Gregory's ServerWhen 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
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4 comments
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 
Roger Sen
“Relay logic activates the supplies in order, checking voltages before activating more supplies.” I learnt it from my operating systems teacher: turn on the devices IN-ORDER.
kitlovesfsharp
@kenshirriff 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 |
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