Regarding the recent discussion about unplugging/turning off
a terminal and having the sparc break to the prom. I found 
an interesting diagram from reading the SunSpot digest 
from 1992.

Not being electrically literate I cannot comment on the validity
of this circuit. Though I would like to know if this does the
trick.

Jody

>Date:    Fri, 25 Sep 87 17:23:06 bst
>From:    Malcolm Harper <mkh%prg.oxford.ac.uk@nss.cs.ucl.ac.uk>
>Subject: Re: Sun-3 ASCII console problem (2)

The circuit we use to prevent the Watchdog Reset which normally happens when
the terminal is unplugged from the Sun Console port (ttya/ttyb) is as follows.

To terminal                                                             To Sun

pin 3 <----:-----------------------------------------------------------< pin 2
           |    ________      _________________         ____________
           |   |        |    |                 |       | 2200uf 16v |
           :---| 470ohm |----| <- IN4002 diode |---:---| capacitor  |--- pin 7
               |________|    |_________________|   |   |____________|
                ________      _________________    |    -ve      +ve
               |        |    |                 |   |
           :---| 470ohm |----| <- IN4002 diode |---:
           |   |________|    |_________________|   |    Types of diodes and
           |    ________      ___________________  /    transistor, and values
           |   |        |   b|                   |/c    of capacitor, are not
pin 2 >----+---| 4K7ohm |----| BC212L transistor |      critical.
           |   |________|    |___________________|\e
           |                  _________________    \
           |                 |                 |   |
           :-----------------| IN4002 diode -> |---:-------------------> pin 3
           |                 |_________________|                             
           |    ________      _________________                              
           |   |        |    |                 |  
           :---| 22Kohm |----| IN4002 diode -> |----------------------- pin 25
               |________|    |_________________|                             
                                                                             
pin 7 ------------------------------------------------------------------ pin 7
                                                                             
We use only three wire connections, so connect pins 5, 6, 8 and 20           
together at the Sun end, and appropriate control line connections at the
terminal end.  This circuit is permanently connected to the Sun end; any
disconnection must be at the terminal end.                             
                                                                       
If the terminal is unplugged, the transistor is turned on by the negative
reference voltage present at pin 25 of the Sun serial port, and hence   
pulls pin 3 of the Sun port negative.  This causes the Sun to believe   
there is still a terminal plugged in.                               
                                                                    
Acknowledgements to Andrew Newman and Paul Williams who designed and built it.