jw> can you provide more examples?

    Yes.  I will post more later today.  I have some more data,
    but I still need to convert it to hex format for posting.
 
jw> can you influence the transmitted data (i.e.  chose the
jw> content or a part of it)?

    Yes and no.  I can make messages that are different from one
    another at will.  But I cannot create very short messages, or
    force a specified byte to have a specified value.  I wish I
    could.

jw> couldn't you simply ask the author?  :-)

    I am trying to find the author, believe me!

    For anybody interested in attacking the problem by
    disassembling the firmware, <a href="http://www.8052.com/users/rmac/mtc.hex">here</a> is the image in Intel hex
    format.  I don't know where anything is within the image, but
    it might be possible to start at the UART interrupt vector and
    find where it computes the check bytes on the messages it
    receives from the PC side.  (This hex file is for an 8051
    derivative, and I have successfully loaded it into an C8501F120
    chip and poked around a bit with the Silicon Labs IDE.)

 -- Russ