Note that when coss_i = 0 = coss' while diq and dq' are positive s_i = 90º (½p) = s' and cscs_i = 1 = cscs' while when diq and dq' are negative s_i = 270º (3p/2) = s' and cscs_i = -1 = cscs'.  Furthermore, when coss_i = ±1 = coss', while diq and dq' are positive s_i = 0º (0) = s' and when negative s_i = 180º (p) = s', in both cases cscs_i = ±¥ = cscs'.