Introduction to Absolute Encoders and SSI.

The electronic interface SSI or Synchronous Serial Interface was designed for use with absolute encoders and patented by Stegnian. A number of manufacturers of PLC s and intelligent controllers provide for SSI which is now an industry standard.

Generally, absolute encoders give a parallel output in Gray code which allows only a one bit change between adjacent resolvable steps and reduces the maximum reading error to one step.

However, as far as most people are concerned. all that needs to be known is that Gray code is more secure than binary in encoder applications.

Absolute encoders fall into two groups according to their resolutions: encoders with a resolution of up to 360 degrees are called single turn encoders and those which can revolve many times (usually up to 4096 turns) are called multi-turn encoders. Normally a single turn encoder gives a 12 bit parallel output (4.096 different codes per revolution of a coded disc) while a multi-turn encoder uses the same single turn disc as the single turn encoder but it is coupled to a second disc via a 16: 1 reduction gearbox so that the second coded disc revolves once for every 16 turns of the first disc.

The second disc gives 4 bits output and if only two coded discs were used. the total 16 bits gives 65.536 different codes for 16 complete revolutions. ASM use a 24 bit absolute encoder which has 4 coded discs.

This is all very desirable but if the parallel output was used it would mean at least 24 wires would be required to read the output. SSI however offers a much better solution whereby the parallel output is set into a shift register
(parallel to serial converter) and then shifted out serially by pulses supplied by an external interface (SSI).

The speed of the transmitted output is therefore dependent on the frequency of the pulses supplied so if fast update is required, the speed of the pulses would need to be adjusted accordingly.

The main features of SSI compared with other interfaces are:

* Low component count

* Irrespective of the encoder resolution, only 4 wires required

* Secure data output in single step gray code

* Easy electrical isolation between encoder and controller

* Data transmission between encoder and controller is synchronised by the controller clock signal

* Depending on the transmission distance, baud rates of up to l.5MHz can be achieved.


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