The Serial RX and TX pins of an Arduino are not RS-232 compatible without using some type of circuit to give you the proper voltage levels.
**RS-232 logic and voltage levels**
0 (space) Asserted +3 to +15 V
1 (mark) Deasserted −15 to −3 V
Connecting the RX/TX pins of an Arduino directly to an RS-232 device may fry your Arduino in addition to just not working for communication.
Jeff Wahaus
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