PDP-8-Serial Discrete

PDP-8 Serial All-in-One

A W070 compatible serial interface for the original DEC PDP-8, providing both teletype current loop and RS-232 compatible interfaces.

Background

The original PDP-8 was commonly used with an ASR-33 Teletype connected through a DEC W070 interface. The ASR-33 provided a 110 baud printing terminal together with paper tape punch and reader facilities.

The Australian Computer Museum Society PDP-8 is missing its W070 interface. This project started as a RetroChallenge 2026/10 project to build a practical replacement, while adding an RS-232 interface so the machine can also be used with terminals such as a DECWriter or VT220, or with a modern computer through a USB to RS-232 adaptor.

RetroChallenge 2026/10 project thread

Design goals

• Provide a functional replacement for the serial and current loop functions of the DEC W070.
• Retain support for a period current loop Teletype, including reader enable.
• Provide an RS-232 DCE interface on a DE-9 connector.
• Allow the PDP-8 transmit, receive and reader enable paths to be independently selected between current loop and RS-232.
• Preserve the PDP-8 negative logic interface levels.
• Provide visible TX, RX and Reader Enable status indicators.
• Use straightforward, readily available components and remain practical to diagnose and repair.

There is deliberately no onboard USB interface. A conventional RS-232 interface keeps the board useful with both period equipment and modern USB to RS-232 adaptors. I've compromised on going with the the more 1980's era DE-9 connector to accomodate the screw terminals for the current loop.

Interfaces

W070 compatible current loop

The six position terminal block provides the Teletype connections:

The connections are arranged as three pairs:

• TTI / keyboard: 3 (-15 V), 1 (return)
• TTO / printer: 2 (TTO), 6 (return)
• Reader enable: 4 (+10 V), 5 (enable)

RS-232

The DE-9 connector is wired as DCE and is intended for use with a straight through cable to a DTE terminal or computer. If you are using a USB-RS232 adaptor, it should plug in directly.

Interface selection

Three selector links choose the interface used for the relevant PDP-8 signals:

Indicators

• TX indicates transmitted SPACE states.
• RX indicates received SPACE states.
• RE indicates Reader Enable.

Power

The board uses the supplies already available in the PDP-8:

• +10 V
• -15 V
• GND

A local 78L05 regulator provides the +5 V supply required by the interface logic.

Design files

The project is designed in KiCad. The repository contains the schematic, PCB layout and associated project files required to reproduce or modify the board.

Before manufacturing a board, check the current revision notes and run KiCad ERC and DRC. As with any interface connected to vintage hardware, verify supply voltages and connector wiring before installation.

Licence

This hardware design is licensed under the CERN Open Hardware Licence Version 2, Weakly Reciprocal.

SPDX-License-Identifier: CERN-OHL-W-2.0

See LICENSE for the full licence text.