Serial port logic analyzer
http://www.consistent.org/serial-la/

The serial-la suite are a set of programs designed to use the four
status lines on a serial port as a sort of “logic analyzer”. Programs
are also provided to decode the data as RS232 of an unknown nature
(assuming you’ve connected the data pin to one of the status pins).

lasample is the program which actually reads data from the serial port
and writes it to a file.

lachart produces a text bar chart of the signals.

In some sense the flagship of the LA suite, lars232 determines timing,
word size, and parity information for any RS232 data which is recorded
using one of the status pins on the port. This allows for rapid and
fairly reliable determination of the parameters of an unknown signal,
without resorting to trial-and-error methods.