I have the new Yaesu FT-991 radio and my desire is to be able to key the radio PTT via the DB-9 connection on the radio, using the RTS line. On the radio the assigned pins are RTS-7, CTS-8, DTR-4.

The USB-63 does not make these connections as pin 4 is not wired in the connection.

The RTS-03 will work for this configuration. You need only add a standard straight through DB-9 Female to Female cable (which is offered by RT Systems as the RT-45 at 6-feet and RT-46 at 10-ft) to reach the radio from the adapter (the adapter is male as is the radio). The RTS-03 has pins 7, 8 and 4 wired.