transmitting BREAK

[Jerry Aguirre]

With regard to sending a NULL at 50 BAUD to simulate a break:
mcferrin at inuxc.UUCP (P McFerrin) says:

    A NULL is all zero's.  You really want a DEL (0377).

Actually you do want a NULL.  The idle state for an rs232 line is the
mark or all ones state (for "historical" reasons which I suspect date
back to the days of current loop).  A break consists of forcing the
line into the all zero state for more than one character time.

------        ......................................------//--
      | start .                                    . stop     | next
 idle | bit   .   your character  with parity      . bit      | start
  1   |  0    .                                    .  1       | bit
       -------......................................           -----

The only thing sending a 0377 would do is maybe send a break because of
the zero start bit.  I have tried the approach of sending a NULL at 50
BAUD and it works well enough to toggle the 1200/300 BAUD rate.
Actually an '@' (0100) will work nearly as well.

But keep in mind that a break is not always a break.  Some hardware
requires a longer break before it will take notice.  Our PDP-11/70 will
toggle BAUD rate on a 50 BAUD NULL but our Micom port selector will not
break the connection.  It apparently requires a longer break.  Check out
any modem or other data communication equipment that requires a break.
They may have a minimum break time specified in the specs.

					    Jerry Aguirre
    {hplabs|fortune|ios|tolerant|allegra|tymix}!oliveb!jerry