Binary synchronous serial output (SSI)

Absolute position data can be transmitted in a number of ways. One of the most popular interface standards is SSI as it requires only a minimal number of wires, namely 6. Serial data from the magnetic encoders is available in natural binary code through SSI protocol.

By default, with CW rotation of the magnet the value of output data is increasing.

SSI timing diagram

The controller interrogates the encoder chip for its positional value by sending a pulse train to the CLOCK input. The CLOCK signal must always start from high. The first falling edge (point 1) stores the current position data in a parallel/serial converter and the monoflop (timer) is triggered. With each transition of the CLOCK signal (high/low or low/high) the monoflop is retriggered. At the first rising edge (point 2) the most significant bit (MSB) of binary code is transmitted through the DATA output. The next falling edge shifts the next bit (MSB - 1) which appears on the DATA output at the next rising edge. At each subsequent rising edge of the CLOCK the next bit is transmitted to the controller. While reading the data the t_CL must be less than t_MF to keep the monoflop set. After the least significant bit (LSB) is output (point 3) the DATA goes to low. The controller must wait longer than t_MF before it can read updated position data. At this point the monoflop time expires and the DATA output goes to high (point 4).

It is possible to read the same position data several times to enlarge the reliability of transmitted data. The controller must continue sending the CLOCK pulses and the same data will be output again. Between the two outputs one logic zero will be output.

SSI multi-read of the same position data

NOTES:

• rising edge = low to high transition
• falling edge = high to low transition