This is the place to leave well-written magazine-style articles.

If you've written something which sounds like a Wikipedia-entry, place it here.

(Be aware: inside of Joomla, every piece of text is called an "article")


  • Trying to fix a DEC PDP-11/34 backplane

    This articles describes manufacturing failures on a PDP-11/34 backplane, and some strategies to repair it.

    (There is also a German version, thanks to Mark Gerber for translating it to English)

  • EDDA 3 - An Early Transistorized Process Computer

    (This article is also available in a German version, thanks to Mark Gerber for translating it to English).

    In the midst of the 1960s the process computer "EDDA 3" was developed by Carl-Schenck-AG in Darmstadt, Germany. The field of operations was the industrial weighing and dosing technology and the computer’s design engineer was Rudolf Meyer. The author of this essay is a friend of Mr. Meyer’s son, and already during the age of 15 both were granted to unsolder precious transistors from used EDDA boards. In February of 2010 contact arose again, and Mr. Meyer kindly provided valuable original documents and personal memories. He passed away in April 2020 in age of 88.

    EDDA3-Label  Schenck logo

    The following article pages are gathered from the EDDA 3 user manual with 160 pages. These few pages contain everything you need to build and run an EDDA computer: Theory of digital process control, digital number representation, data transport inside the processor, implementation of an adder, complete architecure of the processor, processor operation in each clock time slice, instruction opcodes, data address space, i/o registers, programming examples, instructions how to connect different types of i/o devices, principals of an hierarchical i/o expansion module system, function of different i/o modules, debugging features, electrical checkout procedures, programm debugging procedures, and technical parameters for every module.

    Document language is generally German. Behind all original scans I've given translation to English.

    These are the title pages of the user's manual:

     manual klein 

    Systembeschreibung zum digitalen Prozeßrechner EDDA 3

    Dipl. Ing. Rudolf Meyer

    Carl Schenck Maschinenfabrik GmbH - 61 Darmstadt
    Darmstadt, im Nov. 1969


    1. Einleitung
    2. Der Prozeßrechner EDDA 3
    3. Elemente der programmgesteuerten digitalen
    4. Aufbau und Arbeitsweise der EDDA 3
    5. Die Programmierung der EDDA 3
    6. Anschluß äußerer Schaltungen an die Eingabe-
       und Ausgabekanäle
    7. Erweiterung der Kapazität der EDDA 3
    8. Prüfung der EDDA 3
    9. Technische Angaben


    Translated this reads:

    System description for the digital process computer EDDA 3

    Table Of Contents:

    1. Introduction
    2. The process computer EDDA 3
    3. Elements of program controlled digital data processing
    4. Construction and principals of operation of EDDA 3
    5. Programming the EDDA 3
    6. Connecting external peripheral circuits to input and output channels
    7. Enhancing the capacity of EDDA 3
    8. Checking EDDA 3
    9. Technical properties


  • Time scales of a PDP-11/34

    Take a journey into a running PDP-11/34!

    A logic analyzer is connected to various CPU signals, and the '34 is powered on.

    la propbes

     Then we zoom into different time scales between 100 millisconds and 10 nanosconds and watch what the CPU is doing.

    Of course this article is influenced by the famous video "Powers of Ten", which was created only a few years after the PDP-11/34. (Must link to the xkcd version here!


    This article is about the "DIGI-COMP I" device ... a mechanical calculating toy from 1963.

    DIGI-COMP was designed for manual operation. For better visibility, an Arduino-based motor mechanism is added here.

    digicomp1 noglass

    On first look DIGI-COMP appears to be quite outdated and limited, even for retro-enthusiasts.

    But all these levers, sliders and rods implement a finite-state-machine with flip-flops and a combinatorical AND/OR logic matrix, so it is quite timeless and is a great model to teach logic design.

    In this article series you find videos showing DIGI-COMPs operation, the manuals, links to other pages, operation details, theory of logic functions in normal form, similarity to modern FPGAs and a discussion about Turing completeness.