TTL RS232 driver circuit.
TTL driver circuit consists of 2 transistors and a few other passive components, a negative voltage for RS232 logic generated by the capacitor C7.
TTL to RS232 driver circuit and a variable voltage output driver has a parallel termination impedance to match cable and TTL driver circuit, and provides the best waveform.
When driving to the RS232 TTL driver circuit or a variable input voltage, a source termination impedance matching the cable will give the best results.
RS232 TTL driver circuit works quite well, as previously noted the parallel driver, RS232 TTL driver circuit has very low intrinsic output impedance, so that this component brings up the source impedance of 50-55 ohms, and work better