Digital Electronics: Principles, Devices and Applications

642 Digital Electronicsdevices use IEEE 1284-A and IEEE 1284-B connectors, while IEEE 1284-II devices use IEEE-1284-Cconnectors. The type C connector is not very popular.15.8.3 Internal BusesInput/output ports are used to connect the computer to external devices. Input and output standardsdescribed in the previous sections are referred to as external bus standards. In addition to these externalbuses, computers also have internal buses that carry address, data and control signals between the CPU,cache memory, SRAM, DRAM, disk drives, expansion slots and other internal devices. Internal busesare of three types, namely the local bus, the PCI bus and the ISA bus.15.8.3.1 Local BusThis bus connects the microprocessor to the cache memory, main memory, coprocessor and PCI buscontroller. It includes the data bus, the address bus and the control bus. It is also referred to as theprimary bus. This bus has high throughput rates, which is not possible with buses using expansionslots.15.8.3.2 PCI BusThe peripheral control interconnect (PCI) bus is used for interfacing the microprocessor with externaldevices such as hard disks, sound cards, etc., via expansion slots. It has a VESA local bus as thestandard expansion bus. Variants of the PCI bus include PCI 2.2, PCI 2.3, PCI 3.0, PCI-X, PCI-X2.0, Mini PCI, Cardbus, Compact PCI and PC/104-Plus. The PCI bus will be superseded by the PCIExpress bus. PCI originally had 32 bits and operated at 33 MHz. Various variants have different bitsand data transfer rates.15.8.3.3 ISA BusThe industry-standard architecture (ISA) bus is a computer standard bus for IBM-compatible computers.It is available in eight-bit and 16-bit versions. The VESA local bus was designed to solve the bandwidthproblem of the ISA bus. It worked alongside the ISA bus where it acted as a high-speed conduit formemory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O. Both thesebuses have been replaced by the PCI bus.15.9 Input/Output DevicesInput/output devices are human–machine interface devices connected to the computer. Input devicesare used for entering data into the computer. They convert the raw data to be processed into a computerunderstandableformat. Output devices convert the processed data back into a user-understandableformat. This section briefly describes the commonly used input/output devices.