Most external buses (e.g. USB) transfer data in a serial manner. However, a machine like a PC needs to turn this into parallel data to interpret it? How does it do so?

Edit: If most computers operate using parallel data buses, how do they catch the data from the USB?

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    Using the clock? – Daniel B May 25 '18 at 12:15
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    What makes you think, Universal Serial Data bus data, is converted to parallel data? USB data is converted to packets. It isn’t serial data either, the protocol name, was primarily chosen for marketing reasons – Ramhound May 25 '18 at 12:16
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    @Ramhound What's the difference between a packet transmitted through a single wire and serial data? – Emil David Lopez May 25 '18 at 12:19
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    @Ramhound: What makes you think, it's not serial data? Packet or not, if the bits are transmitted sequentially through a single line, that's by definition "serial". – user1686 May 25 '18 at 12:24
  • @grawity - My vague memory from a course I took nearly 12 years ago. – Ramhound May 25 '18 at 12:46

Briefly, this is usually done with a device generically called a "deserializer". The core component of a deserializer is a circuit called a "shift register" (or firmware or software that performs the same function).

You can think of a shift register as being a little like an old-timey firefighting bucket brigade. Now imagine that instead of buckets of water being delivered to the far end and emptied one at a time, the buckets are kept in the brigade as the "slots" are filled, then emptied all at once.

The first bucket - bit - that's entered into the s.r. gets passed down the line and ends up at the far end. The next bit ends up in the position next to the far end. Etc. When 'n' bits (for an n-bit parallel bus) have been clocked in, the register contents are copied out, all at once, to the parallel bus. (As if the volunteers in the bucket brigade all dumped their buckets out at the same time, in the same direction.)

If you want to know more about this, learn about "D" flip-flops and then shift registers. I suggest Don Lancaster's classic TTL Cookbook, chapters 5 and 7. Don has now made this available as a free download: https://www.tinaja.com/ebooks/TTLCB1.pdf

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