PORT and Its Different Types

The port is a predefined junction on the bus where a resource or an additional circuit canbe placed. A computer port is an addressable location with a specific address by which it isaccessed. Usually, a resource (peripheral) is made connected with the port by using a specific cable, one end of which is connected with the resource (peripheral) and the other endwith a connector is to be plugged into the predefined port. In some cases, the peripheralcircuits are hard-wired to the port. Software of the peripheral device controls and monitors the port circuits by reading and writing to the port's address. There are various typesof ports available, but we will restrict our discussion in brief only to: i. Serial port; ii. Parallel port;}} [1]

between wires is not required here, and as such, it is free from this problem that gets even worse with increased cable length. The disadvantage is that it takes (8 times) longer time to transmit the data than it would be if there were 8 wires. Serial ports use a well-defined protocol and a standard connector of 9 pins or 25 pins, in which each pin is assigned for different purposes to establish the desired communication. Serial ports, also called COMmunication (COM) ports, are bidirectional that allows each device to receive and to transmit data, either using the same pins that would limit communication to half-duplex (one direction), or using different pins that would allow communication to full-duplex, in which information can travel in both directions at the same time. A special controller chip (Intel's 8251A, the universal synchronous asynchronous receiver/transmitter, USART) can be used which is then attached to the system by plugging RS-232 interface cards into the bus. Although many of the newer systems have discarded the serial port completely in favour of USB connections, still few computers provide serial ports for some other reasons. All contemporary OSs in use today still support serial ports.

A brief detail of the standard serial port is shown with figure in the website: http://routledge.com/9780367255732.

Parallel Port

Most of the computers (also PCs) have a parallel port as a commonly used interface to mainly connect a printer and also other devices to the computer, in spite of USB being increasingly popular. With a parallel port, a character of 8 bits (1 byte) is sent at a time in parallel to each other. A typical parallel port, however, is capable of sending 50-100 kilobytes of data per second. Although parallel ports were originally specified as unidirectional, IBM offered a new bidirectional parallel port design, known as standard parallel port (SPP) with the introduction of the PS/2 in 1987. In 1991, Intel, Xircom, and Zenith created the enhanced parallel port (EPP) with data transmission rate ranging from 500 kilobytes to 2 megabytes per second. It was targeted specifically for storage devices (non-printer devices) that needed the highest possible transfer rate. In 1992, Microsoft and Hewlett-Packard jointly announced a specification called extended capabilities port (ECP). While EPP was geared towards other devices to attach, ECP was mainly designed to provide improved speed and functionality for printers. In 1994, the IEEE 1284 standard that included both EPP and ECP specifications for parallel port devices was released.

A brief detail of the parallel port is shown with figure in the website: http://routledge. com/9780367255732.

USB Port

USB port, though not very old, is increasingly popular for being built up with advanced technology to offer a faster and more flexible interface for connecting nearly all devices to the computers, thereby almost replacing both serial and parallel ports completely. It requires almost nothing as such for configuring the hardware or software, and is very different from the legacy (serial and parallel) interfaces it is replacing. While accommodating various types of devices, it may use any of three speeds, namely low speed, full speed, or full and high speeds. While in operation, there are four transfer types, namely control transfer, interrupt transfer, isochronous transfer, and bulk transfer. USB ports differ from many other ports because all ports, each with its own connector and cable, with a host controller here on the bus share a single data path to the host (see USB bus, described earlier). Here, all ports (devices) share the available time, but only one port (device), or the host, is operative at a time. A single host may also support multiple USB host controller; however, each will do so with its own bus. Other types of interfaces that support multiple ports sharing a single data path include IEEE-1394 and SCSI. In contrast, with the traditional serial interface, each port is independent from the other ports, and can independently send and receive data simultaneously with the other ports. For more details, see USB bus already discussed earlier.

A brief detail of the USB port is shown with figure in the website: http://routledge. com/9780367255732.


