Your P3 must be connected to a computer serial port before firmware can be sent.
The P3 back panel has two 9-pin D-shell connectors. Connect the Computer to the upper (female) DE-9 connector.
A 9-pin "straight through" male-to-female DB-9 serial cable is recommended.
Do not use a "crossover" or "null modem" cable.
The cable must have a minimum of three wires: Transmit Data, Receive Data, and Ground. Other RS-232 lines (e.g., Request to Send, Clear to Send, Data Set Ready, Data Terminal Ready) are not used by the P3 Utility.
Other programs may use the DTR and RTS lines for PTT and CW keying. See the Rear Panel and the CONFIG:PTT_KEY sections of the Elecraft K3 Owner's Manual for details.
The female 9-pin D-shell RS-232 connector on the P3 back panel has jack nuts, so the male end of your serial cable should have screws.
If your computer does not have an available serial port, a USB to Serial Adapter such as the Elecraft KUSB may be used. The Elecraft KUSB has screws that attach directly to the jack nuts on the P3 back panel RS-232 connector.
If your USB to Serial Adapter has jack nuts on its 9-pin connector, a short serial male-to-female DB-9 serial cable with screws on each end, such as the one pictured above, may be necessary between the USB to Serial Adapter and the P3 back panel RS-232 connector.
