Basic principles of SSI communication

SSI principles

SSI communication sequence

The SSI master starts pulsing on the clock line with a fixed cycle into the shift register of the SSI slave.
The slave generally "pushes back" data with a width of 25 bits on the data line. An SSI encoder should determine its position with the first falling edge of the signal at the Clock input ("latching"), which is then transferred.
Once the specified number of bits was pushed, the clock signal is terminated.
After a pause, polling by the SSI master recommences.

The last data bit can be a PowerFail bit, i.e. the slave signals a power failure. This output depends on the slave.

The number of bit changes equals the clock frequency, i.e. the maximum data transfer rate for a 1 MHz cycle is 1 Mbit/s.

Different SSI slaves have different communication parameters. The communication parameters of the slave must be set in the SSI master:

Baud rate (e.g., 500 kBaud)
Coding (e.g., Gray code)
Data frame type, e.g., multi-turn 25 bits
Data frame size, e.g., 25 bits
Data length, i.e. how many bits in the data frame represent the actual position data, e.g., 24 bits.

The communication parameters can be found in the data sheet of the SSI slave.

Basic principles of SSI communication 1:

Referencing an SSI signal

An SSI encoder is an absolute encoder, i.e. the position value is available without referencing immediately after switch-on.
Many SSI encoders offer the option of referencing or zeroing the position value via an additional digital input. This can be set via the digital output “Set”: CoE parameter 800D:03.