[rescue] Sun serial port woes

[jodys at helluin.org]

Ok, my IPX server running NetBSD/1.5.2 recently had it's battery
die. Lacking a replacement chip (for now), I decided to upgrade the
server to a weird classic clone (SuperWorkstation LS). Fine, easy
enough, drop the drive in and go. Alas I've run into a 
catch 22. As a headless machine, it won't boot unless I have a 
serial console plugged in (WTF?), but if I boot with a serial console
and then either unplug the terminal (wyse 60) *or* turn it off
the machine drops back to the prom. I can't leave the terminal
connected as it has other uses. So what the hell, and who the hell
thought it was a good idea to make break drop the machine to prom?

I am familiar enough with the inanity of this design on other 
real sun's, but they only drop to prom if I power off the terminal,
which is ok, a little price to pay for convience, but this is just
plain stupid. I have found a circuit that says it will stop the
problem, but I don't know anything about electronics. Anyone care
to comment?

(at least I found something interesting reading usenet from 1989)

Jody

>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.