Very simple serial snooping program I wrote to monitor/tap an RS232 port
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serialsnoop.c

README

This code is designed to perform hardware RS232 snooping. I decided to write it when I realized that how expensive commercial
solutions were.

===How it works===

Its really not even that complicated! In short, it is reading from two different serial interfaces at the same time and logs the
results with a timestamp.

===Building your own Cable===

The most complicated part of this is to build the custom cable. You could buy one off-line, but they're crazy expensive for such a
simple circuit. RS-232 transmits data over pin 3 and receives data over pin 2. You must build a cable that is a simple male-to-female
passthrough. Then you must put the "tap" ports in series. One side must connect the pin 3 to the pin 3 of the receiving connector, and
the other must connect pin 2 to pin 3 of the receiving connector. Both connectors must also connect pin 5 to pin 5.

A diagram is full-duplex-cable.gif.

===Compile and Running===

Plug in two USB-to-serial devices into your Linux box.
(It should also work with a /dev/ttyS0 device, but my ttyS0 is acting odd.)

cc serialsnoop.c -o serialsnoop
./snoop /dev/ttyUSB0 /dev/ttyUSB1