Encoders may be either incremental or absolute.
Incremental encoders may be rotary or linear. The first key specification is the number of pulses per revolution (PPR) or pulses per inch (or centimeter). PPR of 250, 512, 1000, 1024, or even up to 100,000 pulses per revolution are available.
Incremental encoders emit either uni-directional or bi-directional pulses. The most common form of uni-directional pulse is a simple square wave voltage pulse formed as a slit moves between a fixed photo-transistor and an LED.
Bi-directional pulses are most commonly available as quadrature pulses. A quadrature encoder has two seperate sets of slits and electronics, offset 90 degrees from each other in phase. Thus, one photo-transistor/LED pair will always come ON before the other when the movement is in a given direction. For example, Channel A will always come on before Channel B when rotated in the clockwise direction, but Channel B will come on before Channel A when rotated in the counter-clockwise direction.
Another form of bi-directional pulse system simply allows only one channel to transmit in each direction.
Bi-directional encoders will commonly have outputs labeled A, B, C, /A, /B, and /C. (Sometimes the manufacturer will substitute either Z or I for the C channel.)
C, Z, and I represent an index or marker pulse. This third channel most commonly outputs a single pulse each revolution (if a rotary encoder), or each 20 cm or so of linear movement. Index pulses are a big help in giving a position reference to an incremental encoder-based system.
The /A, /B, and /C channels are simply inversions of the A, B, and C channels. In other words, when A is ON, /A is OFF and vice versa. This is very handy in electrically noisy environments. It is also a built-in redundancy which is quite useful to have when equipment is failing. This inverted signal is referred to as a differential output.
Because of the special multiple slit arrangement, a unique ON and OFF output configuration is found for each specific location on the encoder. But there is a significant cost for this capability. Unlike the simple two or three channel incremental encoder, an absolute encoder must transmit its information over multiple parallel output lines. Thus, to get a resolution of 1 part in 1024, the absolute encoder requires 10 output lines (or bits), while a resolution of 1 part in 4096 requires 12 output lines (or bits).
Resolutions of up to 1 part in 23 bits (0.00034 degrees) are available in some rotary encoders. Other rotary encoders offer multiple turn capability, with the ability to determine 1 part in 4096 per revolution over a total of 4096 revolutions.
Most absolute encoders offer the ability to determine the absolute position immediately after power up. No homing moves need to be programmed into the system as is commonly done with an incremental encoder.
Some absolute encoders offer a high-speed serial communications capability.