First bit in a packet - interoperability issues

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Posted by Jayesh T. Patel on February 03, 1998 at 22:20:49:

I would like to make sure that I understand what is going on with the NRZI encoding, and LSBs and MSBs.

1) LSBs, MSBs, MSGs, etcs...
Suppose that our transmitter is about to start sending a packet.

I think it should START with the LEAST significant BIT of the LEAST significant BYTE of the header. Then the SECOND LEAST significant BIT of the LEAST significant BYTE of the header.
And so on, with each subsequent bit being more significant that the previous until the end of the header is reached...
at which point it will send the LS BIT of the databyte and then the SECOND LS BIT of the databyte etc. and it should cycle through the databyte in this manner for however many iterations are specified.
Then it should send the LS BIT of the LS BYTE of the CRC and incrementally proceed to the MS BIT of the MS BYTE of the CRC (in the same manner as was done with the header) at which point the packet will have ended.

(All of the bits mentioned above of course have to be sent through necessary bit stuffing and NRZI encoding before being driven onto the differential outputs).

Correct?

2) NRZI encoding
Suppose the first two bits of a packet to be sent are 00.
Then the transmitter output should be:
D+: 01
D-: 10

Suppose the first two bits of a packet to be sent are 01.
Then the transmitter output should be:
D+: 00
D-: 11

Suppose the first two bits of a packet to be sent are 10.
Then the transmitter output should be:
D+: 10
D-: 01

Suppose the first two bits of a packet to be sent are 11.
Then the transmitter output should be:
D+: 11
D-: 00

In words, the first bit out of the NRZI encoder should have the same value as the input and subsequent bits should be encoded according to the NRZI rules.
(This is important because the receiver needs to either ignore or interpret the first bit of the packet and I'm assuming that we shouldn't ignore it).

Correct?

Thanks,

Jayesh.

Follow Ups:

Never mind, I figured it out (I was wrong). Jayesh 17:09:23 2/05/98 (0)

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Comments:
: I would like to make sure that I understand what is going on with the NRZI encoding, and LSBs and MSBs. : 1) LSBs, MSBs, MSGs, etcs... : Suppose that our transmitter is about to start sending a packet. : I think it should START with the LEAST significant BIT of the LEAST significant BYTE of the header. Then the SECOND LEAST significant BIT of the LEAST significant BYTE of the header. : And so on, with each subsequent bit being more significant that the previous until the end of the header is reached... : at which point it will send the LS BIT of the databyte and then the SECOND LS BIT of the databyte etc. and it should cycle through the databyte in this manner for however many iterations are specified. : Then it should send the LS BIT of the LS BYTE of the CRC and incrementally proceed to the MS BIT of the MS BYTE of the CRC (in the same manner as was done with the header) at which point the packet will have ended. : (All of the bits mentioned above of course have to be sent through necessary bit stuffing and NRZI encoding before being driven onto the differential outputs). : Correct? : 2) NRZI encoding : Suppose the first two bits of a packet to be sent are 00. : Then the transmitter output should be: : D+: 01 : D-: 10 : Suppose the first two bits of a packet to be sent are 01. : Then the transmitter output should be: : D+: 00 : D-: 11 : Suppose the first two bits of a packet to be sent are 10. : Then the transmitter output should be: : D+: 10 : D-: 01 : Suppose the first two bits of a packet to be sent are 11. : Then the transmitter output should be: : D+: 11 : D-: 00 : In words, the first bit out of the NRZI encoder should have the same value as the input and subsequent bits should be encoded according to the NRZI rules. : (This is important because the receiver needs to either ignore or interpret the first bit of the packet and I'm assuming that we shouldn't ignore it). : Correct? : Thanks, : Jayesh.

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