Keyspan USA-19 - Manual (Page 37)

Select the correct protocol for your deck (your).
Under Port, select the Keyspan serial port your deck is connected to. (see above)
Select OK and now you should have deck control with Final Cut Pro!
Keyspan:USB Serial Adapter for Mac OS X-v2.x User Manual
This User Manual applies to the Keyspan USB Serial Adapter Software for Mac OS X.
Rev 05jul15 Page 37