A 1960s PLC Gives Up Its Secrets

When it comes to process automation, the go-to part in most industrial settings is a Programmable Logic Controller, or PLC. These specialized computers will have a modern microcontroller running the show, but surprisingly the way they are programmed still has echoes of a time before electronic PLCs when such control would have been electromechanical.

[Thomas Scherrer] has an interesting design to tear down, it’s a Siemens electromechanical motor controller from the early 1960s. It’s not quite the huge banks of relays which would have made a fully-blown PLC back in those times, but it’s a half-way house with some simple programming capability in the form of several channels of adjustable time delay.

We’re partly sad to see this unit being subjected to a destructive teardown, but nevertheless it’s interesting to see all those very period components. The current sensor has a mechanism similar to a moving coil meter, and the four-channel timer is a mechanical sequencer with four adjustable cam-driven switches. We’re not sure we would be cracking open selenium rectifiers with such nonchalance though.

These units were built to a very high quality indeed, and though it’s obvious this one comes from a decommissioned installation it’s not beyond possibility to think there might be some of them still doing their job over six decades after manufacture. Have any of you seen one of these or something like it in operation recently? Let us know in the comments. Meanwhile the video is below the break.

7 thoughts on “A 1960s PLC Gives Up Its Secrets

  1. There are still old substations using mechanical protection relays in NA. The rule in these is “walk softly”- if you stomp your feet, slam your toolbox etc. they can false trip (breakers) due to the vibration. Some old electrical techs even didn’t like people yelling in the building for fear of vibration causing an outage.
    They keep running old transformer and distance protection relays until the budget is there for replacing the entire building and controls, and sometimes it never happens.

    It’s very difficult making a mechanical integrator and time delay that doesn’t drift with temperature, humidity. I thought the V2 rocket guidance system is an example of extreme mechanical computing with German precision machining. They were very expensive.

    That old pellet bridge rectifiers measured Vf=0.20V, might be copper-oxide or germanium instead of selenium.

  2. When you are working inside a substation, anything that goes wrong gets blamed on your actions. Even if a relay just vibrated and caused a breaker trip.
    I accidentally joined the 1MW Club by accident plugging in a connector labelled with a stupid Lamicoid font where “5” and “6” looked identical. Caused a breaker to trip. Oops and sorry to that village, it was only out for a few minutes lol.
    Note the bad connection in the PLC he finds, that would have been a nightmare to find.

    This is PLC is more sophisticated than (current) overload protection. It has mechanical P, I, D mechanisms and really nobody could service them, let alone pull them out to send off for service and repair. You could not have the plant/process down, not working for a length of time. Very hard to work on.

  3. An EE coworker used to work on large switchgear. They had what they called the ‘load-shedding club’. Say you plunge Denver into darkness, well you just shed that much load. Membership was not a good thing. My coworker never elaborated but I gathered he was a member.

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