SSI - Synchronous Serial Interface

Industry is a sector of the economy, which is divided into many branches. Although these branches at first sight may look very different, in most cases they have one thing in common: They use complex installations. The use of such machines and highly intelligent technology brings not only the benefits of manufacturing products in large quantities and thus in the best case increasing profits, but also the disadvantage that it becomes necessary to let the machines communicate with each other, so that work processes can take place virtually by themselves. This required a data transfer that can create a robust and reliable connection within a network. The Synchronous Serial Interface is especially well known. The SSI, the technical abbreviation for the Synchronous Serial Interface, is an interface that is being used with absolute value transmitters, for example position sensors. This interface makes it possible to create a serial data transfer where absolute information concerning a position is transferred.

The Advantages of SSI

The Synchronous Serial Interface generally is used for point-to-point connections, especially as here a data transfer is required that takes place reliably and securely. For this reason, this interface is found mainly with installations in industry. As the construction of this engineering application is very simple, requiring only a wire pair each for the clock and the data, and as the sensor further requires rarely more than a shift register and a monoflop for control of the shift register, the main advantage of this interface is the cost-efficiency. No special equipment is required, and the installation can be done quickly, so that money can be saved here, which again supports the main application in industrial installations. In addition to cost savings, the Synchronous Serial Interface offers other positive benefits.

  • Simultaneous connection of up to three transmitters to one clock, so that multiple sensors can be read at a specified time.
  • Low expense for wiring, as this does not depend on the length of the data word.
  • Screening against influences causing interference by synchronous and symmetric data and clock signals.
  • Various functions like the number of data bits or setting of the clock frequency can be controlled via the software.

The Flow of a Data Transfer

As the name states, the Synchronous Serial Interface enables a serial transfer where the position value is transferred synchronous to a clock provided by the control. This transfer of the position value always starts with the MSB, which is the most significant bit. When the clock and the data line are still at rest, initially they are still at high level. The clock switches this high level to the low level and then back in a loop. During this process, the parallel-serial converters are stored by a shift load within an input latch by the shift register. The transfer of data by the SSI can be explained basically as follows:

  1. The clock signal changes from high level to low level
  2. Freezing of the bit-parallel data
  3. Change from low level to high level
  4. Transfer of the position value with the most significant bit (MSB)
  5. With each further clock change, transfer of the least significant bit (LSB) is performed.
  6. The last change from low level to high level means end of the transfer
  7. The data line is now at low level

Transfer Types

The Synchronous Serial Interface offers two different options for the transfer of absolute information:

  • Single Transfer
  • Multiple Transfer

To be able to transfer a position value at all, a certain quantity of clock pulses must be connected to the clock input of an encoder. In case of a single transfer, this requires 13 clock pulses, while 25 clock pulses are required for multiple transfer. The single transfer, which can also be called single turn execution, takes place in the classic way. However, with the multi-turn execution, the Synchronous Serial Interface makes use of the option to at least double the clock, although multiplication is also possible. At the end of such a 25-clock sequence transfer, change is performed from low level to high level, which causes output of a low signal to the data output. The information contained in the signal is then used to divide the doubled or multiplied position value again.

The Conclusion for the Synchronous Serial Interface

With the Synchronous Serial Interface, industry, which depends on communication between machines, receives an engineering application that in comparison with its predecessor, the parallel interface, has brought clear improvements to the aspects of flexibility, simplicity, and cost savings. Namely, the Synchronous Serial Interface needs fewer components than its classic predecessor and for this reason also is essentially more robust against possible interference. The transfer requires considerably fewer lines, while even longer cable lengths can be used. Because of these advantages, the Synchronous Serial Interface has become an important part of many industrial systems.