New RS-485 IC Increases System Reliability and Fault Detection in Motor-Control Circuits - AN578

Applying a unique 3-channel RS-485 transceiver displaying many unusual features increases reliability and fault-detection in motor-control applications. Designed specifically for motor shaft encoder applications, the MAX3097 and MAX3098 feature three differential channels to handle optical encoder data, ±15kV ESD protection, increased common mode rejection to -10 and +13.2V, three separate fault-alarm flags, and fail safe open and short detection. Alarm flags indicate open or shorted line conditions, excessive commonmode voltage range, and low-signal strength. Modern factories use large electric motors to control machines that are necessary for the operation of the plant. For increased flexibility and accuracy, these electric motors are sometimes processor-controlled. It is imperative that the electronics controlling these large motors are designed in such a way that, if a problem occurs, the motor is safely and quickly shut down. Failure to detect a problem and to sufficiently control the motor can result in the destruction of expensive equipment, as well as in injury or possibly even death. The MAX3097E/MAX3098E were specifically designed to be used with motors in absolute positioning control loops. These parts have unique features which enable a safer and more robust design in harsh industrial environments. To better understand the unique features of the MAX3097E/MAX3098E, let's first review the basics of motor control. Figure 1. The major building blocks of processor-controlled electric motors. The blocks in Figure 1 include a processor, motor-drive circuitry, an electric motor, and an encoder. The processor decides how hard and where the motor should be driven. The motor-drive circuitry contains all the necessary power electronics to drive the motor and the encoder provides position, direction, and velocity information to the processor, thus closing the loop. To understand the operation of the MAX3097E/MAX3098E, it is necessary to have a good grasp of the operation of the encoder.