DEC PDP-12, System Number 696

This December 1973 vintage PDP-12 was originally purchased by the Leesona Corporation in Warwick, RI. It was used to test a pneumatic computer (ALPS) that was an accessory to a weaving machine. It has some special I/O that was added by Leesona and is connected to the LINC Sense Lines. The in 2015 dollars this machine would cost $310,000.

The architecture of this system is a merge of a LINC (Laboratory Instrumentation Computer from MIT) and a PDP-8/I, so it has two modes for the processor, LINC and PDP-8/I. A single I/O instruction changes the instruction stream from one processor mode to the other. The LINC processor manages the digital and analog I/O, graphics, and the LINCtape. The PDP-8/I processor handles the disk, diskette, and other peripherals.

This system was built very late in the life of PDP-12s, so it has the Priority Interrupt and Data Multiplexer options pre-wired in the backplane. It also has an Omnibus expansion chassis so that it could have a large (2.5MB) disk drive.

The RX02 diskette drives and the RX8-E controller, and the paper tape reader/punch and the PC8E controller came from Brown & Sharpe.

Current Status:

    • The processor and memory have been working well for more than six months.

    • The RK05 disk drive, RX01 floppy disks, VR14 display, paper tape reader/punch, both serial ports, analog inputs, relay outputs, and speaker all work.

    • It will boot and run OS/8 from the RK05 or the RX01, including the special handlers for the PDP-12 devices.

    • We are still having problems with the TC12/TU56 LINCtapes, but we are working on it.


The console for the PDP-12.

The indicators and switches are a combination of PDP-8/I and LINC processor functions.

Features and serial numbers from the labels in the system:

M36 #696 (System Number)

MC #619

MEM #822

PC8E #SP57779

PC04 #SP15627

TU56 #8698

KW12A #646



M22VR14 #WF 558

LP01FA #739, 5060 DP2310 80 Column, 64 Character Line Printer

Installed Options:

AD12 Analog to Digital Converter

AG12 A-D Additional PreAmps

AM12 A-D Expanded Multiplexer

DP12-B Teletype Data Phone, EIA Levels, Missing M706 in MN9, M707 in MN10

DR12 Relays and Control

EM12 Basic 4K Memory

EP12 PDP-12 Processor

KE12 Extended Arithmetic Element

KF12 Automatic Priority Interrupt

KW12-A Programmable Real-Time Clock

MC12 Memory Extension Control

TC12 LINCtape Control

VC12 Scope Control

Wired But Not Installed Options:

DM12 3 Channel Data Break MUX

KP12 Power Fail/Restart

KT12 Timesharing

TC12-F DECtape Control (This needs just 2x M216 modules so we will install this)

XY12 Plotter Control

This is the swing-out frame that holds the processor electronics.

The upper part is the 8kW core memory, analog input, and LINC tape controller.

The lower part is the combined PDP-8/I and LINC processor, the serial console interface, and the priority interrupt controller.

DW8E-PC Omnibus Expansion Chassis populated for Posibus I/O Bus Converter

4x M7102 Omnibus-Posibus Interface

M7171 Bus Control

M8357 RX8-E RX01/RX02 Diskette Controller (From Brown & Sharpe) Added

M841 LE8-0 LE8-FA LP01 Printer Controller (From Brown & Sharpe) Removed

M7104-YA, M7105, M7106, RK8F RK05 Disk Drive Controller

M840 PC8-E PC04 Paper Tape Reader/Punch Controller (From Brown & Sharpe) Added

M8320 Omnibus Bus Loads

This is the DW8E-PC Omnibus Expansion Chassis.

The boards at the top convert the Posibus I/O signals from the PDP-12 to Omnibus signals on the backplane.

The Omnibus contains an LE8-FA LP01 controller, RK8-F RK05 controller, and a PC8-E PC04 paper tape reader/punch controller.

It only supports one data-break controller per Omnibus.

We removed the LE8-FA printer controller to make room for the RX8-E controller for the RX02 floppies.

To Do:

  • Install the batteries in the RK05 disk drive.

  • Try powering the TC12 from a laboratory power supply to see if a really clean power source helps with the TC12 LINCtape controller problem.

  • Put the logic analyzer on the M222 modules and see what the tape block number signals look like.

  • Maybe replace the SN7453 E5 on the M222 module from slot AB21.

  • Look at the signal outputs from the VC20 that drive a VR14. We should be able to display the output in X-Y mode on the oscilloscope.

  • Clean the switches in the TU56 DECtape drive.

  • Run more processor and TC12 DECtape controller diagnostics. Need documentation.


    • Scan and publish more diagnostic documents. (Done)

    • Clean the fingers on the M222 modules, reload MAINDEC-12-D3AD, and try it again. (Done)

    • Bolt the two cabinets together and connect DW8E-PC Omnibus Expansion Chassis to the PDP-12 CPU. (Done)

    • Remove the Omnibus boards from the chassis, reform the caps in the DW8E-PC, and power it on. (Done)

    • Reinstall the PC8-E PC04 paper tape reader/punch controller, reform the caps in the PC04, and see if the reader/punch works. (Done)

    • Reinstall the RK8F disk controller, reform the caps in the RK05 and see if the disk works. (Done)

    • Test the RX8-E in my personal PDP-8/e. (Done)

    • Install the RX8-E in the Omnibus Expansion Chassis and connect to the RX02. (Done)