Michelsen winter 2008 10 ucsd physics 121 2008 ucsd

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Unformatted text preview: 32 serial asynchronous communications data sample times centered in data bits Winter 2008 9 time slide courtesy E. Michelsen Winter 2008 10 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 No Clock: Do You Know Where Your Data Is? • • • Asynchronous: Up Close and Personal Most long-distance, high speed, or cheap signaling is self timed: it has self no separate clock; the receiver recovers timing from the signal itself Receiver knows the nominal data rate, but requires transitions in the nominal data transitions in signal to locate the bits, and interpolate to the sample points Two General Methods: – Asynchronous: data sent in short blocks called frames – Synchronous: continuous stream of bits • Receiver t racks the timing continuously, to stay in synch • Tracking requires sufficient transition density throughout the data stream • Used in all DSLs, DS1 (T1), DS3, SONET, all Ethernets, etc. • Asynchronous – technical term meaning “whenever I feel like it” • • • • Start bit is always 0. Stop bit is always 1. The line “ idles” between bytes in the “ 1” state. idles” This guarantees a 1 to 0 transition at the start of every byte After the leading edge of the start bit, if you know the data rate, you can find all the bits in the byte transition locates data transitions locate data one byte idle idle Lecture 11 stop bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 0 interpolated sample times (bit centers) Winter 2008 bit 0 data start 1 slide courtesy E. Michelsen time time 11 Winter 2008 interpolated sample times (bit centers) slide courtesy E. Michelsen 12 3 Computer Interface 02/21/2007 UCSD: Physics 121; 2008 Can We Talk? RS-232: most common implementation • RS-232 is an electrical (physical) specification for communication stop bit 7 bit 6 idle bit 5 bit 4 bit 3 bit 1 bit 0 start bit 2 ASCII “A” = 0x41 9600, 8N1 idle UCSD: Physics 121; 2008 – idle, or “mark” state is logic 1; 1 bit @ 9600 bps = 1/9600th sec • • • If we agree on 4 asynchronous communication parameters: – “space” state is logic 0; – Data rate: Speed at which bits are sent, in bits per seconds (bps) – Number of data bits: data bits in each byte; usually 8 ...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.

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