SSI Absolute Encoders

The SSI protocol, its full name is Synchronous Serial interface, is a widely used serial interface between a position sensor and a controller. SSI is a one-way serial protocol on RS-422. The one-way clock is generated by the main frequency and is specified from 80khz to 2mhz. The received data is also one-way, and SSI does not support propagation delay compensation. The clock is high when inactive. To start the data transfer, the clock goes low and the location is stored. On the first rising edge of the clock signal, the MSB is shifted out at the SSI encoder. On the second rising edge, MSB-1 is shifted out, and so on, until the last bit (LSB) is shifted out. After another clock cycle, the clock stays high until the next data transfer starts. According to the timeout, the current latch position or the new position value will be transmitted in the next transmission. 

Hardware interface
The 422 differential interface is used to connect to the absolute encoder. Generally, the SSI encoder must have 6 wires, and the CALTSENSOR encoder has 8 wires (additional direction and reset function).
SSI data format
Generally there are binary format and Gray code format.
Binary, that is, communication data.
Gray code is a coding method that minimizes errors. It needs to be decoded to convert the Gray code to binary.