L3LocalAsync6 - Part 1 Data Transmission Part 1 Data...

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1 Part 1 – Data Transmission Local Asynchronous Communication: RS-232 and USB Gail Hopkins Part 1 – Data Transmission Introduction ± Using electric current to transmit bits over short distances ± Present two mechanisms for sending characters over short distances Ð RS-232 Ð USB ± Baud rate and bandwidth ± Nyquist Sampling Theorem & Shannon’s Theorem Part 1 – Data Transmission Asynchronous Communication ± Where the receiver does not know when the sender will transmit Ð transmit when data are ready Ð variable delays between transmissions Ð no sender-receiver coordination beforehand ± E.g., keyboard connected to a computer ± Technically, the electrical signal does not contain information about where individual bits begin and end Using Electric Current to Send Bits ± Use a wire to create a circuit between the sender and receiver ± Negative voltage on the wire could represent a 1 and positive a 0 ± Waveform diagram shows variable delay Part 1 – Data Transmission Part 1 – Data Transmission Communication Standards ± Standards ensure that hardware from different vendors can inter-operate Ð Voltages used, length of time a voltage should be held, how rapidly the voltage should change ± Scope of “physical layer” standards Ð Mechanical Ð Electrical Ð Functional Ð Procedural ± Standards are published by standards organisations - ITU, ISO etc. Part 1 – Data Transmission The RS-232 Standard ± To connect keyboards, terminals etc. to computers over copper wire ± Concerned with 7-bit characters ± Details of physical connection (maximum length, plugs and sockets) ± Electrical details (voltages) ± Serial communication ± Asynchronous (for each character)
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2 Part 1 – Data Transmission Example RS-232 Waveform Part 1 – Data Transmission RS-232 ± Never leaves 0 volts on the wire - an idle line is the same as a 1 bit ± Sender and receiver agree how long a bit lasts - receiver uses a local timer ± A 0 start bit signifies the start of a character and is followed by 7 data bits ± A minimum gap of 1 bit between characters (a phantom stop bit of 1)
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L3LocalAsync6 - Part 1 Data Transmission Part 1 Data...

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