CW_Transmit

CW transmit using flrig:

flrig can generate key-line signals (equivalent of straight key) signals on a DTR or RTS serial port. That serial port can be either

shared with the CAT data-in/data-out port, the IC 7300 for example
on a separate transceiver serial port, the FT 991A for example (sorry no screen shots)
shared with the auxiliary serial port,
or via a completely separate serial device, USB or a part of the computer. Amazon sells USB serial devices which can be used for both CAT and CW keying.

CAT interface cable

USB serial to key-line interface. Connects to the transceiver CW keyline input.

Both of these are used to connect to a QRP Labs QCX+ CW transceiver.

fldigi has several methods by which a transceiver can be keyed in CW:

as a digital audio waveform in the same manner as transmitting PSK, MFSK or any other digital signal

set transceiver to USB or USB-DATA
select CW as operating mode
use the transmit text panel, the T/R button, macros, and/or the Pause/Break keyboard button to generate and send the CW signal
advantage is that the CW can be sent at any point on the waterfall. Rx and Tx can be at separate audio frequencies.
disadvantage is that transceiver narrow band CW filtering might not be available, but narrow digital filter will still be available

detection of a stereo right channel on/off keyed audio signal with conversion to a hard line keying signal

Note that the right audio channel can be used for either the CW keying waveform or a QSK control signal, but not both simultaneously.
as a DTR or RTS control line

Note that you can interface to the shared CAT port (IC 7300), or a separate serial port (FT 991A and QCX+)
by sending unique CAT commands which contain CW TEXT which is then independently transmitted by the transceiver
by interface to the flrig CW keying system

by interface to a stand-alone CW terminal unit such as nanoIO, MORTTY, or WinKeyer.