K3 and digital modes

Using WSJT-X as my example digital program, here are the steps to getting digital modes to work with the K3 using an external sound card, the ASUS U7.
The process is almost identical to the one I use with the KX3, an external soundcard dongle and a MacBook Pro.
First, it is simple and easy to adjust and repeatable.
Line in on the K3 rear panel is connected to the Headphones socket on the front of the Asus U7 (using red patch cord with ‘Line in’ label).
Line out on the K3 rear panel is connected to the Microphone socket on the front of the Asus U7 (using black patch cord with ‘Line out’ label).
RS232 on rear of K3 is connected via USB adaptor to unused USB connector on the PC.
USB socket on rear of Asus U7 is connected to another USB connector on PC. (this also provides power for the Asus U7).
Asus (Xonar U7 Audio Center) to confirm selection of Headphones and Line-In, and to adjust levels
WSJT-X File > Settings or F2 >
General tab: Enter your callsign and grid square
Radio tab: Select Elecraft K3/KX3 in Rig drop-down list
Set serial port to same COM Port number revealed when you are using
Elecraft K3 Utility (But both programs cannot be run at same time)
Set baud rate to 38400
Data bits 8     Stop bits One    Handshake  None
PTT  – select CAT     Port should be USB
Transmit Audio Source is Rear/Data
Mode is Data/Pkt
Split Operation is set to Rig
Use Test CAT and Test PTT button to confirm correct connections. CAT will turn Green and PTT Red after clicking, indicating all is well.
Audio tab:
Under Soundcard select the appropriate option from the drop down lists:
Input – ‘Line (2-Xonar U7)’
Output – ‘Headphones (2-Xonar U7)’
Under the Reporting tab you can enable or disable uploading of spots to PSK Reporter and linkages to other programs include logging software.
Under the Frequencies tab – if you find there are no frequencies listed for the different modes, position cursor in the Working Frequencies window and right click and select ‘Reset’. This should populate the window with all the frequencies.

Use the PC’s regular audio device controls or the soundcard’s controls to adjust input and output levels. For example WSJT-X likes to have the green input bar graph on the lower left of the application window indicating about 30dB. Adjust the Headphones level to achieve this. I operate routinely with it hovering between 30 and 60dB.
The K3 is particular about the level of ALC on its transmit signal. You can use the microphone level to easily achieve the Elecraft ideal of 4 solid bars with the fifth bar flickering. This ensures a clean splatter-free signal.

Other aspects you need to consider with applications such as WSJT-X include installing an application to keep the PC clock accurate, as well as eventually checking frequency alignment.