How do the TX/RX/CTS/RTS/DSR/DTR/DCD/RI/GND signal monitor LEDs on these odd FTDI-based USB-RS232 converters work?
Most TTL-level UART signals are persistent enough that they can be viewed directly on an LED tied high or through an op-amp (to reduce loading). TX and RX, however, usually need support from the converter chip so that they don't just flicker dimly. The chips have pins that output the TX and RX states, usually open drain. You simply connect a LED and resistor from the positive supply to the pin, and the chip does all the hard work.