The IBM 1401 was the most popular computer of the early 1960s, with over 10,000 built. Renting for $2500 a month, it made a computer affordable to medium-sized businesses. You can see one in operation twitter.com/ComputerHistory. But what if the computer suddenly stops turning on? 1/10
When you push the POWER ON button, the computer springs to life. But starting last week, it would power off as soon as you take your finger off the button. Something was going wrong with the power-up sequence. 2/10
The computer uses numerous power supplies (some visible in the photo) to provide different voltages. Relay logic activates the supplies in order, checking voltages before activating more supplies. If a voltage is bad, the power-up sequence stops. 3/10
These eight relays control the power-up sequence, closing circuits in order. The SMS card to the right checks one of the voltages. (Why relay logic? Probably because the power supplies for the transistor logic isn't energized yet. Chicken and egg.) 4/10
This slow-motion video shows the relays operating, with the contacts moving to open and close circuits, moved electromagnetically by the white coils. The right relays are activated at the right time, so the voltages seem right, but something was still wrong. 5/10
I checked the signal at various points in the relay chain, hampered by the tangle of wiring on the back of the relays. Relay #8 had -20 volts in and nothing out, despite the contacts being closed. The multimeter showed 10 MΩ across the closed contacts. Not good! 6/10
I cleaned the relay contacts and the computer powered up successfully! Next step, figure out why the card reader sometimes gives errors. With old computers, there's always something breaking. 7/10
The computer has an Emergency Power Off (EPO) handle for emergencies. People have lurid tales of how EPO mechanically guillotines the power cable or blows all the fuses but the truth is less dramatic. It simply cuts current to the main power relay, disconnecting the power. 8/10
You couldn't power the computer back on after pulling the Emergency Power Off handle, though; you had to call IBM to get it reset. A little metal tab on the back of the switch pops out so the handle won't go back in unless you push the tab. 9/10
You can see the IBM 1401 in operation at the Computer History Museum in Mountain View, CA. Demos are on Wednesdays and Saturdays. computerhistory.org/exhibits/i… 10/10
@valhalla Hopefully the Emergency Power Off never needs to be used, but a motor could short out and start smoking for instance, in which case you'd want to power off immediately.
Ooo this reminds me of an anecdote which hardly anyone other than you will appreciate Ken!
When I worked in the City of London in the 80’s/90’s, Wang systems were common. The disk drives were separately powered, with big push off/push on buttons. They didn’t actually power down until you pressed *and released* the button. 1/2
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Ken Shirriff
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Computer History MuseumElena ``of Valhalla''
in reply to Ken Shirriff • •@Ken Shirriff I do wonder what kind of emergencies they'd expect to happen that would require such a drastic measure
but if it's going to be in the blog post I'll just read it there :)
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Ken Shirriff
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kitlovesfsharp
in reply to Ken Shirriff • • •Ooo this reminds me of an anecdote which hardly anyone other than you will appreciate Ken!
When I worked in the City of London in the 80’s/90’s, Wang systems were common. The disk drives were separately powered, with big push off/push on buttons. They didn’t actually power down until you pressed *and released* the button. 1/2