The TXD signal (pin 3) drives an NPN transistor (Q1) to get the 'open collector' aspect of the output side of the bus. The normal output voltage range for current USB to RS232 adaptors seems to be about -6V to +6V. The diode D1 is used to prevent the positive power supply from going negative if the DTR signal goes low. Looking at the schematic for the 9 Pin RS232 to CAT / CI-V interface, you can see that the board is powered from pin 4 (DTR), so DTR must be set high for the board to operate. Both interfaces use a similar physical layer design. The name CAT seems to be in wider usage for this purpose. ICOM calls their communication system CI-V and Yeasu calls it CAT. The design for the RS232 to CI-V adapter was shamelessly copied from, thanks!
