Rob Ruark and Nick Andre Reverse Engineer Classic Military Telephones — and Build a Modern Switch

Vintage communications enthusiasts Rob Ruark and Nick Andre have been hard at work reverse engineering a military phone — and have built a compatible telephone switch, powered by a Raspberry Pi Pico, for any civilians interested in getting the devices up and running again.

"For an awkward period between the era of analog telephones and VOIP telephony, digital field telephones connected by twisted pair wires carrying unframed differential Manchester-encoded CVSD (Continuously Variable Slope Delta) modulated audio found use with the US military and NATO allies. These are DNVTs (Digital Non-secure Voice Terminals)," Ruark explains of the project.

"I've always through that field telephones are neat. In July 2022, these phones happened to come up in a conversation with my friend Nick Andre who was also interested in trying to get these phones working with a modern system."

While the hardware itself is readily available in the surplus market, they're not of much use without the antiquated military communications infrastructure for which they were designed. What followed was considerable effort in reverse engineering their operation — the military being less than generous in providing documentation to civilians — and the development of a replacement switch to get the phones back in operation.

"I first figured out how to physically interface with the phones and then built a custom circuit board to interface the phones with an STM32F103 microcontroller running crappy Arduino code to encode and decode data in Differential Manchester Encoding," Ruark explains.

"From there, I figured out the in-band control codes used to control the state of the phones and the sequencing of these codes needed to transition the phone through its various states. Nick then stepped in and implemented a switch on a Raspberry Pi Pico based on these findings. Serendipitously, the Raspberry Pi Pico's RP2040 microcontroller contains PIO (Programmable Input/Output) blocks capable of easily decoding differential Manchester code. After all of this was sorted out with single-line test boards, I designed a four-line switch board."

The resulting switch board, housed in an attractive chassis with a color OLED display, provides connectivity for up to four DNVT phones. In stand-alone mode, the Raspberry Pi Pico allows one of the phones to dial up another directly — while a clever USB mode, described as "more of a proof of concept," allows a host computer to interface with the handset over a USB 1.1 connection.

"Why did we make it? It's pretty fun," Andre explains. "[I] wanted to try my hand at embedded development, and Rob is great at hardware. Also nobody has been able to figure out how to reverse engineer the DNVT protocol and implement it on hardware, so we thought we'd take a stab at it, and it works. So now these doorstops can be used again."

Ruark's write-up of the project is available on his website, while Andre is selling the completed four-line switches on Tindie for $131 fully-assembled with power supply — though you'll have to provide your own military telephones.