The Synchronous-Serial-Interface (SSI) is a digital interface which is mostly used in length and angle measuring systems. It is especially suitable for applications where reliability and signal robustness is very important. The Max Stegmann GmbH, which is now a part of the company group SICK, was the developer of the SSI. Meanwhile, SSI is also used by other manufacturers. Unlike a parallel interface, where the bits are transmitted in various lines, transmission here is serial – which means successively.