Connection to a USB to TTL dongle
If the Dongle output is TTL and no RS232 converter IC is expected between the dongle and target controller then positive logic is usually used.
 The TX line from the dongle = RX line into target will be high when idle. The dongle rx line = target tx line expects a high = +5V in idle state. Start bits are 0V and stop bits are 5V. The dongle TX line = target RX should be "stiff enough" to drive say a 1kohm load and still return say 4V+ when idle. Loading the dongle rx line with a 1k to ground or +5V should pull the line to the appropriate level. 

If the Dongle output is true bipolar RS232 (+/- relative to ground) and negative logic is used then the TX line from the dongle = RX line into PC etc should be negative when idle. The RX line of the dingle = TX line of target expects a low = say -5V to -12V when idle. Start bits are positive wrt ground. Stop bits are negative wrt ground.

To make the dongle work you may need to loop back RTS to CTS.