SSI interface is known as a serial synchronous interface that is used in encoder in cases when a simple parallel interface is not acceptable. It means that when there are several encoders providing signals at one time, connecting them in a parallel way with multiple cables will be non-efficient.
In this case, the use of a synchronous-serial interface (SSI) can solve the problem. In this method of data transmission, only two twisted pairs are needed to connect each sensor, i.e. one for the clock signal and one for the data signal. The power connection requires (as well as the parallel interface) two wires.
Special differential drivers for RS 422/485, which generate a balanced output signal, allow the signal to be transmitted over a distance of up to 1200 meters at a transmission rate of up to 10 Mbit / s. This is sufficient for most applications. The maximum baud rate depends on the cable length.
SSI transmits positioning data synchronously with the received clock signal from the control system. At rest, the signal level on both the clock wire and the data wire is 1 (High). As soon as the clock signal for the first time at the beginning of each clock burst changes its level from high to low, the bit-parallel data inside the sensor in the parallel-to-serial converter Input - Latch of the shift register is "frozen" by the in-circuit generated signal.