CS 2734 Computer Organization II

Read pages 60-65 (Sections 2.3.1 and 2.3.2) of Tanenbaum

• Serial communication:
  A bus is an electrical pathway for connecting multiple devices. It is meant for devices in close proximity of the CPU (Figure 2-21 of Tanenbaum). What about other devices (e.g. printers and modems)?
  • Serial I/O card connects to the bus and handles serial (one bit at a time) communication with slow devices.
  • UART = Universal Asynchronous Receiver Transmitter.
  • RS-232-C
    • Standard defines 25 signals.
    • Pins 2 and 3 define transmit and receive.
    • Original standard's model was modem to data terminal. There was confusion for devices such as printers which are hooked up directly.

• Transmission over phone lines:
  • Analog Conversion:
    Digital signal must still be converted to analog (since the the datapath from point A to point B is probably not all digital):
    • Two-level voltages
    • Amplitude modulation
    • Frequency modulation
    • Phase modulation
    (See Figure 2-25 of Tanenbaum.)

    Frequency or phase modulation are most commonly used because they suffer the least attenuation.

  • Example:
    A transmission line can be driven at each of 8 possible frequencies. If the line drivers can change the frequency once every 100 microseconds, how many bits/second can the line deliver?

  • Synchronous:
    • Interval between bytes is constant.
    • Modems synchronize by continuously sending a special IDLE character.

  • Asynchronous:
    • A character can be sent any time.
    • Normally line held at 1.
    • Start of character marked by a start bit of 0.
    • End of character marked by one or more stop bits.

Revision Date: 4/25/97 at 7:30 a.m.