What does the Big Red Button on IBM mainframes do?

You see that button above? Hard to resist right? I first saw one of these when I was a kid and my father took me to work. The machine was impressive, but that button…oh that button was irresistible and I wanted to pull it more than anything. My father told me, “No way!”, but never confirmed what would happen if I did (besides breaking my hand).

As it turns out, according to the IBM System/360 Model 65 / Functional Characteristics that I found posted on ycombinator (download here and go to page 13):

Pulling this switch turns off all power beyond the power-entry terminal on every unit that is part of the system or that can be switched onto the system; therefore, the switch controls the system proper and all off-line and shared control units and 110 devices.

The switch latches in the out position and can be restored to its normal position by maintenance personnel only.

When the EMERGENCY PULL switch is in the out position, the POWER ON pushbutton is ineffective.

Ah! It is just as mighty as I hoped it would be! It is a shame they don’t install these anymore, I want them on everything.

(Photo taken at Seattle’s amazing Living Computer Museum)