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From: toma@tekgvs.UUCP
Newsgroups: comp.misc
Subject: Re: IBM 1130 Emergency Pull switch
Message-ID: <2062@tekgvs.TEK.COM>
Date: Mon, 9-Feb-87 11:15:38 EST
Article-I.D.: tekgvs.2062
Posted: Mon Feb  9 11:15:38 1987
Date-Received: Wed, 11-Feb-87 19:21:19 EST
References: <1092@sfsup.UUCP>
Reply-To: toma@tekgvs.UUCP (Thomas Almy)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 57

In article <1092@sfsup.UUCP> jeffj@sfsup.UUCP (J.S.Jonas) writes:
>I never pulled the 'emergency pull' switch for fear of crashing the disks.
...
>Professor Costello (of The Cooper Union), however, always wondered what happens,
>so he pulled it.  The power went off to everything.
>You see, the switch turns off the main power relay,
>and turns everything off without regard to the sequencing.
...
>In <706@argus.UUCP>, Art Marriott says that the IBM CE had to reset the switch.
>Yup, that's the function of the spring and collar.  I guess that the power
>sequencing was important enough that maybe things would lock up or get
>damaged (like the disks?).  Or it was a gimmick to get extra service
>calls (if you're desperate enough to pull the Emergency Off switch
>instead of just turning it off, you deserve to be punished).
>

The power-down sequence for these old machines left the cooling fans on for
a long period to remove the residual heat.  Pulling the Emergency Off switch
cuts all the power, even the fans, causing the temperature in the cabinets
to *rise*.  This could cause damage to sensitive components such as memory
cores (or drums, etc).  The switch would lock up forcing the service call
and the (more important) running of diagnostics to insure no damage was 
caused.  To quote from IBM Form 22-6060-1, IBM 650 Manual of Operation, 1953
(portions marked with "*" were italisized in the manual):

Power-Off Button

	When this button is depressed, the following sequence of events occurs:
	1. DC power is turned off.

	2. AC power (except that supplied to the blower motors) is turned off.

	3. The blower motors are turned off after a five minute delay.

DC-Off Button
	
	...Depressing the DC-OFF button will cause all DC power to be turned
off and the ready light will go out.  The DC-OFF button has no effect on the 
AC power.  When this button is used, the machine can be shut down for
comparitively short periods (e.g. an hour) without necessitating cooling off
and reheating the electronic tubes in the machine.

Master Power Switch

	The master power switch is for *emergency use only*.  When this
switch is depressed [sorry, predates "Emergency PULL"], all power, including
that to the blower motors, is removed from the machine *immediately*.

	The depression of this switch also activates a locking device which
makes it *impossible* for the operator to turn the machine back on.  A
customer engineer *must* be called in order to get the machine back in
operation.


Tom Almy
Tektronix, Inc.
tektronix!tekgvs!toma