Re: PLease clarify

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Posted by John Chappel on January 25, 1998 at 14:17:13:

In Reply to: PLease clarify posted by Jim on January 25, 1998 at 12:33:15:

:
: : : The NRZI encoding depends on knowing what the previous
: : : value is (0 or 1). In the lab, we know that the
: : : beginning of the header is preceeded by se0, which is
: : : neither 0 or 1. So what should the NRZI encoding start
: : : off with?

: : Start with a 0.

: Meaning what?

: Regardless of weather the data starts with a 0 or 1? That doesnt make sense, because if your first bit is a zero or a 1, then the first transmitted bit will be zero?
: Then you dont know what was sent, unless you send n+1 bits. Ie always atrt with a zero, then procced from there. But that violates the rule of header size being 4 bytes.

: If you say start a 0 with a 0, and start a 1 with a 1, then you get

: 101.... NRZI-> 100
: 010.....NRZI-> 001

: Jim

What I meant was, assume that the NRZI encoder is in the state where
its last output was 0. I think this is the simplest thing to do
because during Se0, D+ is 0.

If the first bit you are going to transmit is a 0, the encoder
flips its output and D+ goes to 1. If the first bit you are
going to transmit is a 1, the encoder leaves D+ at 0 (although D-
moves to 1 since you are no longer transmitting Se0.

When in doubt, make your circuit as simple as possible.

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Comments:
: : : : : : The NRZI encoding depends on knowing what the previous : : : : value is (0 or 1). In the lab, we know that the : : : : beginning of the header is preceeded by se0, which is : : : : neither 0 or 1. So what should the NRZI encoding start : : : : off with? : : : Start with a 0. : : Meaning what? : : Regardless of weather the data starts with a 0 or 1? That doesnt make sense, because if your first bit is a zero or a 1, then the first transmitted bit will be zero? : : Then you dont know what was sent, unless you send n+1 bits. Ie always atrt with a zero, then procced from there. But that violates the rule of header size being 4 bytes. : : If you say start a 0 with a 0, and start a 1 with a 1, then you get : : 101.... NRZI-> 100 : : 010.....NRZI-> 001 : : Jim : What I meant was, assume that the NRZI encoder is in the state where : its last output was 0. I think this is the simplest thing to do : because during Se0, D+ is 0. : If the first bit you are going to transmit is a 0, the encoder : flips its output and D+ goes to 1. If the first bit you are : going to transmit is a 1, the encoder leaves D+ at 0 (although D- : moves to 1 since you are no longer transmitting Se0. : When in doubt, make your circuit as simple as possible.

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