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MODCOMP Classic Control Panel

Category: Articles
Written by Administrator

Here comes the description of MODCOMP and its control panel, by Josh Heaton.



The MODCOMP Classic was built by MODCOMP (Modular Computer Systems, Inc) in 1978. The Classic replaced the MODCOMP IV, offering full support for 32-bit addressing. The platform offered 240 general purpose registers, addressable as 16 banks of 15 registers. The machine also had a special register zero that reflected the state of the data switches on the front panel.

full installation manual

MODCOMP was popular in process control applications, offering deterministic real-time interrupts and real-time control. The architecture was modular in nature (hence the name), allowing systems to be built to match the intended application. The system was even adopted by NASA, tracking space probes and collecting data and ultimately controlling the Space Shuttle launch at Cape Canaveral.

For details see this handbook.

The MODCOMP was somewhat unique in that it had a phased clock signal. The full fetch/decode/execute/store cycle was completed with each CPU clock. So for example, the 7850 had a CPU frequency of 66 MHz and was capable of 66 MIPS.

7830 Control Panel



The Model 7830 had a control panel with 16 data switches (that set the value in register 0), the ability to step the processor one instruction at a time, and inspect/modify any value in memory at any point in time. A security key disables the HALT/RUN switch for an extra layer of physical protection in critical applications. MODCOMP panels were tied directly to the CPU, so panels could not be swapped between different models.

The panel itself operates with an 8-bit bi-directional data bus and 3-bit address bus. There are also several bus pins dedicated to hard-wired switches such as halt, master clear, and console interrupt. The entire panel runs on 5VDC, consuming approximately 1500mA of current.

All of the panel’s functionality is accessible through a 40 pin header, “J5”. Half of J5 is tied to ground, leaving only 20 true I/O pins. Save for one chip, all of the logic is stock 74LS-series. Ground is logic TRUE.


For more detailed pin mapping, see attached PDF.


Download this file (modcomp.pdf)modcomp.pdf[MODCOMP Classic 7830 Control Panel]3125 kB

PDP-11 restoration by simulation

Category: Articles
Written by Administrator


Those who can, repair. Those who can't, simulate.

For almost all vintage computers software simulations exist. The „SimH“ suite is well  known for classic mainframes and minis.

Early consumer micros (Commodore, Atari, Amiga, etc.) can be run on „MAME“ or Raspberry Pi „RetroPie“ for gaming fun.

On the other side, old discrete computers like DEC UNIBUS/QBUS PDP-11s must often be repaired. Then we replace defective parts sometimes with ancient or modern hardware.

A special case are disk drives, which are most sensitive and always likely to fail. SD card solutions exist for many systems like C64, Amiga, Apples or SCSI.


So today a vintage computer can be a mix of software simulations, recreated hardware and original parts.

PDP-11 continuum

Uncountable repair and simulation projects exist for DEC PDP-8s and -11s, making these machines the perfect model case for "restoration by simulation".

See several PDP-11 projects arranged on a scale for their „virtuality“ or „physicality“ (are these even english words?)

virtual physical tableclick to enlarge

Project Link collection:

Living Museum PDP-11s: "Miss Piggy" at LCM+L Seattle, LSIBox

The UniBone and QBone projects are special, as they allow to simulate arbitrary parts of a PDP-11 in hardware.

DECBox, PDP-11 in a VT100 terminal

BlinkenBone for reanimating console panels

PDP2011, Sytse van Slootens FPGA recreation with PiDP11 interface

PiDP11, Oscars Vermeulens famous PDP-11/70 replica

SCSI2SD, SCSI "disk drive" based on SDcards


Well, sometimes we have that strange part mix, with repair parts fetched from dead PDPs, assembly often done in a dark night in a sinister lab, and life was given by toggeling the big power switch ... See here one of these in action:






  • 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.