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Thread: Mechanical memory (and logic gates)

  1. #11

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    It is hard to ignore the Babbage Analytical Engine. On the remake of the Time Machine movie they created a machine that was similar looking with the many gears and rotating shafts, but it had a date display of course, picture attached.

    The best description I ever heard of the machine in the first movie from the 1960's, which was much more decorative, was that it looked like something Elton John would have driven through the Everglades.
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  2. #12
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    Well, in truth, a hard disk is a "mechanical" memory, meaning that mechanical motion is required for it to function.

    Another early machine to consider is ABC from the 1930s.

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    Quote Originally Posted by Dwight Elvey View Post
    The change from things like analog adders and subtractors mixed in with digital machines didn't go away until the cost of using diodes with transistors instead of resistors with tubes became cheaper. With tubes it was easier to make an AND function with two resistors creating a sum that was greater than the threshold of a tube ( an analog function ). It was all about minimizing the number of tubes.
    Dwight
    I realize this and should have said. You can build an analog adder with fewer valves than a pure digital adder, and it will be almost as reliable. The Manchester Baby and the Cambridge used a lot of single A-Stable circuits as temporary latches rather than flip-flops, again as it saves a valves, although the Baby also used EF50 pentodes in "Dual Control" mode as AND gates.
    Dave
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    Quote Originally Posted by Chuck(G) View Post
    Well, yes and no. One of the problems with tubes was reliability (ask any of the old SAGE people). Transistors were initially suspect, so a lot of diodes were used instead (e.g. PB250)--and alternate technologies stepped in to fill the gap. Parametrons (NEC systems) and core logic (Univac Solid State) were considered state-of-the-art at one time because their extreme reliability.

    The big advantages that silicon transistors had over tubes that really closed the books was speed, space and consistency in manufacture. When that materialized, the alternate technologies were dropped.

    I used to have some old logic boards with the little wire-leaded "peanut" tubes on them. PCB in the middle, with aluminum "wings" on either side. 6 tubes; 3 on each side, held in clips; every board that I bought had double triodes for all the tubes. They were very cheap on the surplus market; I was told that the only cost-effective way to troubleshoot these was to replace the whole assembly. I used several of the little triodes in some high-frequency experimentation.
    Most of the problems with tubes/valves are caused by heaters popping at switch-on. On the Baby replica in Manchester we run the heaters on half voltage for a few minutes. We run the machine several days per week, and only experience a couple of valve failure a year. Its got over 500 valves/tubes, so not as many as a Sage...

    http://www.cs.man.ac.uk/CCS/SSEM/vol...valvetypes.htm

    but we only get one or two failures a year....
    Dave
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    I recall that there was, back in the 1950s, an add-on TV and radio kit consisting of a large carbon-block NTC resistor ("Globar") to soften the inrush current. I don't know if any manufacturers at the time employed it in stock sets. Interestingly, you can still find them sold today.

    Running filaments/heaters at 90% of rated voltage can result in greatly increased life, particularly in power tubes--but you have to be careful to degrade the ratings somewhat.

    You can still find inrush protectors on modern equipment--mostly to save the diode bridge in the power supply.
    Last edited by Chuck(G); April 16th, 2019 at 08:21 AM.

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    Quote Originally Posted by g4ugm View Post
    You can build an analog adder with fewer valves than a pure digital adder, and it will be almost as reliable.
    I am restoring a DEC-PDP-9 and found a problem with the adder. The transistorized adder circuit is far from digital and has four possible logic voltages on the summer. We made an LT-SPICE simulation of the adder because we really didn't understand how it worked, and were a little challenged to fix it.
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