What is the difference between RS-232 and RS-232-C?
The RS-232 Standard has gone through several revisions over the years. In the original RS-232, the signal would flip between positive and negative 25 volts.
Since that point there have been three revisions, RS-232A, RS-232B, and RS-232C; these revisions lowered the acceptable voltage from 25 to 12 to 5 volts respectively. In 1969, the Electronic Industries Association Standards Committee adopted RS-232C as a standard for computer manufacturers.
Because this adoption date was so long ago, most manufacturers, including National Instruments, have dropped the &quot;C&quot; from the name and simply refer to the protocol as RS-232. In common, modern usage, there is no difference between RS-232 and RS-232C, protocol.

Source (NI RS-232C Standard Conformance)