I'm planning to buy servos for a humanoid robot, is there any disadvantage of the TTL vs RS-485 versions? EMC/Electric disturbance / how many servos on one line/bus are possible... others?
At the electrical level, RS485 will allow you to send the signal over longer distances (10's to 100's of metres; if the data rate is suitable low then over 1km is possible. It is more immune from interference as it with two differential signals (one goes high while the other goes low) which helps eliminate common-mode noise.
However for short range signals as in a robot, I would have thought that these would not be considerations. On the other hand TTL uses 3 lines between the servos rather than the 4 for RS485 so it might be a little easier to fit the cables into small spaces.
For me he bottom line for such an application would not be the comms technology but rather the size of the servos, their torque and the controller and other items you may already have.
RS485 is definitely recommended in noisy environment as Susan mentioned above.
I'd say TTL would be good for less than 1 meter distance with no electric disturbance.
Both RS485 and TTL support daisy chained connection so you can theoretically connect as many Dynamixel until there isn't any redundant ID, but I'd recommend less than 32 Dynamixels in one channel.
If you are worrying about electric disturbances, you can also consider using custom twist-shielded cables for RS485.
Please note that each connector will act as a resistor and affect to the signal level, therefore having multiple Dynamixel in a single channel would require to compensate the voltage level by jumping(supplying additional) power cable every 4~5 Dynamixel.
If you are using high current products that continuously draw more than 3A each, you need to supply the power separately to each Dynamixel.