Synchronous serial interface (SSI) is a commonly used point to point, serial communication standard for data transmission, between the absolute rotary encoder (slave) and the controller (master). SSI is based on RS422/485 standards, has high protocol efficiency in addition to its implementation over various hardware platforms, making it very popular. It is very suitable for applications demanding reliability and robustness in measurements under varying industrial environments. SSI has a very simple design consisting of just two pairs of wires, one for the clock signals from the controller and other for position transmission from the encoder. Differential signalling improves the resistance to electromagnetic interference (EMI), hence making it a reliable communication channel over long transmission. Different clock frequencies can be used ranging from 100 kHz to 2MHz and the number of clock pulses depends on the number of data bits to be transmitted. Reduced wiring in addition to the simplicity of SSI design due to use of minimal number of components helps in reducing the cost and created more transmission bandwidth for message bits. There is complete protocol flexibility for the number of bits transferred and has an arbitrary choice of message size.
Transmission of Data
When not transmitting, the clock and data lines are HIGH (idle mode). The current position value is frozen when the clock pulses from the controller are received (1). The MSB of the position is transmitted at the first rising edge of the clock (2) and the rest of the bits are sequentially transmitted with consecutive rising edges. After transmission of a complete data word, the data line remains low (3) for a period of tim (transfer timeout, tm) until the encoder is ready for updating (Single Transmission). If the controller sends another data-output request (CLOCK Signal) before transfer timeout, the same data will be transmitted once again (Multiple Transmission). The transmission of data is controlled by the master and it can be interrupted at any time just by stopping the clock sequence, for a period longer than transfer timeout. The slave automatically will recognize the transfer timeout and go into idle mode where it keeps updating its value. Additional information like CRC bits or parity bits can be added to the SSI protocol to ensure secure transmission. In simple words, they are used for identifying if the byte has been correctly interpreted and received in case of very sensitive applications. For more detailed information on Synchronous serial interface: SSI on WIKIPEDIA To know more about our products with SSI interface visit: Optocode (OCD)- Absolute Rotary Encoders MAGNETOCODE (MCD)- Absolute industrial rotary encoders MAGNETOCODE (MCD Heavy Duty)- Absolute heavy duty rotary encoders For detailed information on SSI in applications:
