Useful PCL links
HP Support links for PCL
Differences between PCL 3 and PCL 5/6
List of printers supported by PCL 5 and PCL 6
Other useful PCL links
Gracious thanks to Karthik Punja of Robonik India for sharing these useful links:
PCL Paraphernalia is a free utility which provides a small collection of tools of use to technicians supporting printing on PCL5 and PCL6 printers.
Contains basic information for multiple printing protocols. Has not been updated since 2011 but is still very useful.
This section of the github repository contains c code for PCL and other printer languages