Borrowed from the datasheet:

Description:

 The HEDS-65xx/HEDL-65xx are high performance two and three channel optical incremental encoders. These encoders empha­size high reliability, high resolution, and easy assembly. Each encoder contains a lensed LED source (emitter), an integrated circuit with detectors and output cir­cuitry, and a codewheel which rotates between the emitter and detector integrated circuit. The outputs of the HEDS-6500 are two single ended square waves in quadrature. The HEDL-65xx outputs are differential. The HEDS-6540 / HEDL-6540 also have a third channel index output in addition to the two quadrature outputs. This index is an active high pulse that occurs once every full rotation of the codewheel. Resolutions up to 1024 Counts Per Revolution are available in the two and three channel versions. The line driver option offers enhanced performance when the encoder is used in noisy environ­ments, or when it is required to drive long distances. The line driver option utilizes an industry standard line driver IC AM26C31Q which provides com­ple­mentary outputs for each encoder channel. Thus the outputs of the line driver encoder are A and – A, B and – B, and I and – I for three channel versions. Suggested line receivers are 26C32 and 26C33. The quadrature signals are ac­cessed through a cable and 10-pin female connector. Please refer to the ordering information at the end of this data sheet for a selection matrix.

See the full datasheet here: