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In UCIe whitepaper, there is one paragraph describing the lanes in a cluster, as below:

The unit of construction of the interconnect is a cluster which comprises of N singleended, unidirectional, full-duplex Data Lanes (N = 16 for standard package and 64 for advanced package), one single-ended Lane for Valid, one lane for tracking, a differential forwarded clock per direction, and 2 lanes per direction for sideband (single-ended, one 800 MHz clock and one data).

I don't understand the emphsized phrases "single-ended, unidirectional, full-duplex" in the quote above: if a communication line is unidirectional, how it can be full-duplex? I guess I may be missing some background knowledge here.

PS: the following picture from UCIe spec 1.0 suggests that: a lane is full-duplex and is composed by two single-ended uni-directional connections in different directions.

enter image description here

bruin
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1 Answers1

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I agree it isn't clear, but I read it as:

There are N lines, each can work in either direction, but only sending data in one way at a time. You can transfer N bits in one way, or X in one direction Y in the other (where X+Y=N). But X and Y can be any number, from 0 to N. Depending on how you want to transfer the data, you may want them balanced: X=Y=N/2, or you may want Y=1 and X=N-1.

So these lines are able to transfer data in a plethora of ways.

While a line could be full duplex (using frequency modulating for the two devices for example) this is very unlikely to be mentioned in a off-hand manner like this. Duplex (half, full or not) is a BUS level distinction, singled ended and unidirectional are line level distinctions.

In summary: the lines are singled ended, can transfer data in either direction and in any combination.

Puffafish
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  • That may be the answer, but it would be strange, as "bidirectional" has always been used to describe what you are talking about. For example, we'd happily say "in a memory bus, address lines are unidirectional (MCU->memory) and data lines are bidirectional (MCU <-> memory)". Nobody in their right mind would imagine that those bidirectional lines are able to carry data both ways at the same time. In fact, I'm having hard time trying to find examples of actual digital lines able to carry data both ways at the same time, except for some stuff that have a heavily analog physical layer like DSL. – dim Apr 04 '22 at 12:42
  • Another interpretation could be: suppose that a "lane" is full-duplex by definition, such that a lane contains two uni-directional connections. An uni-directional connection can be implemented by one single-ended connection, or by a pair of differential wires. So " N singleended, unidirectional, full-duplex Data Lanes" can possbily be interpreted as N full-duplex lanes, each is implemented by two single-ended uni-directional connections. – bruin Apr 04 '22 at 14:18
  • In this picture, the PCI-e lane is full-duplex "by definition". – bruin Apr 04 '22 at 14:23