As used in MMSSTV, CPIX, AGWPE, PSK31 etc

Transformer: The type of transformer is determined by the source of the RX audio. If you take it from the radio's microphone or data jack, you'll use a 600 to  600 Ohm (1:1) transformer. If you take it from the radio's speaker jack (8 Ohm impedance), you'll need a 1000:8 transformer.

R1 =  Resistor, e.g. 1K2, to reduce voltage on the IC pin 1
IC Integrated Circuit; this sketch shows an IC, such as a 4N33
or PS2603 Optocoupler, which uses a Darlington pair transistor.
(Note: to identify pin #1, look for a small embossed circle on
the top of the IC above pin 1; or looking into the notch in one
side of the IC with the pins down, pin #1 is to the right of the notch.)
D1 = Diode, e.g. IN4001, would shunt any potential reverse voltage
that might damage the sensitive diode/emitter in the IC.
(Note: the band printed on the diode marks the cathode end,
which attaches to the Serial Port/IC Pin 1 line in the sketch above.
The opposite/anode end attaches to Ground.)

The ideal components for your circuit may vary because of your radio's requirements. For example, you might be able to use a different type of IC, such as a 4N25 or PS2601, but that may require a different value for R1.

Use the RTS ( Request to send ) pins NOT the DTR ( Data terminal ready ) pins

On a DB 9 Serial connector use RTS pins 7 ( PTT ) & 5 ( Ground )

On a DB 25 Serial connector use RTS pins 4 ( PTT ) & 7 ( Ground )