SUMMARY: A/B console switching

From: Celeste Stokely (
Date: Mon Aug 16 1993 - 00:50:32 CDT

Many thanks to everyone who responded to my request for some form of A/B
switch to allow 1 dumb terminal to be the console of many Suns.

The answers fell into 3 categories:
        1. Where to buy an A/B type switch that does all or most
           of what I need. (What I was looking for.)
        2. How to build an A/B switch.
        3. Using terminal servers and other means as a way to have a
           terminal be a console. (unusual, but worth mentioning)

[Summary data in square brackets like this are my own comments.]

A tip of this sysadmin's hat to those who helped answer the question:
      Gene Rackow <>
      Martin Lewald <mol@NSD.3Com.COM> (Kevin Sheehan {Consulting Poster Child})
      Mark Herberger <> (Ray Smith) (Barnes William)
      kmah@DCS-Systems.COM (Kevin Mah)

My original question:
> A few weeks back I saw an advertisement in some Sun trade mag for this device
> and now can't find it. Any help would be appreciated.
> The device is a fancy A/B/C/D switch box that allows multiple Sun machines
> to connect to a single terminal as a console on all the Suns' ttya ports.
> You select the machine on the box, and it switches the terminal's serial
> connection to that machine, without causing a glitch on the serial lines.
> (It obviously must condition the lines in some appropriate way.)
> If anyone knows the company name and phone number, please email it to me.

1. Buying an A/B switch that should do this (without glitches on the lines
when the input is switched):

- Peter T. Henderson
  Lightwave Communications, Inc.
  84 Research Drive
  Milford, CT 06460
  ph# 203 878 9838
  fax 203 874 0157
  [I haven't called they yet, but I believe this is exactly what I'm
  looking for!]

- Nu-Data has a box which uses the actual MiniDIN 8 cable for the
  normal keyboard and the monitor. I.E. it doesn't use any of
  the tty ports. Nu-Data's catalog number for the switch is 4266 and their
  phone number is 908-842-5757.

2. Creating an A/B switch which will work

- You don't need any fancy a/b box specifically for this
  purpose. I use just a regular RS232 a/b box and it works
  fine for SS1/SS2/SS10 units. I *believe* the problem of
  units halting when the connection to the terminal went
  down only plagued older 3-series Suns. Note however if
  you power cycle a terminal acting as the console terminal
  you WILL halt the machine - requiring you to type 'c' to
  continue the processor. This is due to many terminals sending
  a break signal when powered on. The break signal is sent
  in-line (pin 2 TD) and cannot therefore be avoided unless
  perhaps you have such a box you described that can filter
  this out. To avoid the problem simply disconnect the console
  terminal data cable before cylcing power then reconnect after.

  [The only thing is that you need a resistor on the conector at each
  server to keep the server from halting when you switch to it.]
- [I'm including this next one, but I don't believe it because I can't
  imagine how pins 4, 5, and 6 would be involved. Try this at your own risk.]

  You should be able to do it yourself if you get the cables
  custom made. I believe you can keep the lines ok if you
  bridge pins 4, 5 and 6 in the db25 at the ttya port side. Then switch away.

3. More uncommon (to me) methods:

- You can use a 3Com Communications Server to enhance the connection.
  Simply hook up all the Sun console ports to the RS-232
  connections on the comm server
  and you can now telnet in to any console port! This means
  also you can bring up x-number of windows on any X-terminal
  and have a single window/telnet for each console, you'll never miss
  an error message! WOW! Then put a modem on the comm server
  and you can have out-of-band access from *YOU GUESSED IT*
  _home_ ! If you get that 3:00am call about so-n-so machines
  being down, dial in, reboot/fix and go back to sleep.
  [Using a 3Com comm server is a VERY interesting idea!]

- Hooking consoles up via the "conserver" program. This allows you to
  wire all other consoles into the serial ports of one machine where you
  have logging and remote connection capabilites. It's great.
  If you want/need details, let me know. It's available from ohio-state,
  purdue, and others. [Gene Rackow <>]

- a serial board on one box or a terminal server works pretty well.

Thanks again, folks!

..Celeste Stokely
Program Mgr for Unix Systems Administration Consulting, Expert Support Inc.
Voice Line: 415-917-7893 / FAX: 415-941-2898
USMAIL Address: Expert Support, Inc.
                  201 San Antonio Circle, #115 / Mountain View CA 94040

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