Remaking the PDP-8/I using the Pi, simh & a replica front panel: a true catching project!

Goal: a (very) low cost full replica of the PDP-8/I.

Design: use the $20 Raspberry PI Model A+, a custom front panel PCB to mount it behind, and 26 nice-to-use toggle switches. Extend the simh emulator to drive the front panel.

Reason: PDP-8 replicas can get very expensive very quickly. The idea was to do the exact opposite: design a faithful replica using minimum parts. Made possible by the fact that the computing engine is available for $20 (Pi model A+) and in software, there’s no better replica of PDP-8 functionality than the simh emulator.

Result: Pi drives the 92 leds and 26 switches using a multiplexing process running in parallel to the simh emulator. Only a few ARM instructions are necessary to update the front panel during a PDP instruction cycle. An acrylic front panel faithfully reproduces that of the original.

Components

• 1×Raspberry Pi (Plus or 2)needs 40 pin GPIO connector

• 26×Toggle switches

• 12×390 Ohm resistors

• 26×4148 Diodes

• 1×UDN2981A (optional)Interface and IO ICs / Peripheral Drivers and Actuators

• 1×PCBDesigned in Kicad/Open source hardware design

• 89×LEDs (5mm, High Brightness)Electronic Components / Misc. Electronic Components

• 3×1K Ohm Resistors

• 1×2*20 pin header for mounting the Raspberyr Pi