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The "SD Specifications Part 1 Physical Layer Simplified Specification Version 2.00" states on page 93:

The SPI standard defines the physical link only, and not the complete data transfer protocol. The SD Memory Card SPI implementation uses a subset of the SD Memory Card protocol and command set.

What does it mean by "physical link only"?

When it says that SPI will only support part of the SD memory card protocol, what part is it that is not supported?

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    \$\begingroup\$ physical link refers to the actual wiring and associated components such as pull-up resistors and connectors and to the voltages used to transfer data \$\endgroup\$ – jsotola Jul 1 '18 at 1:52
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    \$\begingroup\$ @jsotola I believe physical link also includes signalling levels and timings, i.e. "how" you pass the data. The protocol begins at "what" that data means. \$\endgroup\$ – Maple Jul 1 '18 at 4:47
  • \$\begingroup\$ Physical Layer is lowest of 7 OSI "Layers" in an application. "Please Do Not Throw Sushi and Pizza Away." geekstarts.info/osi-model-simplified \$\endgroup\$ – Sunnyskyguy EE75 Jul 1 '18 at 5:14
  • \$\begingroup\$ The second question asks what it means by SPI only supporting part of the memory card protocol. What is missing? \$\endgroup\$ – quantum231 Jul 1 '18 at 7:19
  • \$\begingroup\$ @TonyStewartolderthandirt from your link: "(OSI) is a conceptual model which was introduced to standardized the way data is transferred and received on the network". This model has very little to do with short-distance interfaces. Also... it feels like we discussing off-topic stuff because we don't know the answer :) \$\endgroup\$ – Maple Jul 1 '18 at 8:01
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When it says that SPI will only support part of the SD memory card protocol, what part is it that is not supported?

In "SD Specifications Part 1 Physical Layer Simplified Specification Version 2.00", look at table 7.3 starting on page 102, it shows clearly in the second column whether SPI mode is supported or not for the particular command.

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