Responses from the radio go through Q2 to be inverted. When the host is not actively talking to the radio, the COM IO signal idles in the high state. The COM_IO signal is pulled up to about 5V by the voltage supplied by the DTR pin. D2 prevents the TXD signal from driving the base of Q1 below ground. It also logically inverts the RS232 levels before applying them to the COM_IO signal to the radio. 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!
