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Multi Tech MT5656ZDX modem

Discussion in 'Embedded' started by Hul Tytus, Apr 11, 2014.

  1. Hul Tytus

    Hul Tytus Guest

    comp.arch.embedded, sci.electronics.design
    Multi Tech MT5656ZDX modem

    A Multi Tech MT5656zdx modem is now connected to a computer here and is
    working great - at 2400 baud! The AT$SB38400 command, which is reported to
    change the serial baud rate for the computer interface, produces "ERROR"
    which, according to a manual for a different Multi Tech modem, indicates an
    invalid command.
    A modem limited to a 2400 baud computer connection seems unlikely so,
    hopefully, there is a way to change it. Anyone familiar with these modems have
    a suggestion?

    Hul Tytus, Apr 11, 2014
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  2. Hul Tytus

    hamilton Guest

    Google found this:

    Page 15:
    Command: $SBn Serial Port Baud Rate
    Note: Use this command with MT5600BA and MT5600BR only. Not supported by
    the MT5656ZDX/MT5656ZDXV

    Page 36:
    Command: +VPR Select DTE/Modem Interface Rate (Turn Off Autobaud)
    Description: This command selects the DTE/modem interface rate.
    Syntax: +VPR=<rate>

    Your Welcome.
    hamilton, Apr 11, 2014
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  3. Hul Tytus

    Hul Tytus Guest

    Hul Tytus, Apr 11, 2014
  4. Hul Tytus

    Hul Tytus Guest

    Hamilton - no luck, that's the modem rate rather than the rs232 rate. Said
    differently, the data rate between the computer and the external modem is
    only 2400 baud and, apparantly, can't be changed. The VPR command alters
    the data rate between the external modem and whatever's on the other end
    of the telephone line.
    Thanks for looking, though.


    Hul Tytus, Apr 11, 2014
  5. Hul Tytus

    mroberds Guest

    Quite a lot of modems will automatically set their serial port bit rate
    to match what the host computer is doing. (That was part of the
    original purpose of the 'AT' command prefix - it gave the modem a couple
    of bytes to sync up to.) Just change the bit rate on the computer to
    whatever you want and the modem should change itself to match.

    If you want to "lock" the speed in the modem for some reason, there is
    usually an AT command to do it, but I don't think it's standard - check
    the manual for your modem.

    Also note that if the modem does data compression, it's pretty common
    to set the serial port bit rate on the computer to 2x - 4x the phone
    line speed, to let the compression work. The modem uses the flow
    control lines on the serial port to stop and start the data from the
    computer, so the modem only ends up with as much data as it can handle
    at the moment. Some modems with phone line speed of 9600 bps did
    compression, and I think all modems with phone line speeds of 14400 bps
    and up do compression. Just multiply the phone line speed by 2 or 4 and
    then round up to the next "standard" bit rate.

    Matt Roberds
    mroberds, Apr 11, 2014
  6. Hul Tytus

    upsidedown Guest

    Are you sure that the modem does not support autobauding ?

    Set the serial port to some higher speed, such as 115200 bit/s, then
    turn on the modem and send some <CR> characters or for instance the
    command. Many modems autobaud with the "AT" characters.

    Please note that when using simple FSK/PSK protocols without ECC or
    framing, the serial speed _must_ be the same as the line speed, so
    that 2400 bit/s makes sense for simple PSK.

    In order to use different serial and line speeds, you need to
    configure the modem to some error correction mode. On the line, blocks
    of data are transferred usually as synchronous frames, handling error
    correction (or resends), data compression as well as flow control
    between modems.

    With these assumption, the serial line speed can be higher (and should
    be, if compression is used) as the line speed.

    Figure out what error correction modes are supported, select one of
    them, save settings and then try autobauding the serial line.
    upsidedown, Apr 11, 2014
  7. I think I had one of those back in the day, and I'm pretty sure it does.
    Been a long time though, I might not have the model number exactly the same.
    Many of them use the Rockwell V92 chipset, so lots of models are very
    similar in functionality.

    Theo Markettos, Apr 11, 2014
  8. Hul Tytus

    Hul Tytus Guest

    Typically, any rate other 2400 showed a partial response or none at
    all. At 2400, "AT<cr>" returned OK, at 9600, if memory serves, just K and 38400
    showed nothing.
    It does smell like an "autobaud" of some sort that
    requires info about it's operation but none is shown. The "user's
    guide" mentions a cdrom which wasn't included; maybe information was
    originally placed there. Overall, Multi Tech appears to be producing a
    "windows only" piece of gear and the mentioned cdrom may have contained
    installation software.


    In comp.arch.embedded
    Hul Tytus, Apr 11, 2014
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