Why PWM anyway?

[subpilot]

Here's something to ponder. Why did Hitec go with PWM control of the RN servo motors? Why not go with a serial data control scheme like RS-485, CAN or LIN etc bus? Much less overhead I/O wise and the servos could be daisy chained to cut down on the wiring wuzzle.
I would think the servos have a built in micro controller since they can be programmed so lack of smarts shouldn't be a big factor. I suspect it's because they are in the RC business and that's how things are done.
Some of the other Robo One class robots appear to have gone with serial control and offer more in the way of feedback than the Nova system. You can get motor current along with position. That would be very useful for feedback on a gripper for example.

[limor]

You are right about their attitude of sticking to what they know and complex programming not being on the top of that list. Kondo invented the trick of using the lag time between PWM cycles to send back a PWM signal to the controler and Hitec coppied it. However, they have engineered this trick for a single "use case" - capturing servo positions in 'teaching-mode'. It isn't clear if it is possible to receive the position data with every PWM cycle to allow for gripper functions and other feedback-control functions. Getting the motor power consumption is fairly simple with a free A/D on the controler board.

Robotis and Megarobotics have gone the way of serial control and daisy chaining. What i haven't figured out is why stick to the bulky servo box instead of opting for other shapes that are more streamlined with biological looking limbs.

[Richard]

I guess that with Bus systems, you can only control one servo at a time. I'm not sure how the PWM outputs are managed on thr RN controller, but presumably it can control multiple servos simultaneously.

Having said that, I think the Robotis bus is pretty high speed, over 700k bps, so any lag is small.

Does the Robotis controller run asynch data on it's bus or does it use a sychronus TDM type system?