Interfacing with an SSI sensor?

Question

I'm trying to interface with version RM08S of this rotary encoder using an Arduino Mega. The sensor supports a "Synchro serial interface (SSI), single ended" interface. I'm unfamiliar with this protocol.

Despite the name, it doesn't appear to match the description of the formal SSI or RS-422 specification, since that requires two wires for differential clock and data lines, whereas this sensor only has a single wire for each. This makes it sound more like IIC, but without the bus-like functionality.

Googling finds very few results on interfacing the Arduino to SSI devices, much less any standard libraries. This forum thread asks a similar question, but doesn't seem to resolve the issue.

Since there's just a clock and data line, would reading from this be as simple as toggling clock, and then reading the 10-bits of data from the data line, sequentially with each clock tick, accounting for whatever bits are used to pad the packet?

Answer 1

It's just SPI.

SPI.begin();
SPI.setDataMode(SPI_MODE2);

// ...

uint8_t h = SPI.transfer(0x00);
uint8_t l = SPI.transfer(0x00);
delayMicroseconds(20); // Indicate EOT
uint16_t v = (h << 8) | l;
v >>= 4; // for 12 bit resolution, 5 for 11 bit, 6 for 10 bit etc.

For 8 bit resolution or less you only need one SPI.transfer, and subtract 8 from the right-shift amount.

Wiring is simple: clock to SCK (52 or ICSP 3) and data to MISO (50 or ICSP 1).

The timing diagrams in the datasheet (page 5) describe it all in detail.