This utility is designed to work with a specal piece of cabling in order
to allow you to non-intrusively monitor an RS-232 interface. The syntax
is:

serialdump <speed> <BPS> <DCE device> <DTE device>

<speed> is the baud rate, which can be any value defined in termios(3)
(for example, 9600 or 115200)

<BPS> represents the number of data bits, the parity, and the number of
stop bits on the line. B can be one of 5,6,7 or 8; P can be N for none,
E for even or O for odd; S can be 1 or 2. Beware that if you serial port
hardware doesn't support the settings you choose, what you will recieve
is undefined.

The two devices are the paths to the device files that will be used to
monitor the DCE end and the DTE end of the line, respectively.

The output consists of modem status and data lines. Modem status lines
look like this:

DTR DSR RTS CTS CD RI

This example shows all the modem control signals on. Any subset is 
possible. Data lines look like:

0d                                      0d
4154 2053 373d 3435 2053 303d 3020 4c31 4154 2053 373d 3435 2053 303d 3020 4c31
2056 3120 5834 2026 6331 2045 3120 5130 2056 3120 5834 2026 6331 2045 3120 5130
0d                                      0d
                                        0d0a 4f4b 0d0a

The left side (up to 16 bytes/line) is data sent by the DTE and the
right side is data sent by the DCE. Every time data becomes available,
serialdump will drain both the DTE and DCE-side buffers, with output
continuing for as many lines as necessary. So, for example this:

41                                      41
54                                      54
49                                      49

shows that either the DTE or the DCE is sending data one character at a time,
and that the other end is echoing. Whereas this:

                                        6f62 6f74 6963 7320 3536 4b20 4641 5820
                                        0a0d 0a20 2020 4230 2020 4531 2020 4631
2020                                    4d31 2020 5130 2020 5631 20
2020 5930 0d0a 2020 2042 4155 443d 3131
3532 3030 2020 5041

Shows a burst of data from the DCE followed by a (partially overlapping) burst
from the DTE. 

Here is a crude ASCII-art diagram of the cable you'll need:

  /  CD-------------------X---CD  \
  |  RX-----------X-----------RX  |
  |  TX-X---------------------TX  |
D | DTR-----X-----------------DTR | D
T | GND-------X---------X-----GND | C
E | DSR---------------X-------DSR | E
  | RTS---X-------------------RTS |
  | CTS-------------X---------CTS |
  \  RI---------------------X-RI  /
        R C D G   R C D G C R
        X T S N   X T S N D I
          S R D     S R D    
        \-----/   \---------/
         DTEM        DCEM

X-s represent connections.

DTEM and DCEM are the DTE and DCE monitor ports respectively. You give 
serialdump the paths of the ports these 2 cables are plugged in to. The 
DCE and DTE cables plug in to your DTE and DCE as usual. You'll need 3 
female ( DTE, DTEM, DCEM ) and one male ( DCE ) DB9 or DB25 connectors. 
The statserial(1) utility will give you the needed pinouts. Good Luck!