page_title: Synchronous Serial Interface
section_title: Cabling – according to RS-422 standards
hierarchical_section_title: Synchronous Serial Interface / Cabling – according to RS-422 standards
caption_attribution_description: English: RS422/RS485 characteristic graph for cable distance and data rate variation
context_page_description: Synchronous Serial Interface is a widely used serial interface standard for industrial applications between a master and a slave. SSI is based on RS-422 standards and has a high protocol efficiency in addition to its implementation over various hardware platforms, making it very popular among sensor manufacturers. SSI was originally developed by Max Stegmann GmbH in 1984 for transmitting the position data of absolute encoders – for this reason, some servo/drive equipment manufacturers refer to their SSI port as a "Stegmann Interface".
  It was formerly covered by the German patent DE 34 45 617 which expired in 1990. It is very suitable for applications demanding reliability and robustness in measurements under varying industrial environments.
  It is different from the Serial Peripheral Interface Bus: An SSI bus is differential, simplex, non-multiplexed, and relies on a time-out to frame the data. An SPI bus is single-ended, duplex, and uses a select-line to frame the data. However, SPI peripherals on microcontrollers can implement SSI with external differential driver-ICs and program-controlled timing.
context_section_description: Since SSI is based on RS-422 standards, it is necessary to select appropriate cables and to stay within the limits of cabling length and clock frequencies.
  The relation between the cable length and clock frequency is shown in the following figure. This can be used as a conservative guide. This curve is based upon empirical data using a 24 AWG Standard, copper conductor, unshielded twisted-pair telephone cable with a shunt capacitance of 52.5 pF/meter (16 pF/foot) terminated in a 100 Ohm resistive load.  The cable length restriction shown by the curve is based upon assumed load signal quality requirements of:
  Signal rise and fall times equal to or less than, one-half unit interval at the applicable data switching rate.
  A maximum voltage loss between generator and load of 66%
  When high data rates are used, the application is limited to shorter cables. It is possible to use longer cables when low data rates are used. The DC resistance of the cable limits the length of the cable for low data rate applications by increasing the noise margin as the voltage drop in the cable increases. The AC effects of the cable limit the quality of the signal and limit the cable length to short distances when high data rates are used. Examples of data rate and cable length combinations vary from 90 kbit/s at 1.2 km to 10 Mbit/s at 5m for RS-422. 
  Cables having characteristics different from the twisted pair 24 AWG, 52.5 pF/meter (16 pF/foot), can also be employed within bounds mentioned above. First, determine the absolute loop resistance and capacitance values of the typical 24 AWG cable provided by the cable length associated with the data signalling rate desired from the figure.  Then convert those values to equivalent lengths of the cable actually used.  For example, longer distances would be possible when using 19 AWG, while shorter distances would be necessary for 28 AWG.
  The maximum permissible length of cable separating the master and slave is a function of data signalling rate and is influenced by the tolerable signal distortion, the amount of longitudinally coupled noise and ground potential differences introduced between the master and the slave circuit.  Accordingly, users are advised to restrict cable length to a minimum.
  The type and length of the cable used must be capable of maintaining the necessary signal quality needed for the particular application. Furthermore, the cable balance must be such as to maintain acceptable crosstalk levels, both generated and received.
caption: Cable length versus signaling rate
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