The Raspberry Pi has a UART interface to allow it to perform serial data communications. 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.
While an industrious person could build their own RS-232 converter, there is little need to do so when cheap converters are available.
Figure 11-1 shows a MAX232CSE chip interface that I use. (This unit supports only the RX and TX lines.) When searching for a unit, be sure that you get one that works with 3 V logic ...