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

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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