What a puzzling world, that of RS232.
RS232 is used for communication with Data Terminal Equipment. It used to be commonly found on the back of Windows PCs in the form of a male DB9 connector. Carrying the following signals
pin sig 1 DCD 2 RXD input from rest of world ) ICOM CI-V CAT control 3 TXD output to rest of world ) 4 DTR 5 GND PC side ground 6 DSR 7 RTS used for switching to transmit (PTT) 8 CTS 9 RIThe signals on RS232 are anything up to +/-13V, not TTL 0/5V.
And the logic is inverted. positive logic '1' RS232 HIGH = negative logic '1' TTL LOW voltage. So,for example, a data byte B10101010 on RS232 and TTL look like those above.
To convert RS232 to TTL logic signals means converting the voltage levels and the logic. This is commonly done with an FT232 or MAX232 chip and an opto-converter, like the 1N35 or the better PC900V.
To interface todays PCs with USB input/outputs a further step is needed. To convert USB to RS232. Here is a typical complete circuit using a FT232R IC to generate an islated CI-V interface:
Another practical way is to use a low cost, ready made cable which has on one end a slightly larger USB plug, containing the "232" chip, and on the other 6 wires carrying TTL level RS232 signals.