Instead of using PWM signals to control the position, smart servos have traded this unidirectional technology for serial communication which allows for bi-directional communication. Unfortunately, the smart servo's software protocol and hardware implementation of the serial communication will vary depending on the manufacturer, the lineup and grade of servos. The most popular hardware protocols are: TTL Half-Duplex, TTL Full-Duplex and RS-485. This means that robot designers need to choose servos of the same brand and lineup because they are not all compatible with each other.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/smart-servo-the-difference-between-smart-and-regular-servos