  The LIGO Interferometer Sensing and Control System (ISC) is a large and
highly distributed Multiple Input Multiple Output (MIMO) control system that is
used to control the length and alignment degrees of freedom of the
interferometers. The 4 kilometer Fabry-Perot cavity lengths are controlled to
better than 10-13 meters (rms) and the angular degrees of freedom are
controlled to better than 10-8 radians. This paper will describe the real-time
digital servo control systems that have been designed, developed and
implemented for the LIGO Length Sensing and Control (LSC) [1] and Alignment
Sensing and Control (ASC) [2] systems. In addition, the paper will describe how
these controls, along with the suspended optic controls [3], have been
integrated into the overall LIGO control and data acquisition system [4].
