DNVT Military Phone Switch for TA-1042

Stock Update: we have all DNVT switch models and the adapter boards in stock. We have now exhausted the entire (reasonably priced) supply of the required transformer in the US; we have enough for about 50 more units after which lead times are into 2024. If we run low, we will switch the kit over to the bare board version as there is one supplier for the transformers at higher cost.

An open-source switch to connect to digital non-secure voice terminals (DNVTs like the TA-1042). It has two main modes:

  • “Line Simulator Mode” which will allow you to connect 4 phones and dial in between them (also R key plays rickroll feature)
  • “USB Mode” allows a program running on a host (e.g. raspberry pi or other linux device) to pass bidirectional data and control packets to the device. This will in the future be extended to support VoIP in various fashions (asterisk, custom solutions).

This is what it looks like:

There are two variants offered:

  • RJ45 uses 2x RJ45 ports with 2 DNVT phone lines on each port. There’s a RJ45 to TA-1042 adapter board that works well with these, or you can hack off an ethernet cable and clamp them down in the normal wire terminals of the DNVT.
  • Terminal block which will allow any arbitrary field telephone or other wire to clamp directly into the switch.

These switches are available for sale, see below:

Quick Links

Buying Option:

Video demos:



  • Firmware:

Also featured in a Hackaday article

Get in touch/nerd out/support:

Project Summary

This project began as a reverse engineering of the switching requirements for a TA-1042 (digital non-secure voice terminal). The result was the implementation of:

  • a PCB and circuit interface for the Raspberry Pi Pico to interface with the 4 phones over the standard DNVT interface (and supply power to the phones)
  • the software necessary for a Pico to perform basic switching
  • a custom USB protocol used to communicate with a host application
  • a demo host application to enable switching between multiple 4 phone switch boxes

The project is entirely open source (firmware, software, and hardware). Some future goals:

  • connecting these switches to Asterisk to enable VoIP using DAHDI or SIP
  • Implementing the audio codec on the Pico to enable standard USB audio and keyboard (fixed point CVSD, which requires IIR low pass filter)