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I want to connect an absolute encoder that has SSI interface to a raspberry pi. Is it possible?
Do i need some additional hardware?

2 Answers 2

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I guess by SSI you mean Synchronous Serial Interface.

The Pi has no differential inputs so your first problem will be converting the clock and data signals into a single ended form. I guess a RS-422 chip would help there.

The next problem will be decoding the signal. The Pi should be able to handle 100-200 kbps but anything over and you'll need to find additional hardware support.

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  • Hi Joan, Thanks for your answer. Yes i do mean Synchronous Serial Interface. Can i use a standard RS-422 port (like USB -> RS422)? the thing i can't understand is why "SSI is based on RS-422 standards and has a high protocol efficiency" (from wikipedia) and then you cannot uses it with standard RS-422 port.
    – Itay Gal
    Commented Sep 30, 2016 at 6:42
  • You need someone who understands the electrical properties and differential protocols. I think you'd be better off posting this question on something like the electrical stack exchange.
    – joan
    Commented Sep 30, 2016 at 8:06
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    normal RS422 uses start and stob bits to frame the data, SSI does not, thats why you can use level converters designed for RS422 to get them at pi compatible levels, but a USB->RS422 converter will not be able to do anything with it due to the lack of the start/stop bits
    – James Kent
    Commented Aug 24, 2017 at 13:04
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SSI is a protocol that uses the RS422 transmission standard. The easiest way to read this is by using an RS422 Line driver IC. This one is available from Digikey.

A differential signal is a way of reducing noise on the line by transmitting a logical high and a logical low simultaneously on a twisted cable. In the case of SSI it is two twisted cables because one has the signal and the other the clock.

You need to do two things to interpret the SSI signal. First, you need to shift the voltage and recover the data from the differential signal. Then you need to real the signal. It might be possible that it could be read as an SPI signal.

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