Sun 3 console question

[Steve Jay]

The circuit to keep a Sun from halting when the console generates a break
accidentally (like powering the terminal off or disconnecting the cable)
was described in sun-spots v5n45, by Malcolm Harper
<mkh%prg.oxford.ac.uk at nss.cs.ucl.ac.uk>, way back in Sep. 87.  If our
moderator is willing to send it out again to everyone rather than having
interested parties retrieve it from the archives, I have appended it to
the end of this message.  I have never tried this circuit.

Steve Jay                       domain: shj at ultra.com
Ultra Network Technologies	Internet: ultra!shj at ames.arc.nasa.gov
101 Daggett Drive               uucp: ...ames!ultra!shj
San Jose, CA 95134		
408-922-0100

>Date:    Fri, 25 Sep 87 17:23:06 bst
>From:    Malcolm Harper <mkh%prg.oxford.ac.uk at 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.