Using Screen as a terminal emulator

I've been using ZTerm and various terminal emulation software on the Mac for a while.
Most have been a bit clunky, so I've reverted back to screen.
Came across an issue with it not connecting properly using my Keyspan USB to Serial adapter.
When I run the command:

 screen /dev/tty.USA19Hfa14P1.1

I get the following error:

  Cannot open line 'dev/tty.USA19Hfa14P1.1' for R/W. Resource busy.
  Sorry, could not find a PTY.

After some digging around turns out that when I last used screen, I didn't exit out of it properly.
Out of doing a: ps -f

  ps -f

  UID   PID  PPID   C     STIME TTY           TIME CMD
  501   666     1   0   0:00.00 tty.KeySerial1   0:00.00 SCREEN /dev/tty.USA19Hfa14P1.1
  501   670   669   0   0:00.01 ttys000    0:00.02 -bash

I killed the process:

 kill 666

Tried to use screen again and it worked this time around.
This is a reminder, more to myself, that you should always exit screen using the CONTROL-A followed by the CONTROL-\.