Shipping times: I ordinarily ship within 1 business day but I'll have some travel this summer and if I cannot ship within a week I'll email you after your order. I'll be at Toorcamp the last weekend of June and I'll have some of these with me for purchase there :)

RJ45 to TA-1042 adapters: we now have a separate listing for RJ45 adapter boards. You should be able to order these in the same order as the switch. If you order an assembled RJ45 I'll include one of them at no cost.

Kit Version: the new kit version will require soldering the DC barrel jack and 8x transformers (and terminal blocks for that version). Video instructions are here, no written instructions yet sadly.

DNVT Phone Switch

Is your TA-1042 military phone collecting dust or sorely missing its ability to dial other phones? Well you're in luck! This nifty little device can interface with 4 DNVT terminals and provide a Line Simulator mode or interface them with your computer over a custom USB protocol. All code and hardware are open source. The product page here has links to all relevant resources.

This is an entirely open source project, hardware by Rob Ruark and software by Nick Andre. Featured in this YouTube video, the first of a series covering this project.

Rob has an excellent writeup on his website here as well.

The IEEE paper that enabled the reverse engineering is here.

Functionality

Firmware Version 0.31

Video Demo of functionality

The switch integrates with 4 DNVT phones. When powered up without a host program running, the switch operates in "Line Simulator" mode to demonstrate the capabilities. An indicator on the screen will show USB status (n/c - not connected, inact - usb device is enumerated but host program isn't running, and activ - usb host program is running and switch will proxy requests to the computer.

Line Simulator Mode: This mode allows the user to dial any of the 4 phones using a simple dial plan (press key 1-4 for line 1-4). The remote phone will ring and connect. When the second phone hangs up, the phone will return to dial mode and can call another phone.

USB Mode: a host program is available that uses the libusb binary to interface with the phone over USB 1.1. This host program currently allows switching between multiple switches, but should be considered more of a proof of concept. Several future improvements are planned (support for an AWS service that will switch between phones, proxying the data to Asterisk, etc.).

The display provides:

1. Firmware version.
2. A line for each phone with state and dialed extension.
3. An approximation of load using a timer and some math (observed to be a little buggy in v0.3 in USB mode).
4. The current USB state.

Purchasing Options

There are four current options:

• Fully assembled and tested or a kit with everything you need to solder yourself
• RJ45 (eg ethernet cable) of Terminal Block (allows individual stripped wire insertion)

In addition you may order the boards directly from a manufacturer using the design files should you wish.

1. Partially assembled board. The boards have all SMT components soldered by JLCPCB and require soldering of all through hole parts (several headers, barrel jack, ground lug, transformers, RJ45 or terminal blocks).
2. Fully assembled and tested board.

In addition, we have a few assembled "Blinkenlights" versions (the first version we produced) that we will list. These do NOT include a display and instead have a light array. They are also missing DIP switches for configuration, so will use the default settings, but they do look cool.

Note: terminal blocks expose 4 wires for each phone. RJ45 exposes two phone interfaces per port; they do not follow the standard 10/100 ethernet wiring so if you want to split them you'll need a custom jack. The cheapest solution is to hack the end off an old CAT5 or higher cable and follow the pinout below. We will provide TA-1042 to ethernet adapter boards in a separate listing which allows you to plug directly into a phone with RJ45 and daisy chain a second phone.

The pinout of each RJ45 jack is printed on the end PCBs for assembled units and is as follows:

The default choice is fully assembled RJ45 switch, which includes the following:

• DNVT switch board hat for Raspberry Pi Pico with headers and has drivers/transformers for DNVT line circuit.
• i2c display for status information.
• USB interface to custom host software through RPi Pico micro usb port
• support for standalone dialing between 4 phones (add power and call between 4 phones)
• ability to rickroll you or unsuspecting friends (press the R key in line simulator mode)
• power supply to all 4 phones via 48v DC barrel jack (approximately 250ma at 48V required for 4 phones)
• rather slick metal enclosure
• silkscreened PCB end plates with helpful explanation of all pinouts
• 3.3V + 4 GPIO pin expansion header supporting SPI with UART debug console enabled by default
• There is no DC/DC converter so the switch requires 5V over micro USB (host computer or phone charger) and 24-56V over DC barrel jack.

Why did we make it?

It's pretty fun. 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 :)

Gory Technical Details

For a complete write-up see Rob's site here.

For the fun backstory watch this YouTube video.

I'll record a more in-depth demo soon.

Some Additional Notes

This device uses the RP2040/Pico PIO state machines to perform differential manchester encode/decode. As a result you won't be able to use this as is on the Pico W which requires 1 state machine for wifi.

This device will only support either 16 or 32 KHz operation at one time; as a result, if you want to use TA-1035s, you must set all phones to 16 KHz and all phones will operate at that rate. For some more illumination:

• The DNVTs support two signaling rates with most phones supporting 32 and 16 KHz with the exception of the TA-1035U which seems to only support 16 KHz mode.
• As a result of the shared clock in the PIO we can only configure signaling rate on pairs of machines without some clever implementation.
• A feature is available in these phones for the switch to request a 32KHz phone fall back to 16KHz for the text phase; however, a) we have yet to find this codeword, b) the implementation would likely need to change to support this adequately, and c) the paper seems to specify that audio data will switch to 16Khz but the switch will still have to read codewords at 32KHz simultaneously which presents a quite difficult implementation challenge. It's probably doable if we dedicated the second core to a software implementation but would substantially complicate the implementation.

Also the USB PHY is 1.1 which is plenty of bandwidth for DNVT in interrupt polling but worth noting.

The estimated range of the total wire length switch to phone is on par/better than the DNVTs. We estimate about 4km total with CAT5 compliant cabling. The main limit is attenuation.

For more software details, please review the github link. This code currently runs on one of the two cores in a processing loop. The second core is currently used for display updates due to the time requirements for the i2c display readout, but a transition of i2c to the DMA facilities would free the second core for additional functionality.

Future ideas

• USB host program connectivity to Asterisk with patch for asterisk to handle CVSD
• On-board CVSD codec functionality in fixed point
• USB modes for audio and keyboard i/o for Zoom use

If you have ideas or want to make a PR, feel free!

We're also investigating the development of a similar analog 8 port FXS device with autovon DTMF decode to interface analog telephones, which will include a radio interface which can be enabled instead of the 8th line.