In the ARRL's August 2015 edition of QST (page 58), there is a Q&A in the monthly column "The Doctor is In" concerning the confusion about and between the TNC and the SCI. The discussion begins at bottom left column.
If you are an ARRL member, you may access this article at:
www.nxtbook.com/nxtbooks/arrl/qst_201508/index.php?startid=58

While this all sounds marvelous, there are some things that should be said.

Sound Card Interfaces (SCI)
These provide isolation from the computer and PREVENT all those annoying and unwanted audio tones and other sounds from coming across on your transmitted signal.
- I am told that X- Rated movie audio has actually been heard.

Computer "ON"
Although it can be inferred, it should be noted that by restricting all TNC/SCI functions to within the computer and not having a stand-alone TNC, you MUST always have a computer and it must be turned ON to run Digital Modes.

However, when using Classic Packet Radio or RMS Express with an external TNC (e.g. Kantronics KPC 3 or 3 Plus), you may leave your Radio 'ON" and remove the computer.
- It is able to continue to act as your personal mailbox and a digipeater WITHOUT the computer,

SCENARIO 1:
You are leaving the house and still want to keep the TNC in operation with the radio, but you need to take your laptop with you.

Just turn off the computer, and disconnect it from the TNC.
Leave the radio and TNC operating on their own to receive mail and digipeat.
Then, come back, when ready, and connect the computer to the TNC and see what's new.


SCENARIO 2:
Line Of Sight BLOCKED by HILLS

You need to set up a digipeater on a ridge to access a primary Node or Gateway.

No Computer Required...
Just bring your "Digipeater Go Box" and its "Go Kit".
Go Box: Radio, TNC, connecting cables, (Battery?)
Go Kit: Battery, Antenna, Antenna Mast, 1/2" Coax, Guy Ropes/stakes.

Here is a site that covers using Packet with a sound card.
- Everything is explained in a step-by-step manner on the pages of this site.

It is a very comprehensive site and although it seems as if it hasn’t been updated in a while, that should nott be cause for concern.

LINK
www.soundcardpacket.org


Thanks to Dana - KA1WPM for providing this information.