[rescue] Break signals [was: Booting from LOM?]

[Scott Newell]

>What's peculiar about Sun hardware is also peculiar about the original
>IBM PC serial ports, and many of the later clones.  I've not looked at
>the exact specs of any recent ATX-like machines.

All the clone hardware I've used puts out +/- 12V, but I don't have a
laptop (I suspect that's where you'd see the really marginal levels).


>75154 which normally only operates at 5vdc.  They are in effect using
>the RS-423 electrical standard with RS-232 signalling!

Scary, but it makes sense--the MAX202 series datasheet mentions that it
will work with this signal, so maybe it is common.  (They're even ok with
0/+5 logic level 'rs-232' signals...handy for some ultra-cheap projects.)


>Some Sun machines have partially "fixed" this problem by optionally more
>closely following the RS-232 specifications, but it's still usually a

>user-settable jumper -- the serial ports can be configured as either
>RS-232 (+/- 12V) or RS-423 (+/- 5V).  Be sure you select RS-232!  Lots

I've just finished measuring the outputs with jumpers in both positions.  I
think all my Suns have had the jumpers in the RS-423 position.  I wonder
what they were set to when Sun shipped 'em.


>Note that some terminals and some terminal servers, and indeed some
>cabling implementations, may generate spurious BREAK signals when
>powered off (or even just when they're powered on again).  The following

Now that makes sense.


>> For what it's worth, I hot plug my serial console on IPX and SS2 machines
>> all the time, and I've yet to see 'em halt.  But I'm running OpenBSD and
>> NetBSD--maybe they don't listen for break signals.
>
>You're probably just lucky....

I don't think so.  The line receiver chip used in the SS2 and IPX has a
failsafe pin that controls the logic output when the diff input pins are
open or shorted.  It's set to output a idle line, not a break, when the
input is floating or shorted to ground.  And that's what I observed with
the 'scope.  That's not to say it wouldn't read a burst of static as a
break signal.

Besides, if a grounded or floating input was interpreted as a break signal,
you'd have to have a terminal plugged into the console 24/7!


newell