A  datascope looks to both the RXD and the TXD line of a
              V24/RS232C  line.  So only follows what happens  on  the
              line. There are very nice commercial packages as FELINE,
              HP  etc.,  but they are expensive and they use mostly  a
              special POD to connect to the V24 line. Most of them you
              have  to  preset  on one speed and  than  to  start  the
              logging.
              If  you  have  an  old computer with  better  two  speed
              programmable UART or a SIO chip and you can program them
              in  Assembler/basic/pascal or C,  you can make your  one
              one.
              The  next  story  is a simplified logunit  only  to  log
              datastreams  preceded  with a  AT-string.  Most  of  the
              application  programs  works in terminal  session  aswel
              filetransport  in a half duplex mode.  Or better,  never
              the TX and RX line transports a character.  So basically
              in default you have to look to the TXD-line, and look at
              2400 baud for a character x'41','06',  '78' or '01'.  At
              that  moment you know that probably the speed  is  2400,
              1200,  600  or 300 baud.  You get on line speeds 600 and
              1200  a  framing  error,  because the  stop-bit  is  not
              detected.  Now depending the detected speed you have  to
              wait  (time-out)  till  the  rest of  the  character  is
              passed,  then you have to change the speed of your comm-
              port,  and  await for the 'T',  If it is a 'T' than  you
              have  the  speed,  if  not wait on  2400  baud  for  the
              posibilities  as described above.  The basic idee behind
              this is really simple.  If you listen with 2400  bit/sec
              to  a  300  bit/sec send character,  the  start  bit  is
              detected  and  will  serve for the startbit and  7  bits
              char.  bits (so 7 zero bits).  The eights char.  bit  is
              formed  of the LSB bit and well 1/8 portion of  it.  The
              second  portion will be the stop bit.  So if on the line
              really is transmitted a 600 bit/sec char.,   the same is
              true with 1/4 portions.  Furthermore you have to look to
              both  the  TXD  and RXD line,  if  on  one  a  char.  is
              received. If so you put this char. in its own buffer and
              in the other one a x'00'.  Lateron you can print it out,
              one  line from buffer RXD,  one line from buffer TXD  in
              Hex or ASCII (or both) outfit.
              Go  on on the same speed till you didn't receive for  30
              sec. any character on both lines and start again with AT
              synchronisation. In fact the HAYES inside micro computer
              is doing the same. Don't try to write the buffer to disk
              at first instance,  because you need a DMA at least, but
              also 2 buffers. So buffer it in memory only. You can try
              to  write to disk in the silence just before the new  AT
              string.