The 8520-SM4 is an analog servo module for the 8520 9-series, manufactured by Allen Bradley. This 8520-SM4 is a high-speed servo motor processor, which is enclosed in the component’s enclosure. It receives various data from the control module about the velocity or positioning, which is then received by the analog servo amplifiers, in order to power the control servo motors. A total of four closed-loop axes and one open loop axis can be controlled by one analog servo module. The 8520-SM4 consists of a 4-axis servo module. The connectors (J1, J2, J3, and J4) receive data that is related to the axes. Order of the connection of the axes is not necessary. These connectors transmit the data to the servo amplifiers. Another connector, TB2, is used to provide commands for the analog spindle drive system and should not be used with a feedback device.

The 8520-SM4 provides an output voltage in the range of 10 Volts and its analog output driver is single ended. The resolution of the 8520-SM4 is 1.22 mV (13bits plus one sign bit) while the sampling frequency is 500 Hertz. The maximum output current that the 8520-SM4 can provide is 5mA with the load range of 2 kOhms and a load capacitance of 0.01 mF. To replace this servo module, ensure that power to any and all the peripherals is disconnected and that the cables are removed from the connectors of the servo module; then, remove the mounting screws from the board, and slide the board rack from the enclosure From the board rack, remove the six screws to remove the board and replace the servo module. Then insert back all the screws and tighten them, reconnect the connectors, and turn on the power.