I think RRobbins has given you a good answer.
However, over the last three years or so, I have only ever used Profibus to load the speed (and other) information directly into the VSD. Since you are using a DP equipped PLC, you may find this an option.
I have worked on a 314C-2DP only once (I generally use either the 315 or 414), but using DP we could control 6 servo drives, directly writing position and speed information to the drive, and receiving diagnostic information along with position and current feedback.
Of course, if you need no more than two VSDs, the direct analog output method is much cheaper, and often easier to diagnose for the average sparkie.