Making digital circuits with only discreets

Tip Most picture are available in high resolution when clicked


The first affordable widely sold computer was DEC’s PDP-8 "Straight-8". It was introduced on March 22 1965 and made of DTL’s packaged on Flip-Chip cards.

PDP-8 image from the SMECC museum in Arizona images/dec/pdp8_LoRes.jpg

DEC’s Flip-Chip R107D card (Inverter) images/dec/R107D_300x213.jpg

Tip DEC: Digital Equipment Corporation, PDP: Programmed Data Processor, DTL: Diode-Transistor-Logic

Complexity of the PDP-8:

  1. 10148 diodes

  2. 1409 transistors

  3. 5615 resistors

  4. 1674 capacitors

I want to recreate a range of DTL’s and use them in several projects in the same way as TTL or CMOS logic. When the DTL’s turnout to be reliable over a board temperature range (5~70°C) I want to build a PDP-8 clone or my own computer from it. I use DECs Digital Logic Handbook 1967 for the idees.

Compared to the original design my design uses less board space. But I’m using double sided PCB’s with plated holes while DEC uses one side etched PCB’s without plated holes. I also use hairpin style assembly

Figure 1. Flip-Chip DEC vs Stack-Chip AE
Figure 2. Hairpin style assembly

Image from Elektor summer circuits 1979

To get more components per m3 I use a board stack system. With this system I also hope to reduce wire length which could lead to higher clock rates.

Figure 3. 16 bit counter

The four upper boards are the basic inverter boards (B010 BasInv). The two lower boards are Diode-Capacitor-Diode boards (B020 DCD). All connections of the eight inverters are on one board side. Four BasInv boards can be stacked together with a 90° twist. The interconnection of the boards can be done with solid tinned copper wire laced to all boards. The lowest board gets a cheap standard pin header connector. In the middle of all boards is a M3 hole so the boards can be firmly assembled with help of a M3 threaded rod to the main board. The M3 threaded rod can also be used with a tool to easily pull out the stack from the main board. The board size is the economy priced fusion PCB 5x5cm size.

Figure 4. Assembled PCB’s for 16 bit counter

When combining a BasInv + DCD we get a JK-FF with direct (asynchronous) preset and clear. A full stack with 4x BasInv and 2x DCD can be used as 16bits register/ripple-counter/shifter.

The DCD board has his own inner connector ring, a little bit like the "Droste effect". Thus the upper 4x BasInv board has a single row 2.54mm pitch through hole pins, the lower 2x DCD boards two row 2.54mm pitch through hole pins,

Top view PCB B010B/BasInv & B020B/DCD PCB’s


Figure 5. Shift register connection example

Test setup

Figure 6. My 5~70°C temperature test setup

For high temperature tests I use a modified bake oven. For low temperature tests I use a peltier effect electric coolbox.