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Thread: Question about 3270 Coax Protocols

  1. #21


    Hi - came across your query while looking for something else and it caught my eye because I used to be an expert - around 30-35 years ago! Regret I have not been able to find the appropriate spec anywhere but fwiw -

    The 3278 coax level protocol was very different from the 3277 in that the screen buffer was managed by the control unit, which polls the terminal for keystrokes and echos the characters to the terminal. (The 3277 managed everything at the terminal until the user pressed an attention key, when the control unit would read the whole screen buffer.) The protocol is very simple, as the control unit does almost all the work, including translating keyboard scan codes to characters for different character sets, and managing the cursor position. As far as I remember almost everything was achieved with 3 commands - POLL, SET ADDRESS, and WRITE. The character set was proprietary and different from the 3277. So you could maybe figure that out by sniffing a wire, you would have a lot of work to get from there to 3270 data stream level.

    We built a pc card to emulate the control unit using the National Semi DP8340/41 chip set. The spec here ftp://bitsavers.informatik.uni-stutt...s_Handbook.pdf

    gives some detail of the physical interface for the coax but unfortunately nothing about the protocol. (you need to go down about 170 pages to find the relevant bit)

    Good luck!

  2. #22


    Further to my last reply - you have me intrigued!

    I did a bit more digging and found a spec for the coax level protocol in the data sheets for the Chips 82C570 here -

    If you had the hardware to interface to the coax at a low level (like the National Semi set) a possible approach to the TN3270 level function would be to use x3270 modified to hook your adapter in at the appropriate level. There is a version with no human interface intended for scripting that could form a good base.

    Good luck again!

  3. #23


    Thanks for the added insight. I hadn't noticed that portion of the 82C570 data sheet before. I guess it didn't occur to me that a description of the built in features in microcode would tell me something about the protocol itself.
    A while back I came across a NS document ( -- starting on page 6) which describes the protocol decently well, but not entirely. Between these two documents the protocol is well defined.

  4. #24


    It all comes flooding back! The 8344 doc has lots of good stuff. (Though a bit confusing when they talk of the coax level protocol as Ď3270 Data Streamí which would generally mean the protocol between the host and the controller, a very different sort of beast.). It talks of Ďformidable design challengesí using the DP8340/41 but these all arise from the 5.5 microsecond response time required from the terminal. The controller has no such constraint so is pretty simple to implement. We drove it with programmed IO on a PCXT. Our application was emulating the controller to test terminals in manufacturing.

    Most of the commands can safely be ignored if you are using a shadow display buffer on your controller. There is no reason you would ever need to read or search or insert into the device buffer. When the protocol was designed (around 1977) RAM was expensive, so they had the capability of not shadowing.

    A couple of other things come to mind -

    The buffer addresssing reserves address 0-79 for the indicator row - the bottom row of the screen. The display area is addressed from 80. (except for the Model 1 when it was 64 - but I donít think we ever actually shipped a model 1).

    The character set is not actually EBCDIC, but a proprietary coding known internally to IBM as NDSIC. (The internal name for the 3274/78 architecture was New Display System (NDS) hence NDS Internal Code.) The special symbols on the indicator row use the codes reserved for attributes in the display. Emulators generally donít use them, but they miss the true 3278 experience!


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