Rotary Dial Tester Adapter

A small connector/adapter for testing vintage rotary dials (e.g., W48/W49) on a computer. It connects via DB9 RS232 (use an RS232-to-USB adapter) and provides both color-coded screw terminals (GE/GR/BR/WE) and a 4-pin connector for the dial wiring. This repository contains the KiCad project files.

Overview

• DB9 RS232 interface for USB serial adapters
• Color-coded screw terminals (GE/GR/BR/WE)
• 4-pin connector for a cable harness
• Tested and working with: DIGITUS USB auf Seriell Adapter - RS232 Konverter - USB 2.0 Typ-C zu DSUB 9M - FTDI FT232RL Chipsatz - 1 m Anschlusskabel (https://www.amazon.de/dp/B07917RBBJ)

Wooden base (CNC)

• Estlcam project: adapter-wooden-base/Adapter_Base.E12
• Estlcam is a CAM and CNC control program; offers inexpensive USB control hardware and easy control via gamepads, handwheel pendants, keyboard, and mouse.

Required materials

• 18 mm wood for the base
• 4x brass wood screws, DIN 96 pan head, slotted, 3 mm x 8 mm
• 4x Keystone Electronics screw terminal 8191-3 (or 7770), M3
• 4x brass knurled screws, M3 x 6 x 9.5
• Adhesive dot set, 5 mm foil K020 (yellow, green, brown, white)
• 4x Wittkower solder terminal, 2.8 mm flat tab, straight, for PCB mounting
• 1x Wurth Elektronik 61800929321, D-SUB receptacle, 9-position, vertical solder

Wiring

Use either the screw terminals or the 4-pin connector. The wiring diagram below shows the pin mapping.

Pinout

• NSI (DCD) -> Yellow (GE) -> SUB-D pin 1
• RTS (common, high) -> Green (GR) + Brown (BR) tied together -> SUB-D pins 2 + 3
• NSA (NSA) -> White (WE) -> SUB-D pin 4
• NSR (DSR) is not connected on this board

NSR (DSR) is the dial’s off-normal/rest contact. It is closed at rest and opens when the dial starts, which can be used to detect start/end of dialing or to mute the speech path during dialing. For simple pulse counting, NSI (DCD) and optionally NSA (RI) are sufficient; if your dial needs NSR, you will need to rewire or modify the adapter.

Non-German rotary dials can be wired differently (e.g., 3-wire Ericsson or some US variants). If your dial needs NSR/DSR, you will need to rewire or modify the adapter, since only DCD/NSI and RI/NSA are routed.

Web-based Rotary Dial Tester

Project: https://github.com/SunboX/Rotary-Dial-Tester (live: https://rotary-dial-tester.com/)

• Web app using the Web Serial API to read rotary dial pulses.
• Requirements: Chromium-based browser, served over https:// or http://localhost, and a serial adapter that exposes modem status lines (DCD/DSR/RI).
• Signal mapping: nsi = DCD, nsr = DSR, nsa = RI; RTS is set high on connect.
• Usage: Connect COM, Start Test, then dial digits 2-0 (1 is too short for frequency measurement). The browser prompts for the port (no auto-detect).
• Debounce (EP): adds a wait time between DCD reads; higher values can reduce bounce but may affect measurements.
• Analysis: after 10 measurements of the same digit, the app shows runtime spread and pulse/pause tables.

Photos

License

This project is available under two licensing options:

1. Open hardware and documentation licenses

Hardware design files in this repository are licensed under the CERN Open Hardware Licence Version 2 - Strongly Reciprocal (CERN-OHL-S-2.0).

Documentation, photos, diagrams, and other media are licensed under Creative Commons Attribution-ShareAlike 4.0 (CC-BY-SA-4.0) unless otherwise marked.

2. Commercial/proprietary license

For use in closed-source, proprietary, or otherwise public-license-incompatible products, a separate commercial license is required.

Commercial licensing contact: mail@andrefiedler.de

Attribution / notices

Copyright (C) 2026 André Fiedler.

Copyright, license, attribution, and source-origin notices must be preserved as required by the applicable public licenses and the notice files in this repository.

The component data in parts/7770.pretty/ contains third-party source-origin metadata for Keystone Electronics / SnapEDA material and is not relicensed by this project. See NOTICE.md and .reuse/dep5 for details.