DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 10: Channel Organization

Parallel | Serial | Summary

Introduction
This section briefly discusses how data might be sent from one point to another, namely via a parallel method, or via a serial method. Each technique has its own advantages and disadvantages.

Objectives
At the end of this section you should be able to

• explain using diagrams the transfer of data using parallel and serial channels
• compare the features of parallel and serial channels
• select an appropriate channel for a given set of criteria
• identify serial and parallel channels in a computer system

Channel Organization
Data may be transmitted between two points in two different ways. Lets consider sending 8 bits of digital data (1 byte). These bits may be sent all at once (in parallel), or one after the other (serial).

• Parallel
  Each bit uses a separate wire. If there is eight bits sent at a time, this will require 8 wires, one for each data bit. The organization looks like,

  

  To transfer data on a parallel link, a separate line is used as a clock signal. This serves to inform the receiver when data is available. In addition, another line may be used by the receiver to inform the sender that the data has been used, and its ready for the next data.

  In the above diagram, the sender places the data on the data lines, then signals the receiver that data is available by asserting (sending a pulse) on the DA (Data is available) line. After reading the state of the data lines, the receiver signals back to the sender that it has processed the data and is now ready for some more data by asserting the DU (Data used) line. The sender, upon getting the DU signal, removes the data and sends the next data element in the same manner.

  This exchange of signals such as DA and DU between the sender and the receiver is called a handshake. These handshake signals allow the sender and receiver to keep synchronized (work on the same data at the same time in the proper sequence).

  Parallel transmission is obviously faster than serial, because more than one bit is sent at a time. Parallel transmission is good only for short links, and examples are found in all computers. The address, data and control buses which interface the processor to other peripherals inside the computer are all parallel buses. In addition, most printers on PC's (LPT1/LPT2) use a parallel interface, commonly called the Centronics Interface.

• Serial
  Each bit is sent over a single wire, one after the after. The organization looks like,

  

  No signal lines are used to convey clock (timing information) and handshake signals. There are two methods (asynchronous and synchronous) in which timing information is encoded with the signal so that the sender and receiver are synchronized (working on the same data at the same time). If no clock information was sent, the receiver can mis-interpret the arriving data (due to bits being lost, going too slow). In asynchronous, each character is synchronized using a start and stop signal. In synchronous, each group or block of characters is synchronized using a synchronize flag.

  In the following sections, serial transmission will be further investigated. A common standard for serial communications is RS232

Summary
Parallel transmission sends each bit using a separate wire. In addition, extra wires are needed to transfer the data between the sender and receiver. These handshake signals allow the data to be transferred in the correct sequence. Computers often send data in parallel form because it is fast. An example of a parallel format is the Centronics parallel interface.

Serial data is slower than parallel, but suited to long distances. There is no need for extra wires to convey handshake signals, as the data is packaged in a variety of different ways (prefixed and suffixed with additional information). An example of a serial format is RS232.

© Copyright B Brown. 1995-2000. All rights reserved.