The Ramachandran plot displays the dihedral angles phi against psi of all residues.
It is a very powerfull tool to identify errors in protein structures.
Nevertheless, some residues (have to) adopt conformations where the dihedral angles are off the well defined regions in the Ramachandran plot.
See our script of how to compute the Dihedral Angle.

G.N. Ramachandran, C. Ramakrishnan, and V. Sasisekharan. Stereochemistry of polypeptide chain configurations.. J Mol Biol, 7:95-9, 1963.
G.N. Ramachandran and V. Sasisekharan. Conformation of polypeptides and proteins.. Adv Protein Chem, 23:283-438, 1968.
URL for direct computation of results of this tool:
http://i.moltalk.org/ramachandran_plot/3/PDB/1TAR/65
parameters: