14 votes

The printer that wouldn't print: Fixing an IBM 1401 mainframe from the 1960s

2 comments

  1. s4b3r6
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    This is the kind of detail I've come to expect from the Computer History Museum, and their investigations of the retro computers under their care... And I adore it. I missed out on mechanical...

    The process to write to core memory may seem backwards, using an inhibit line to write a 0. The inhibit line is used instead of writing the value directly due to how cores function. The key that makes cores work is that they require a high current pulse to flip the core's magnetic state; a pulse with half the current has no effect on the core. Cores are arranged in a grid, with X and Y address lines that are pulsed to select a core. Multiple planes are stacked, one for each bit. Each line is pulsed with half the necessary current, so only the core where both lines cross has enough current to flip to the 1 state. Each plane has an inhibit line that passes through all the cores in the plane. To write a 0 to a plane, the inhibit line gets half current in the opposite direction. The result is that none of the cores get enough current to flip, and remain in the 0 state. Thus, by setting each plane's inhibit line appropriately, the desired 0's and 1's can be written to the address in the core stack.

    This is the kind of detail I've come to expect from the Computer History Museum, and their investigations of the retro computers under their care... And I adore it.

    I missed out on mechanical computers and decimal computers, and reading how they used magnets as bits, and how a wired-OR works... I couldn't get happier.

    3 votes
  2. trecht
    Link
    I love these articles. Thanks for sharing OP!

    I love these articles. Thanks for sharing OP!

    1 vote