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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
Ken Shirriff
in reply to Ken Shirriff • • •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
Ken Shirriff
in reply to Ken Shirriff • • •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
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Ken Shirriff
in reply to Ken Shirriff • • •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
Ken Shirriff
in reply to Ken Shirriff • • •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
Ken Shirriff
in reply to Ken Shirriff • • •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
Ken Shirriff
in reply to Ken Shirriff • • •Ken Shirriff
in reply to Ken Shirriff • • •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
Ken Shirriff
in reply to Ken Shirriff • • •Ken Shirriff
in reply to Ken Shirriff • • •IBM 1401 Demo Lab - CHM
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
in reply to Elena ``of Valhalla'' • • •Elena ``of Valhalla'' likes this.
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