With the present OPHV circuitry and a 24VDC pullup supply, input voltage will drop below 24V as inputs are switched on. This is due to a parallel feedback resistor in the original OPTO pullup circuitry.
Fortunately, this does not affect the inputs. They will still trigger just fine and they will be able to use 24V without an external 12VDC clamping zener (that was the goal of this design).
If this voltage drop is a problem for some applications, contact Compumotor Application Engineering Dept. for possible solutions.