Synchronous Serial Interface (SSI) is a widely used serial interface standard for industrial applications between a master (e.g. controller) and a slave (e.g. sensor). SSI is based on RS-422[1] standards and has a high protocol efficiency in addition to its implementation over various hardware platforms, making it very popular among sensor manufacturers. SSI was originally developed by Max Stegmann GmbH in 1984 for transmitting the position data of absolute encoders – for this reason, some servo/drive equipment manufacturers refer to their SSI port as a "Stegmann Interface". It was formerly covered by the German patent DE 34 45 617 which expired in 1990. It is very suitable for applications demanding reliability and robustness in measurements under varying industrial environments.
It is different from the Serial Peripheral Interface Bus (SPI): An SSI bus is differential, simplex, non-multiplexed, and relies on a time-out to frame the data. An SPI bus is single-ended, duplex, and uses a chip select signal to frame the data. However, SPI peripherals on microcontrollers can implement SSI with external differential driver-ICs and program-controlled timing.