Synchronous serial interface

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The synchronous serial interface . Synchronous Serial Interface (SSI) is an interface for absolute encoders (position measuring systems). It makes it possible to obtain absolute information about the position through serial data transmission . The Synchronous Serial Interface was originally developed by Max Stegmann GmbH (part of the Sick Group since 2002 ), but is used by a number of other manufacturers.

SSI point-to-point connection

It is particularly suitable for applications that require reliability and robustness in an industrial environment. The synchronous serial interface has a very simple structure, only two pairs of lines are required (for the clock and the data), and little more than a shift register and a monoflop is required in the sensor to control it. This enables a cost-effective structure. SSI also enables up to three encoders to be connected to a common clock. This enables several sensors to be read out at a defined point in time. Compared to other interfaces for position measuring systems, the synchronous serial interface offers the following additional advantages:

  • Less cabling effort
  • The effort for cabling and interface technology does not depend on the length of the data word.
  • The shielding against external electrical interference is achieved by transmitting differential clock and data signals via twisted-pair cables .
  • Complete galvanic isolation by optocouplers is used to avoid earth loops .

The clock frequency can be set using software so that the transmission can be adapted to the cable lengths. In addition, the number of data bits can be programmed via software, so that flexibility in terms of resolution is also possible. The data value can either be in Gray code or in binary code . A parity bit can also be transmitted for data backup . Some sensors use a data bit as an alarm bit, for example to indicate a failure of the light emitting diode or an interruption in the supply voltage in optical distance meters.

transmission

Transmission via the synchronous serial interface: 1. The measured value is saved. 2. Output of the first data bit. 3. All data bits are transmitted, monoflop begins to run. 4. Monoflop returns to its basic state, a new transmission can be started.

A shift register in the sensor is permanently loaded with the current measured value. When a data value is to be read, the control outputs a burst of clocks on the clock line. The first falling edge triggers a monoflop in the sensor, which switches the shift register from parallel loading to serial output. A data bit is now output with each subsequent rising clock edge. When the least significant bit has been received, the clock is stopped. The monoflop, which was always retriggered by the clock pulses, returns to the basic state after the switching time has elapsed and allows the measured values ​​to be transferred to the shift register again. The data line is kept at low level until then.

literature

  • Patent EP0171579 : Arrangement for the serial transmission of the measured values ​​of at least one measured value converter.

Web links

  • Posital: SSI Application Note (PDF, English; 523 kB). Application description for setting up an SSI interface with the help of a microcontroller