nuttx-apps/system/zmodem/README.txt

README
======

Buffering Notes
===============

  Hardware Flow Control
  ---------------------
  Hardware flow control must be enabled in serial drivers in order to
  prevent data overrun.  However, in the most NuttX serial drivers, hardware
  flow control only protects the hardware RX FIFO:  Data will not be lost in
  the hardware FIFO but can still be lost when it is taken from the FIFO.
  We can still overflow the serial driver's RX buffer even with hardware
  flow control enabled! That is probably a bug.  But the workaround solution
  that I have used is to use lower data rates and a large serial driver RX
  buffer.

  Those measures should be unnecessary if buffering and hardware flow
  control are set up and working correctly.

  RX Buffer Size
  --------------
  The Zmodem protocol supports a message that informs the file sender of
  the maximum size of dat that you can buffer (ZRINIT).  However, my
  experience is that the Linux sz ignores this setting and always sends file
  data at the maximum size (1024) no matter what size of buffer you report.
  That is unfortunate because that, combined with the possibilities of data
  overrun mean that you must use quite large buffering for Zmodem file
  receipt to be reliable (none of these issues effect sending of files).

  Buffer Recommendations
  ----------------------
  Based on the limitations of NuttX hardware flow control and of the Linux
  sz behavior, I have been testing with the following configuration
  (assuming UART1 is the Zmodem device):

    1) This setting determines that maximum size of a data packet frame:

       CONFIG_SYSTEM_ZMODEM_PKTBUFSIZE=1024

    2) Input Buffering.  If the input buffering is set to a full frame, then
       data overflow is less likely.

       CONFIG_UART1_RXBUFSIZE=1024

    3) With a larger driver input buffer, the Zmodem receive I/O buffer can be
       smaller:

       CONFIG_SYSTEM_ZMODEM_RCVBUFSIZE=256

    4) Output buffering.  Overrun cannot occur on output (on the NuttX side)
       so there is no need to be so careful:

       CONFIG_SYSTEM_ZMODEM_SNDBUFSIZE=512
       CONFIG_UART1_TXBUFSIZE=256

Using NuttX Zmodem with a Linux Host
====================================

    Sending Files from the Target to the Linux Host PC
    --------------------------------------------------
    The NuttX Zmodem commands have been verified against the rzsz programs
    running on a Linux PC.  To send a file to the PC, first make sure that
    the serial port is configured to work with the board (Assuming you are
    using 9600 baud for the data transfers -- high rates may result in data
    overruns):

      $ sudo stty -F /dev/ttyS0 9600     # Select 9600 BAUD
      $ sudo stty -F /dev/ttyS0 crtscts  # Enables CTS/RTS handshaking
      $ sudo stty -F /dev/ttyS0          # Show the TTY configuration

    Start rz on the Linux host (using /dev/ttyS0 as an example):

      $ sudo rz </dev/ttyS0 >/dev/ttyS0

    You can add the rz -v option multiple times, each increases the level
    of debug output.  If you want to capture the Linux rz output, then
    re-direct stderr to a log file by adding 2>rz.log to the end of the
    rz command.

    NOTE: The NuttX Zmodem does sends rz\n when it starts in compliance with
    the Zmodem specification.  On Linux this, however, seems to start some
    other, incompatible version of rz.  You need to start rz manually to
    make sure that the correct version is selected.  You can tell when this
    evil rz/sz has inserted itself because you will see the '^' (0x5e)
    character replacing the standard Zmodem ZDLE character (0x19) in the
    binary data stream.

    If you don't have the rz command on your Linux box, the package to
    install rzsz (or possibily lrzsz).

    Then on the target (using /dev/ttyS1 as an example).

      > sz -d /dev/ttyS1 <filename>

    Where filename is the full path to the file to send (i.e., it begins
    with the '/' character).  /dev/ttyS1 or whatever device you select
    *MUST* support Hardware flow control in order to throttle therates of
    data transfer to fit within the allocated buffers.

    Receiving Files on the Target from the Linux Host PC
    ----------------------------------------------------
    To send a file to the target, first make sure that the serial port on the
    host is configured to work with the board (Assuming that you are using
    9600 baud for the data transfers -- high rates may result in data
    overruns):

      $ sudo stty -F /dev/ttyS0 9600     # Select 9600 BAUD
      $ sudo stty -F /dev/ttyS0 crtscts  # Enables CTS/RTS handshaking
      $ sudo stty -F /dev/ttyS0          # Show the TTY configuration

    Start rz on the on the target.  Here, in this example, we are using
    /dev/ttyS1 to perform the transfer

      nsh> rz -d /dev/ttyS1

    /dev/ttyS1 or whatever device you select *MUST* support Hardware flow
    control in order to throttle therates of data transfer to fit within the
    allocated buffers.

    Then use the sz command on Linux to send the file to the target:

      $ sudo sz <filename> t </dev/ttyS0 >/dev/ttyS0

    Where <filename> is the file that you want to send.

    The resulting file will be found where you have configured the Zmodem
    "sandbox" via CONFIG_SYSTEM_ZMODEM_MOUNTPOINT.

    You can add the az -v option multiple times, each increases the level
    of debug output.  If you want to capture the Linux rz output, then
    re-direct stderr to a log file by adding 2>az.log to the end of the
    rz command.

    If you don't have the az command on your Linux box, the package to
    install rzsz (or possibily lrzsz).

    STATUS
      2013-7-15:  I have tested with the configs/olimex-lpc1766stk
        configuration.  I have been able to send large and small files with
        the sz command.  I have been able to receive small files, but there
        are problems receiving large files:  The Linux SZ does not obey the
        buffering limits and continues to send data while rz is writing
        the previously received data to the file and the serial driver's RX
        buffer is overrun by a few bytes while the write is in progress.
        As a result, when it reads the next buffer of data, a few bytes may
        be missing (maybe 10).  The symptom of this missing data is a CRC check
        failure.

        Either (1) we need a more courteous host application, or (2) we
        need to greatly improve the target side buffering capability!
        Either (1) we need a more courteous host application, or (2) we
        need to greatly improve the target side buffering capability!

        My thought now is to implement the NuttX sz and rz commands as
        PC side applications as well.  Matching both sides and obeying
        the handshaking will solve the issues.  Another option might be
        to fix the serial driver hardware flow control somehow.

      sz has several command line options which one would think would
      alleviate these problems.  But as of yet, I have not found a
      combination of options that does so:

      -L N, --packetlen N
          Use ZMODEM sub-packets of length N. A larger N (32 <= N <= 1024)
          gives slightly higher throughput, a smaller N speeds error
          recovery. The default is 128 below 300 baud, 256 above 300 baud,
          or 1024 above 2400 baud.

      -l N, --framelen N
          Wait for the receiver to acknowledge correct data every N
          (32 <= N <= 1024) characters. This may be used to avoid network
          overrun when XOFF flow control is lacking.

      -w N, --windowsize N
          Limit the transmit window size to N bytes (ZMODEM).

      UPDATE:  I have verified that with debug off and at lower serial
      BAUD (2400), the transfers of large fails succeed without errors.
      You may need the Linux sz -O option to keep it from timing out
      in these low BAUD transfers.  I do not consider this a "solution"
      to the problem.