Karbosguide.com - Module 5a4.

About I/O port, Plug and Play, and PC Cards.


The contents:

  • I/O addressing
  • Plug and Play (PnP)
  • The physical connector on adapters

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  • If the unit is on the ISA bus, it handles 16 bits at a time (words). Then you link two consecutive ports together, to make a 16 bit channel. If we talk about a 32 bit PCI unit, we link four byte ports together to get 32 bits width (called dword).

    The PC has a built in listing of all I/O units, each of which has their own "zip code" - a port address. Since the PC is basically a 16 bit computer, there are 2 at the 16th power possible addresses (65,536) - from 0000 to FFFFH. They are described in the hexadecimal number system as 5 digit numbers. Hexadecimal is a 16 digit number system. Digits go from 0 to 9 and continue with 6 letters A - F. Let me show you some examples of I/O addresses:

    I/O addresses

    Finally, we need to mention how the CPU finds all these units - adapters, ports. etc. They all have an address - an I/O port number.

    Each unit can be reached through one of many I/O ports. Each port is a byte port. That means that 8 bits (one byte) can be transmitted simultaneously - parallel mode.

    Unit I/O ports
    CMOS RAM 0070H
    Keyboard 0060H ... 0063H
    Serial port 1 (COM 1) 03F8H ... 03FFH
    Parallel port 1 (LPT1) 0378H ... 037FH

    Fortunately, you do not have to adjust port addresses too often. Some adapters give room to adjust to user option I/O addresses, but you have to have bad luck to encounter any conflict in this area.


    Plug and Play

    Plug and play (PnP) is an industry standard for expansion boards. If the board conforms to the PnP standard, the installation is very simple. The board configures itself automatically. These are the minimum requirements:

  • The PC system board must be PnP compatible.

  • The operating system must be capable of utilizing PnP, as Windows is.

  • The adapter must be able to inform the I/O bus which I/O addresses and IRQs it can communicate with.

  • The adapter must be able to adjust to use the I/O address and the IRQ, which the I/O bus communicates to the adapter.



    The physical connector

    The different I/O cards each fit with a particular I/O bus. The different buses each have their own system board slot configuration. That is a socket in the system board, in which you press in the expansion board. Here you see three different edge connectors fitting into each their own type of socket. The ISA bus has a total of 98 prongs (31+18 on each side).


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    Learn more

    Module 5b about EIDE, Ultra DMA and AGP.

    Read Module 5c about SCSI, USB etc.

    Read Module 6b with a little about Windows 95/98.

    Read Module 6c about the relationship between BIOS, OS and hardware

    Read Module 7a about the videosystem

    Read about video cards in Module 7b.

    Read about digital sound in Module 7c.

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