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How is binary data 'split'?

up vote 14 down vote favorite

If one computer sends data, for example 001101, how does the other computer know, where one bit starts and the other one ends? If you were just to monitor the change, you would get 0101, which is totally inaccurate. Is it just a timing thing? (I apologise if this is a noob question)

2 Answers

up vote 4 down vote accept

Back in the TTL days, disk drive blocks started with an alternating 0-1 pattern, which we synced to with a PLL. The pattern ended with 11 to indicate the start of the read data. The PLL provided the read clock, and a crystal oscillator (a whole 5MHz) was used for the write clock. The read data was sampled 1/2 clock between edges of the bit frame.

up vote 3 down vote

It's a timing thing (I don't like the word 'just').

In the case of a UART, in which the A stands for Asynchronous, there is a line idle state, then a start bit, which edge the receiver uses to synchronise its timing. After that, all the bits are sent each with an equal period. The word finishes with a stop bit, which ensures there is a transition into any following start bit.


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How is binary data 'split'?

up vote 14 down vote

If one computer sends data, for example 001101, how does the other computer know, where one bit starts and the other one ends? If you were just to monitor the change, you would get 0101, which is totally inaccurate. Is it just a timing thing? (I apologise if this is a noob question)


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up vote 9 down vote

Back in the TTL days, disk drive blocks started with an alternating 0-1 pattern, which we synced to with a PLL. The pattern ended with 11 to indicate the start of the read data. The PLL provided the read clock, and a crystal oscillator (a whole 5MHz) was used for the write clock. The read data was sampled 1/2 clock between edges of the bit frame.

edit

preamble still exists today, mostly to rebalance transformer coupled communications - JonRB Jul 13 at 18:40

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