[ On , May 16, 2003 at 15:23:30 (+0100), Greg Matthews wrote: ]
> Subject: Re: Sun Classic crahes (NetBSD 1.6)
>
> couldnt something similar be implemented internally by sun hardware
> engineers on newer machines?

Certainly Sun could build a proper circuit to prevent "accidental" break
signals, though to do it 100% correctly requires even more discrete
components and thus adds to the cost.  Third-party cable makers (Nudata
at least) were selling the equivalent for ~$100[usa], IIRC.

The rest of this message, originally from SUN-SPOTS v9n20, and
previously quoted in port-sparc as well, details one such circuit:

<URL:http://sunsite.bilkent.edu.tr/pub/sun-info/sunspots/v9n20>
<URL:http://mail-index.netbsd.org/port-sparc/2002/05/16/0000.html>

|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.


-- 
								Greg A. Woods

+1 416 218-0098;            <g.a.woods@ieee.org>;           <woods@robohack.ca>
Planix, Inc. <woods@planix.com>; VE3TCP; Secrets of the Weird <woods@weird.com>