I/O system organisation nowadays is of immense importance, and it critically influences the performance of the system as a whole. The organisations of I/O systems are distinguished by the extent of CPU involvement in I/O operations. Four basic approaches to I/O transfers are of common interest. The simplest technique is PIO in which the CPU is totally involved and performs all the necessary control functions of an I/O operation. The second approach is interrupt-driven I/O in which CPU is partly relieved. The third approach is based on providing I/O devices with DMA in which data transfers can be implemented independently without CPU involvement. Maximum speed and total independence in I/O are, however, achieved by providing I/O channels, and then IOPs, which are capable of executing their own programs to manage the entire I/O operations.

Buses are used as the primary means of interconnecting different resources of a computer system. Fundamental characteristics and different useful parameters of a generic bus system have been explained. Three popular interconnection standards, namely PCI, SCSI, and USB, have been briefly described with their individual strengths and drawbacks. A most-favoured high-speed, high-performance serial bus, FireWire, having high-speed serial bus interface standards, especially required for digital audio and video pieces of equipment, has been described. Easy availability of immensely powerful multiple-processor servers with reduced cost now requires typical I/O specifications having low latency, high bandwidth, high throughput, and low-overhead interconnect that might perhaps only connect servers and storage to one other for building up large commercial datacentres. InfiniBand, a switched fabric communication link using switch- based architecture, satisfies all these requirements and also aims to fulfil the constantly increasing demand that additionally includes quality of service, failover, and also scalability.

Port is used to attach a peripheral device or an additional circuit with the existing system. Popular computer ports such as serial, parallel, and USB are of common importance. They usually exhibit different approaches that meet the needs of various devices and the related interconnecting buses, and also reflect the increasing importance of plug- and-play features that ultimately make it possible to summarily realize users' ease and handiness.


