They return several bits (often 8) that indicate the absolute position of the encoder shaft. One usefull attribute is that you do not need power to know/remember where the shaft is.
The other common encoder sends out one bit that blinks on and off every time the shaft moves a little bit. So a computer has to always be on to count these pulses. If the computer is ever off and the shaft moves a little then the computer will not know about it.
So, one advantage of an absoulut encoder is that the computer can know the absoult shaft angle even if it has been off for a while.
An absolute encoder will give you a binary or gray scale output for absolute position. Whereas an incremental encoder will give you only pulses when the motor is in motion. With an incremental you need to have a device that will count the pulses. If power is loss the register that held the count may be lost and present position will be unknown. With an absolute encoder you will know where you are regardless.
Keep in mind though, an absolute encoder has a limited # of turns until the data rolls over to zero again.
An absolute encoder can store the current position data along with the number of turns the encoder has made. The combination allows the encoder to output a much larger range of position data then one turn.
An absolute encoder requires a power source to store it's position when power is turned off. This is either done with a super-cap inside the encoder or from a back-up battery at the servo-amp. This power source also enables the encoder to work and store position data when the encoder is turned even with the power off.
Useally, when power is first applied to an absolute encoder the current position and number of revolutions turned is transmitted to the servo-amp and is used as a starting point for position data.
Absolute encoders do not require origin setting after turning power on since the position data is not lost.
Since position data is always available, home switches and over-travel switches are not required (although still a good practice to use).
Also, absolute encoders are more expensive, the cables are more expensive due to additional conductors required, and the amplifier itself is more expensive and may require back-up batteries.
