Servo motor controllers can be driven from many microcontrollers however the number of servos tends to be limited and requires using specific timers and pins that may be required for other purposes.
A servo controller takes the burden of controlling a bunch of servos off another system and has specific internal logic to handle the speed, acceleration and position of multiple servos with a much smoother result then basic servo code libraries of general purpose microcontrollers like Arduinos.
Using a simple command set a microcontroller is able to tell the servo controller what to do over a TTL serial interface. In addition the servo controllers can be programmed with there own internal scripting language to perform special movements. Unused servo connections can be repurposed as digital outputs or analog inputs.
Pololu's servo controllers also support USB so they can be controlled directly from a computer (PC, Mac, Linux).
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