SUMMARY: enabling break/STOP

From: David Mitchell (
Date: Tue Aug 26 1997 - 07:48:55 CDT

Original query:

> Contrary to the usual problem that gets reported on this list,
> I *cannot* send a break signal to an Ultra-2 from a Wyse terminal
> functioning as its console. Even pulling out the serial cable at the
> Sun end and plugging it back in has no effect.

As it happens, the problem "fixed itself" (or more probably, I was
just doing something plain dumb the first time round - I suspect there
may be a 'window' during the boot sequence during which the Sun ignores
breaks, and I extrapolated wildly...).

Anyway, thanks to the following, who gave lots of useful info
which may come in handy at some point:

Ric Anderson <>
Jim Harmon <>
Seth Rothenberg <>
Glenn Satchell - Uniq Professional Services <>
Shahrol Halmi <>
David Moline <>

The replies can rougly be summarised as:

* some WYSE models (eg -85) have an option to disbale the break key
* the sequence "<f1>-<a>" or "<esc>-<f1>-<a>" may generate a break
* turn the key from the 'LOCK RUN' to the 'RUN' postition
  (unfortunately this doe not apply to my Ultra-2, which doesnt have a key)
* on some WYSE's the AUX port doesnt generate breaks - use the MODEM port
  instead, with a 2-3 7-7 null modem cable
* leave the terminal attached, but cycle its power

gave some useful detailed info about RS232:

pulling the cable only works on current-loop connected tty's.

"break" in an rs232 connection is done by lifting the line to +12v for
 at least 1/4 second. (the normal "idle" condition for an rs232 data
  line is -12v (well, -48v for rs232, -25v for rs232a, -12v for rs232B,
   -5v for rs232C), the "1" condition is +12v. The -3v to +3v region
  (such as unplugging will achieve) is, by definition, neither 1 nor 0.

control lines (such as RTS, cTS, etc.) have the opposite "idle":
  +5v on CTS means everything is hooked up and the CTS unit is "clear to send".
  Thus unplugging -does- mean something: if it's not +5v (or +12v) then it's
  not Ready.

terminals (such as the Wyse) will usually have a setup control to enable/
 disable the "break" transmission.
if the terminal won't generate a "break", then creating a little box with
 a battery and momentary switch to open the data-to-host line and replacing
 it with (say) +9v during the switch depression should do it.
  the data-to-host line is pin 2 on the back of the Wyse. ground reference
   is pin 7.


>> pulling the cable only works on current-loop connected tty's.
>We used to have a sparcserver 4/330, and whenever the terminal was
>powered off, or the cable unplugged, the server would halt - I could
>have sworn it wasnt current loop, but I guess I was wrong.

it's always possible (Murphy dictates mandatory) that Sun is using
 either the other control signals (if DTR goes away, STOP!), or
 some specific key-code (DEC used control-P on VAX consoles) to
 be "translated" as BREAK.

(i'm far more familiar with Digital's techniques... on one of their
 RS232 boards they had a jumper which went as far as hitting the backplane's
 hardware HALT line if a break happened. And many terminal manufacturers
  provided a BREAK key which held the rs232 signal line to "1" for a quarter-

Now a short digression re: rs232 asynchronous signals: given an 8-bit data
 packet, an asynchronous line usually sits at idle (-12v). When a character
 is sent, first the line goes to "1" (+12v) for the "start" bit. That's
  there to wake up (and synchronize) the receiver's bit-clock.
 The receiver then looks at the 8 sequential bit-times to see if the data
  bits are 1's or zeroes. *then* it looks at the 10th (stop) bit... which
  must be "1". (if it is set to watch for two stop bits, it'll look for
  the second one, too). Many receivers *then* look at the bit-space -after-
  the stop bit.
If either the stop bit is missing, or there's still a "1" beyond the end of
 the stop bit, the receiver hardware declares a "framing error".
(the 10-bit [start-data-stop] sequence is called a "frame")
The hold-the-line-at-"1" for a quarter second is seen as a framing error.

How the -system- handles a framing error is your choice (if you're allowed
 to exercise it)... some systems simply dismiss them (like a parity error),
 some consider it a BREAK, others use it for other nefarious purposes.



* Dave Mitchell, Fretwell-Downing Data Systems:
* The usual disclaimers....
* Standards (n). Battle insignia or tribal totems
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