SSI – Synchronous Serial Interface

The Synchronous-Serial-Interface (SSI) is a digital interface which is mostly used in length and angle measuring systems. It is especially suitable for applications where reliability and signal robustness is very important. The Max Stegmann GmbH, which is now a part of the company group SICK, was the developer of the SSI. Meanwhile, SSI is also used by other manufacturers. Unlike a parallel interface, where the bits are transmitted in various lines, transmission here is serial – which means successively.

In the project “Autonomous Mobile Excavator” several SSI sensors go into action.

In the project “Autonomous Mobile Excavator” several SSI sensors go into action.

SSI – simple structure with many advantages

The great advantage of the SSI is the simple construction. Only two cable pairs are needed: one transmits the clock and the other transmits data. The sensors only need a shift register and a monoflop for the triggering of the shift register. The small number of cables needed reduces the wiring effort. The pairwise twist of the cables and the synchronous and symmetric clock and data signals provides a good shield against interfering influences. Likewise the wiring effort and the cable length is independent of the length of the data word.

The principle of data transfer operations

For transmission of the data from the shift register, which is permanently written with the latest sensor value, at every clock one bit of the latest measurement value is placed on the data line. Up to three encoders can be attached to one clock. This means that also several sensors can be read out at the same time. In the idle state, the clock line and the data line is located on the high level. When the clock signal changes to “low”, first the bitparallel data of the shift register is briefly frozen. Now the actual data-transfer begins. The clock signal again turns to “high” so that the most significant bit (the first bit of the data word) can be transmitted. The following bits are now transmitted at every further level change of the clock line until all bits of the data word are transmitted. Afterwards, the next measurement value can be transmitted.


Today, the Synchronous-Serial-Interface in the sector of point-to-point communication with sensors is widely used. This is especially due to the cost-effective technology and simple construction. Unlike the previous technologies, like the parallel data transmission, the SSI stands out with its high flexibility, simplicity and robustness due to the low number of components. Therefore, the Synchronous-Serial-Interface nowadays is essential for the modern industrial production.

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