KarbosGuide.com. Module 2c.2

About the ISA bus and other old PC buses

The contents:

  • Introduction to the ISA bus
  • MCA, Eisa and VLB buses
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  • Introduction to the ISA bus

    Since about 1984, standard bus for PC I/O functions has been named ISA (Industry Standard Architecture). It is still used in all PCs to maintain backwards compatibility. In that way modern PCs can accept expansion cards of the old ISA type.

    ISA was an improvement over the original IBM XT bus, which was only 8 bit wide. IBM's trademark is AT bus. Usually, it is just referred to as ISA bus.

    ISA is 16 bit wide and runs at a maximum of 8 MHz. However, it requires 2-3 clock ticks to move 16 bits of data. The ISA bus works synchronous with the CPU. If the system bus is faster than 10 MHz, many expansion boards become flaky and the ISA clock frequency is reduced to a fraction of the system bus clock frequency.

    The ISA bus has an theoretical transmission capacity of about 8 MBps. However, the actual speed does not exceed 1-2 MBps, and it soon became too slow.

    Two faces

    The ISA bus has two "faces" in the modern PC:

  • The internal ISA bus, which is used on the simple ports, like keyboard, diskette drive, serial and parallel ports.

  • As external expansion bus, which can be connected with 16 bit ISA adapters.

    ISA slots are today mostly used for the common 16 bit SoundBlaster compatible sound cards.


    The problem with the ISA bus is twofold:

  • It is narrow and slow.
  • It has no intelligence.

    The ISA bus cannot transfer enough bits at a time. It has a very limited bandwidth. Let us compare the bandwidths of ISA bus and the newer PCI bus:

    Bus Transmission time Data volume per transmission
    375 ns
    16 bit
    30 ns
    32 bit

    Clearly, there is a vast difference between the capacity of the two buses. The ISA bus uses a lot of time for every data transfer, and it only moves 16 bits in one operation.

    The other problem with the ISA bus is the lack of intelligence. This means that the CPU has to control the data transfer across the bus. The CPU cannot start a new assignment, until the transfer is completed. You can observe that, when your PC communicates with the floppy drive, while the rest of the PC is waiting. Quite often the whole PC seems to be sleeping. That is the result of a slow and unintelligent ISA bus.

    Problems with IRQs

    The ISA bus can be a tease, when you install new expansion cards (for example a sound card). Many of these problems derive from the tuning of IRQ and DMA, which must be done manually on the old ISA bus.

    Every component occupies a specific IRQ and possibly a DMA channel. That can create conflict with existing components. Read module 5 about expansion cards and these problems.

    The ISA bus is out

    As described, the ISA bus is quite outdated and should not be used in modern pcs. There is a good chance, that this "outdated legacy technology" (quoting Intel) will disappear completely.

    The USB bus is the technology that will replace it. It has taken many years to get this working and accepted, but it works now.

    Intel's chip set 810 was the first not to include ISA support.

    MCA, EISA and VLB

    In the 80s, a demand developed for buses more powerful than the ISA. IBM developed the MCA bus and Compaq and others responded with the EISA bus. None of those were particularly fast, and they never became particularly successful outside the server market.


    IBM's top of the line bus from 1987 is named Micro Channel Architecture. The MCA bus was a masterpiece, unifying the best bus technology from the mainframe design with the demands from the PC. However, contrary to the ISA bus, MCA is patented, and IBM demanded high royalty fees, when other PC manufacturers wanted to use it. Thus the bus never became a great success, despite its advanced design. It ended up being a classic example of poor marketing strategy.

    The MCA bus is 32 bit wide and "intelligent." The cards configure themselves with respect to IRQ. Thus, they can be installed without adjustments of jumper switches or other features. It works constantly at 10.33 MHz, asynchronous with the system bus.

    The MCA bus is also relatively fast with transfer rates of up to 40 MBps in 32 bit mode at 10.33 MHz. MCA requires special adapters. There have never been too many adapters developed, since this bus is by and large used only in IBM's own PCs.


    EISA is a bus from 1988-89. It is designed by the "Gang of Nine:" the companies AST, Compaq, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, Wyse and Zenith. It came in response to IBM's patented MCA bus.

    EISA is built on the ISA bus; the connector has the same dimensions and old ISA cards fit into the slots. To keep this compatibility, the EISA bus works at maximum 8 MHz. Like ISA, the bus bus is synchronous with the CPU at a clock frequency reduced to a fraction of the system bus clock frequency.

    EISA is compatible with ISA in the sense that ISA adapters can be installed in EISA slots. The EISA adapters hold a second level of connectors in the button of the slot.

    However, EISA is much more intelligent than ISA. It has bus mastering, divided interrupts and self configuration. It is 32 bit wide, and with it's compressed transfers and BURST modegives a highly improved performance.

    But, like the MCA, it did not have great success. The EISA bus is still used in some servers.

    Vesa Local Bus

    This Bus called VLB for short. It is an inexpensive and simple technology. This bus only achieved status as an interim phenomenon (in 1993-94). VLB was widely used on 486 motherboards, where the system bus runs at 33 MHz. VLB runs directly with the system bus. Therefore, data transfer is at CPU speed, synchronous and in width. The problem with VLB was compatibility. Adapters and system system boards would not always work together. Vesa is an organization with about 120 members, mostly monitor and graphics card manufacturers. Therefore, most VLB cards were video cards.

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    Learn more

    Read module 5c about the modern I/O bus called USB.

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP and module 5c about Firewire

    Read more about chip sets on the motherboard in module 2d.

    Read more about RAM in module 2e.

    Read Module 4b about hard disks.

    Read Module 4c about optical media (CDROM and DVD).

    Read Module 4d about super diskette and MO drives.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

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