SUMMARY : Shutdown proof console serial cable pinout

From: Niall O Broin - Gray Wizard (
Date: Fri Mar 22 1996 - 06:35:06 CST

I asked whether it was possible to wire a serial cable for a console terminal
in such a way that power cycling on the terminal would not drop the Sun into
the PROM monitor. This generated a lot of interest (so I'm not naming names
except for some specifics) and is also a FAQ and is in the archives. In
fairness, and I try to be always fair (at least to myself), I did look in the
archive but I didn't find the answer because I failed to ask the right question.

The problem arise because power cycling a terminal produces noise which a Sun ZS
(serial port) driver interprets as a BREAK signal, so it does what it's meant to
do, and drops into the PROM monitor. Kevin Sheehan suggested hacking the ZS
driver so that a BREAK doesn't call the PROM and Hal Stern mentioned a Sun product
called CONSULT-ZSBREAK, which disables the halt-on-break behavior (for more info
call +1 415 336 2400). However neither of these would work for me as I need to be
able to drop into the monitor with a deliberate BREAK on occasions. The suggestion
which I intend to try is connecting pin 3 to pin 25 at the Sun end with a 4K7
resistor. This supposedly helps suppress the noise induced breaks while allowing
deliberate BREAKs to still be produced. There was also mention of a circuit
involving a diode . . .

Kindest regards,

Niall O Broin

UNIX Network Administrator, Stations and Communications Engineering Department
European Space Operations Centre
Darmstadt, Germany Ph./Fax +49 6151 90 3619/2179

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