The 4-axis Digital Servo Module 8520-ENC4, from Allen Bradley, is a high-speed servo processor, installed in the component enclosure, that receives velocity and positioning data from the control, processes it, and produces the needed pulse width modulated (PWM) command signals.  The 8520-ENC4 module can also generate analog velocity command signals for the spindle motor that is not using a digital servo amplifier.

It has two typical digital servo drive configurations for the control:  lathe configuration and mill configuration. The former consists of two axes, each with a servo motor and feedback device, as well as a 1-axis analog open loop spindle motor that has a position feedback. The latter has 3-axes, each of which also has a servo motor and a feedback device plus a 1-axis analog open loop spindle motor.  The 8520-ENC4 receives two kinds of feedback; it receives current feedback from the servo amplifier and position data from the feedback devices that are on the axes. The 8520 ENC4 Servo Module input specifications are as follows: the input power has a range of +5V (+10%, -1%), +15V (±7%) for encoders; the highest input frequency for longest cable length is 2MHz; the voltage range for the encoder battery backup is 3.5V to 4.5V.  The Servo Module output specifications are: DAC output voltage range is ±10±, output current is a maximum of 5mA, sampling frequency is 1000 Hz, load capacitance at maximum is 0.01µ, and resolution is 2.44mV (±12 bits); pulse width modulated (PWM) output frequency in approximately 2.0 KHz and signal type is RS-422.A.