May 302013

The TA-1042A/U is a member of the family of Digital Nonsecure Voice Terminals (DNVTs). Basically, this is a field telephone with a 4-wire digital interface to automatic telephone exchanges of the TRI-TAC family. The wireline interface can operate at either 16 or 32 kilobits per second. There is no embedded cryptographic capability, hence the term “nonsecure”.

Two TA-1042A/U field phones may be connected together with 4-wire field line (generally WF-16/U) and operated as plain field telephones, with power supplied by local batteries connected to the terminals on the right side of each phone. They may also be connected to a digital telephone exchange, and receive their power from the exchange over the 4-wire interface. They are not compatible with analog telephone lines, analog field phones, or civilian telephone exchanges. Earlier members of the DNVT family, such as the TA-954/TT, cannot be used without connection to a compatible telephone exchange (i.e., they can’t be used as simple field phones.

Chris Story K6RWJ and I have decided to take on a project to reverse-engineer the digital interface used by these telephones, and create an interface which will allow them to be used as VoIP phones. We have dubbed the project DNVT2IP.

I just received this phone from Murphy’s Surplus Warehouse, along with a few more of them. Let the reverse-engineering begin!

  15 Responses to “TA-1042A/U Digital Nonsecure Voice Terminal (DNVT)”

  1. Hello! I was wondering how your project was coming along. I have a DNVT myself. If you happen to need another let me know. I was just searching the web to try and find out what one could do with one of them. Not much use with just one I suppose.

    • Hi there! I have a few TA-1042 units, and I’ve also gotten a TA-954 and a CA-67 for future compatibility testing. I’d like to find a TA-1035 for compatibility testing, too.

      I haven’t made much measurable progress on the project yet because of other stuff getting in the way. It’s mostly something kicking around in my brain right now, and I’m at the phase of the project where I dread the job of creating the schematic symbols and PCB footprints that I’ll need for the design. That part is always the least fun for me. :)

      I’ve identified some components that I’ll use, such as a DC-DC converter for the 48V supply, a hopefully suitable pulse transformer, a differential comparator, etc. I’ve also been researching line protection devices for telecom lines. I may ditch the analog phone circuitry from the block diagram that I’ve published to simplify things.

  2. I”m also interested in the progress of this project. I’ve been researching a similar concept using a Papilio Pro Xilinx FPGA board, and have found some good information on creating the CVSD encoding/decoding portion and some basic information on the CD/Manchester modulation and signaling. Putting it all together is still an overwhelming idea. I currently have a pair of TA-1042A/U DNVTs, and am hoping to do some signal analysis on how the phones communicate. If I end up with anything useful, I will definitely share. I’ll be following the DNVT2IP closely! Cheers!

    • Good luck with your project! I’ve been distracted by other shiny objects, and I haven’t spent any time thinking about DNVT2IP lately. Maybe you’ll make more progress than I have?

  3. Something to check:
    I believe there is a multipin connector that included the dataport and a direct audio out and direct audio in for connection to another device.
    If you can get direct audio out of the handset and back to the handset via the data jack, then there is no need to mess with the diphase 4 wire interface.
    Direct audio between handset and the R-Pi ?
    Im not totally sure you would need the FPGA either.

    The FXS-FXO stuff is easy to handle with a “SLIC” circuit all in hardware.

  4. Hello, I was wondering if any progress has been make with the phones? I also have a big interest in this project, I used the phones in the Army years ago, and still use them for point to point, would be great to be able to use multiple units.

    Neil Sherman

  5. I just got a Digital Non-Secure Voice Terminal (DNVT) TA 1042A/U for $15. I would like to know how much it is worth and how to get it working if possible.

  6. Can these be made into a regular land line as a conversation piece??

    • Unlike the earlier analog field phones, these digital ones cannot be connected directly to a plain phone line.

      You could connect an EE-8, TA-43, TA-312, TA-838, etc. to a plain phone line. The EE-8 or TA-43 would be answer-only. The TA-312 can accept a touch-tone dialing accessory, and the TA-838 has a built in touch tone keypad. I used to use a TA-838 that way in my apartment in Irvine, a long time ago! But now I have neither a TA-838 nor a land line.

  7. Any interest in finishing the project?

    Very respectfully,

  8. No worries! That would have been a fun project as you and I bought out DNVT when available from the same place.


    Very respectfully,

    • Yes, I would certainly enjoy completing the project, but there are just too many other things in line in front of it! I won’t say that I’ll never work on it again, but it’s far down my list at the moment. Maybe somebody else will take on a similar project?