Mysterious adder from the 1960’s Bendix G20

[David Lovett] Aka Usagi Electric is diving into another old computer design, dating to the early 1960s. He recently received eight mysterious circuit board on-loans for the purpose of reverse engineering. They appear to have come from an older mainframe called the Bendix G20, the successor to the 1965 G-15 vacuum tube model. The cards are:

  • Complete additive
  • And the gate
  • Or gate
  • Emiter follower
  • Flip flop
  • Quad inverting amplifier
  • DLO amplifier
  • Gated CPA

Most of these are quite easy to find, but he ran into some problems trying to understand the full ad board. The first problem is that there is some uncertainty surrounding the reasoning level voltages. This system uses a negative voltage, where -3.5 V represents a logic 1… or is it a logic 0? And even considering this ambiguity, [David] Having a hard time understanding how Adar works. It uses a bunch of diodes to implement an adder’s logic lookup table – except it doesn’t match it with any known addition project. [David] The community has been called in to help with this, and if you have any idea how this adder works, go to her wiki linked above for more information and yell at her.

We do not know how [David] Centurion is spending time for these side projects while he is busy recovering mini-computers and the giant single-bit vacuum tube computers he is building.

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