The Raspberry Pi has a UART interface to allow it to perform serial data communication. The data lines used are 3.3 V logic-level signals and should not be connected to TTL logic (+5 V) (they also are not RS-232 compatible). To communicate with equipment using RS-232, you will need a converter module.
RS-232 Converter
While an industrious person could build their own RS-232 converter, there is little need to do so when cheap converters on a pcb are available.
Figure 10-1 shows a MAX232CSE chip pcb that I have used. This particular unit supports only the RX and TX lines with no hardware flow control. ...
