Building Educ-8 - an Australian computer kit from 1974

VERY EARLY DESIGN PUBLISHED IN ELECTRONICS AUSTRALIA MAGAZINE

This computer is a stripped down version of a DEC PDP-8 minicomputer, comprised of about 100 integrated circuits (and no microprocessors). It is bit serial, like the PDP 8/S which itself was bit serial. At the time in 1974, the parts for an EDUC-8 would cost roughly $300 assuming that the reader made their own PCBs and bent sheet metal for the case. 

Radio-Electronics magazine in the US published their design for a Mark 8 one month prior to the first article for the EDUC8. The Mark 8, however, was built around an Intel 8080 microprocessor. While the Mark 8 did not gain much attention, it did inspire the publication of the Altair 8800 in Popular Electronics in January 1975, kicking off the personal computer era. 

NEW PCB DESIGNS AND UPDATES BY ENTHUSIASTS STIRRED MY INTEREST

Gwyllym Suter has created KiCad files for the boards and ordered a batch of them. I bought one copy from him as I believe my parts cabinets have most of the parts I would need to populate the boards. The original plans from 1974 used single sided PCBs with discrete wires added to bridge signals in lieu of a second side with copper. The new boards preserve the general layout but do employ back side copper instead of wires. 

STRUCTURE OF THE EDUC-8

This machine uses a motherboard with connectors into which a number of PCBs are plugged. The motherboard sits behind and parallel to the front panel board, with its blinking lights and switches. Each PCB plugged into the motherboard is parallel with the ground, sitting one atop another in a vertical stack behind the motherboard.

The boards in the vertical stack are Timing, Decoding, Memory, Program Counter/Adder, Accumulator, and Input-Output Transfer (IOT). A separate power supply board sits in the case as well. The EDUC-8 has a number of peripheral interfaces designed for it, such as Octal Keyboard, Octal Display, Paper tape interface, ASCII Keyboard, Printer and more. 

CONSTRUCTION STATUS

I have soldered on most of the integrated circuits and resistors to the PCBs. I was short a few common ICs (74LS00 and 74LS04) as this used so many that my parts bin didn't have enough, as well as needing the original Intel memory chips and a 74LS154 which I didn't have on hand. 

Next up I will populate the capacitors and the transistors/diodes etc for the power supply. I began testing the front panel board as I added LEDs to it - currently there are ten that I installed and verified are working through the buffer/driver chips. 

The parts I will need to go buy now are mainly the switches and the connectors on the motherboard into which the PCBs plug. 

By Swtpc6800 Michael Holley - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=13525708


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