  • 5.1 The transfer rate between a CPU and its attached memory is higher in order of magnitude than the mechanical I/O transfer rate. How can this imbalance cause inefficiencies?
  • 5.2 What is the function of an I/O interface? What are the responsibilities being performed by I/O modules? What are the criteria that must be fulfilled at the time of designing an I/O module?
  • 5.3 What are the various modes of data transfer between a computer and peripherals? Explain.
  • 5.4 Differentiate between I/O-mapped I/O (isolated I/O) and memory-mapped I/O. What are their relative advantages and disadvantages?
  • 5.5 What do you mean by busy waiting of CPU? Define interrupt. How do interrupts work? Discuss how an interrupt is processed.
  • 5.6 Distinguish between vectored and non-vectored interrupts.
  • 5.7 What is UART (universal asynchronous receiver transmitter)? What is the basic advantage of using interrupted initiated data transfer over transfer under program control without an interrupt?
  • 5.8 Assume a computer without priority interrupt hardware. Any one of many sources can issue the interrupt that interrupted the computer, and any interrupt request results in storing the return address and branching to a common interrupt service routine. Explain how a priority can be established in the interrupt service program. (Hint: Section Multiple Interrupts and Section
  • 5.9 Discuss the role of hardware and software in relation to the actions they take at the time of servicing an interrupt.
  • 5.10 What is DMA mode of data transfer? When is this mode used? Describe the functions of different internal registers used in a DMA controller. How does cycle stealing DMA differ from burst-mode (block) DMA? Why does DMA always enjoy higher priority than CPU while both attempt to communicate with memory?
  • 5.11 A computer uses DMA to read from the disk. The disk has 64 sectors per track, and each sector is of 512 bytes. The disk rotation time is 16 ms. The bus is 16 bits wide, and bus transfers take 500 ns each. The average CPU instruction requires two bus cycles. How much is the CPU slowed down by the actions of DMA?
  • 5.12 Differentiate between polled I/O and interrupt-driven I/O. What are the vectored interrupts? How they are used in implementing hardware interrupts?
  • 5.13 In the daisy chain priority interrupt as shown in Figure 5.6, device I, requests an interrupt after device I2 has sent an interrupt request to the CPU but before the CPU responds with the interrupt acknowledge. Explain with a diagram what will happen.
  • 5.14 What are the advantages of a parallel priority interrupt. Show with a real-life example how interrupt-driven I/O is implemented with an interrupt controller.
  • 5.15 How are multiple DMA requests handled in the implementation of a DMA controller? Give suitable examples. "DMA data transfer may sometimes lead to cache coherence" - justify and explain.
  • 5.16 Why do I/O devices place the interrupt vector on the bus? Would it be possible to store the information in a table in memory instead?
  • 5.17 "IOP is virtually a stand-alone computer"- justify the statement. What are the advantages of using an IOP? How is the IOP organised when attached with a main computer?
  • 5.18 Define each of the following I/O control methods: PIO, DMA controllers, and IOPs. What are the advantages and disadvantages of each method with respect to program-design complexity, I/O bandwidth, interface hardware costs, and system throughput?
  • 5.19 Discuss the role played by the bus when the computer is in operation. Explain the structure of a generalized bus system giving its main constituents. Why is the single-shared bus so widely used as an interconnection medium in both sequential and parallel computers? What are its major disadvantages?
  • 5.20 What do you mean by bus arbitration? Why is bus protocol required? Discuss the key elements (parameters) that are to be considered at the time of designing a bus.
  • 5.21 Analyse the three bus-arbitration methods: daisy chaining, polling, and independent requesting with respect to communication reliability in the event of hardware failures.
  • 5.22 Define each of the following terms in the context of bus design: master unit, handshaking, lock signal, wait state, tri-state, and bus transceiver. (Hint: Section 5.4.6)
  • 5.23 Why do most modern PCs maintain a hierarchy in the buses? Discuss the hierarchy and the responsibilities being handled by each bus in the bus hierarchy of the PCI bus (see the PCI bus given in the website to answer).
  • 5.24 Why is the SCSI bus so popular in today's small systems? State and explain its salient features and distinct advantages. "SCSI is not treated as only a simple bus, but it is called a SCSI network"- justify with reasons.
  • 5.25 What is USB? Discuss its distinctive features that put USB in the forefront of the list of bus systems being used in the small computer systems.
  • 5.26 What is FireWire? What are the salient features that made it more acceptable in the common user market?
  • 5.27 Discuss in brief the protocols that are employed in FireWire serial bus transmission mechanisms.
  • 5.28 "Evolution of InfiniBand is a big step towards a notable development in the information transmission mechanisms": justify the statement in the light of distinguished features that this technology possesses.
  • 5.29 Describe with a diagram the basic components with their interactions that constitute the core architecture of InfiniBand.
  • 5.30 Describe in brief the major features of the protocols that are employed in InfiniBand technology.
  • 5.31 What do you mean by a port? What is the relationship between a port and a bus? What are the differences between a serial port and a parallel port? Discuss the major features that have been observed in SPP, EPP, and ECP
  • 5.32 Describe with a diagram in brief the basic components and the major features of a USB port (hub) (see the websites to answer).

Suggested References and Websites

Abbot, D. PCI Bus Demystified. New York: Elsevier, 2004.

Anderson, D. Fire Wire System Architecture. Reading, MA: Addison-Wesley, 1998.

Gustavson, D. B., "Computer buses — A tutorial." IEEE Micro, vol. 4, pp. 7-22, August 1984.

Kagan, M. "InfiniBand: Thinking outside the box design." Communication System Design, September 2001.

Shanley, T. InfiniBand Network Architecture. Reading, MA: Addison-Wesley, 2003.

1394 Trade Association: Includes technical information and vendor pointers on FireWire.

InfiniBand description, available at www.csdmag.com

PCI Local Bus Specification, available at www.pcisig.com/developers.

PCI Pointers: Links to PCI vendors for additional information.

T10 Home Page: A technical committee on National Committee on Information Technology Standards working on lower-level interfaces, including small computer system interface (SCSI). Universal Serial Bus Specification: available at www.usb.org/developers.

  • [1] USB port.

    Serial Port

    Serial port has been an integral part of most computers from its inception, and the nameserial comes from the fact that a serial port serializes data, taking a byte (8 bits) of data,and transmits these 8 bits one bit at a time. The advantage is that a serial port uses less-wide(needs only one wire to transmit) and less-expensive cables with no additional shielding mechanism to prevent any electrical interference between the wires, and also inexpensive connectors with lesser number of pins to bend or break. Also, synchronization

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