- T: Clock Period (1/T = 100 kHz to 2 MHz)
- Trc: Read Cycle time: This is defined as (n x T) + (0.5 x T)
- Tmu: Message Update time. The time from last falling edge of clock to when new data is ready for transmission
- Timg: Intermessage Gap time. Must be >Tmu otherwise position data will be indeterminate
- n: The number of bits in the message (not including the Error Flag).
After n-CLOCK pulses (rising edges) the data value has been transmitted. With the next CLOCK pulse (rising edge n+1) the sensor output goes to low level. If it is high even after n+1 rising edges then it means that the interface has a short circuit.
Readings from multiple slaves (up to three) can be enabled at the same time by connecting them to a common clock. To avoid ground loops and to electrically isolate the slave, complete galvanic isolation by opto-couplers is needed.
Multiple transmissions of the same data from the position sensor happens only if there is continuous clocking even after the transmission of the least significant bit. The initial sequences are the same as that of the single transmission. In the idle state the CLOCK and DATA lines are high but with the arrival of the first falling edge the transmission mode is evoked and the similarly the data bits are transmitted sequentially starting with the most significant bit with every rising edge of the CLOCK. The transmission of the least significant bit means that the transmission of the data is complete. An additional rising edge pushes the data line to low, signifying the end of transmission of the data.
If there are continuous clock pulses even after the completion (i.e. the next clock pulses comes in time tw (< tm )) the value of the slave is not updated. This is because the monoflop output is still unsteady and the value in the shift register still contains the same value as before. So with the next rising edge, i.e. after the (n+1) rising edge, the transmission of the same data continues and the MSB of data transmitted earlier is re-transmitted. Then, it follows the same procedure as earlier transmissions, leading to multiple transmissions of the same data. The value of the slave is updated only when the timing between two clock pulses is more than the transfer timeout. Multiple transmission can be used to check the data integrity. The two consecutive received values are compared, transmission failures are indicated by differences between the two values. The transmission of data is controlled by the master and the transmission can be interrupted at any time just by stopping the clock sequence, for a period longer than the time out period. The slave automatically will recognize the transfer timeout and go into idle mode.
Some position sensor manufacturers have added additional information to the basic SSI protocol, in various efforts to ensure high integrity data transmission. For secure transmission and to indicate the end of data transmission CRC bits or parity bits can be added. They are used for identifying if the data from the position sensor has been correctly interpreted and received.
Sensors using SSI
The more observant readers may have noted that this article has used the term ‘sensor’ rather than the more usual ‘encoder’. This is deliberate because encoders are often, but incorrectly, thought of as optical devices, producing data in proportion to a measured position. In recent years, a new generation of non-contact encoders – especially absolute encoders – are not optical but rather inductive (sometimes referred to as ‘incoders’). Such devices use printed circuit board transformer constructions rather than the bulky and expensive transformer windings used in traditional inductive position sensors such as resolvers, LVDTs, RVDTs or synchros. These traditional devices have been the engineer’s preferred choice in many harsh environments for many years due to their non-contact, reliable operation and excellent safety record. Incoders use the same basic physics of their traditional counterparts and so, unsurprisingly they are just as reliable and robust but are more accurate and easier to use. Their ease of use partly comes from the fact that they use SSI as a preferred communications methods. They have gained a significant market share in through bore, bearing-less formats favoured in high reliability, precision sensor applications in the defence, medical, aerospace and industrial sectors.