From RC Helicopter Wiki
Servos are connected to their control equipment by three wires. A voltage between two of these wires (usually red and black, or brown and orange) supplies power to the servo. The remaining wire (white or yellow) carries the commands that the servo obeys.
The de facto standard that normal modern servos adhere to is a pulse width modulated signal, with the length of the pulse determining the servo position. A typical servo can be expected to respond to pulses from 1ms long (full one way) to 2ms long (full the other), at a typical rate of 10-11 microseconds per degree. Servos may refuse to respond to pulses longer or shorter than this. The servo will expect to have its position refreshed every 20ms or so (50Hz), known as the frame rate. Too fast a frame rate can confuse the servo; too slow will reduce its holding power, speed and precision.
Within this general scheme, different manufacturers have devised slightly different protocols, mainly relating to the length of the pulse considered 'center' and how far either side of center requests can be made. While theoretically these specifications apply to servos, the specifications mostly relate to what signals the transmitters will create.
- Centre: 1.52ms
- 100% ATV: 1.52+/-0.43ms
- 140% ATV: 1.52+/-0.60ms (Max)
- Centre: 1.50ms
- 100% ATV: 1.5+/-0.40ms
- 150% ATV: 1.5+/-0.60ms (Max)
 Special Servos
There are a growing number of servos on the market that support variations of this protocol, the most common being digital servos. In addition to more precise control of the servo position, digital servos can accept a much higher frame rate (250Hz and beyond), and therefore much faster control response.
There are also a small number of digital servos that accept a scaled-down pulse length, based typically around a 0.75ms center pulse; this is mostly to allow for even higher frame rates.
 See also
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