^Q to break
the problem, your terminal does not transmit it, because output
is inhibited until ^Q is received from the host, which cannot happen,
so you are deadlocked. Fortunately, since I was using a vt100
terminal, I merely pressed the SET-UP key twice, which turned off
the KBD LOCKED light. For an ignorant user, this could be bad.
Best solution: Disable Hayes echoing during command mode, and
this problem cannot occur.
--
Dennis Bednar Computer Consoles Inc. Reston VA 703-648-3300
{decvax,ihnp4,harpo,allegra}!seismo!rlgvax!dennis
dennis@rlgvax.UUCP