BiSS and SSI are actually very similar – both use the concept of a master clocking data from a slave over a serial channel. The slave is the position sensor and the master is a servo drive or other controller. The sensor continuously updates the measured position in an output register. To read the sensor, the master initiates a series of clock pulses. When the slave detects the first clock pulse it freezes the output register and the most significant bit is output on the data line. The master continuous to generate clock pulses until all the bits in the output register have been transmitted. At this point the sensor resumes updating the output register.
The Clock and Data signals are differential on twisted-pair cable for improved immunity to electromagnetic interference. Electrical characteristics conform to RS-422/485 standards. The interface is inexpensive, reliable and capable of speeds up to 10 Mbits/sec (BiSS-C) on 10 meter cable. Much longer cables can be used at lower bit rates.
Synchronous Serial Interface (SSI) is point-to-point so slaves cannot be bussed together. SSI is uni-directional, data transmission being only from slave to master. It is therefore not possible for a master to send configuration data to a slave. Communication speeds are limited to 2 Mbits/sec.
Many SSI devices implement double transmissions to improve communication integrity. The master compares the transmissions to detect errors. Parity checking (Appendix) further improves error detection.
SSI is a relatively loose standard and many modified versions exist including the option for an incremental AqB or sin/cos interface. In this implementation absolute position is only read at startup.
BiSS-C is the latest version of BiSS. Older versions (BiSS-B) are essentially obsolete. BiSS-C is hardware compatible with standard SSI but within each data cycle the master learns and compensates for line delays enabling 10 Mbit/s data rates and cable lengths up to 100 meters. Sensor data can comprise multiple “channels” so both position information and status can be transmitted in one frame. BiSS-C uses the more powerful CRC (Appendix) for detection of transmission errors.
BiSS-C is most often used unidirectionally like SSI. BiSS-C also supports bidirectional communication. Parameters can be exchanged without interrupting sensor data transmission. This is critical for digital control in motor feedback systems. BiSS-C recognizes the end of a cycle and uses the last recognized logic level on the clock line as a control/data bit for the sensor enabling parameter setting over several cycles.
BiSS-C is typically used point-to-point but also supports bussing. In the bus configuration all devices are connected in a chain. Each slave therefore has two connectors – BiSS-In and BiSS-Out. The clock line is pass-through so each slave receives the clock simultaneously. Data-Out of the first slave is connected to the master. Data-Out of the second slave is connected to the Data-In of the first slave and so on. In this way the data from all slaves is clocked out to the master in one continuous frame.