PC Bus

    General information

    Computer motherboards are the cards that house primary electronic components such as processor, memory and chipset. Typical PC motherboard nowadys offers PCI bus for extension cards and AGP connector for the graphics card. Today, the vast majority of PCs shipped use motherboards based on the years-old standards, 12-inch-by-9.6-inch ATX or 9.6-inch-by-9.6-inch MicroATX. Nowadays there are also some smaller form factors becoming popular in special applications like embedded applications and small PCs. At year 2002 Via released a motherboard 6.7 inches on a side, a size it called Mini-ITX. The newest small size from Via is Nano-ITX motherboard, just 4.7 inches on a side. BTX at 10.5 inches by 12.8 inches, MicroBTX at 10.5 inches by 10.4 inches and PicoBTX is 10.5 inches by 8 inches. One of the main reasons for developing some of the larger the new design is to align the three hottest components of a computer--the processor, chipset and graphics card--so that the cooling airflow can travel directly through the machine.

    Plug and Play

    Properly implemented, Plug and Play provides automatic configuration of PC hardware and devices. Plug and Play technologies are defined for IEEE 1394, PCI, PC Card/CardBus, USB, SCSI, ATA, ISA, LPT, and COM. The driver architecture for Windows XP/2000 supports comprehensive, operating system-controlled Plug and Play.

    Card interfaces

      PC Card

      PC Card is a new name for technology which evolved from PCMCIA to provide more advanced card functionaly than just memory cards. This small extension card standard which uses 16 bit bus. The interface is originally known as PCMCIA interface. The cards use 68-pin connector. There are actually two different interfaces that are used with PC Cards today. The first is the older, "original" PC Card or PCMCIA interface, which is a 16-bit interface that runs at 8 MHz. This interface is functionally quite close to the regular ISA system bus found on desktop PCs. Although the the basic functionality is quite close to a subset of ISA, there are many variations what is the actual subset of ISA functions supported. The cards can look like simple memory card, general ISA card (like modem or Ethernet) or card can look like IDE hard drive (True-IDE" mode). Then there are different card operation voltages: 5V and 3.3V. The other newer inerface standard is CardBus, which is a 32-bit interface running at 33 MHz.


      SmartMedia is about one-third the area of a conventional PC Card and only 0.76mm in thickness. This new storage card is expected to help electronic devices - including the digital still camera and various forms of portable information equipment - become even smaller in size.SmartMedia is also known as SSFDC.


      CompactFlash is a very small removable mass storage device first introduced in 1994 by SanDisk Corporation. They provide complete PCMCIA-ATA functionality and compatibility pluse TrueIDE functionality compatible with ATA/ATAPI-4. At 43mm (1.7") x 36mm (1.4") x 3.3mm (0.13"), the device's thickness is less than one-half of a current PCMCIA Type II card.Compared to a 68-pin PCMCIA card, a CF card has 50 pins but still conforms to PCMCIA ATA specs. It can be easily slipped into a passive 68-pin Type II adaptercard that fully meets PCMCIA electrical and mechanical interface specifications. Conforming to PCMCIA ATA specification means that Compact Flash cardbehaves exacly like an IDE disk, so there is no need for any specialdrivers to use it (normal PC operating systems always support IDE drives).So it just looks like a small IDE hard disk, but internally uses flash memory as storage media.You can even buy wiring adapters allowing you to attach a CF to anormal, 40-pin flat ribbon cable connector like any IDE disk. Besides memory cards this card format is also used for some extension cards like modems. CompactFlash is widely used in digital cameras, PDA devices and in embedded PC systems. There are also intrfaces available to interface CompactFlash card to PCMCIA interface. Maximum data speed in CompactFlash specification 2.0 is 16 megabytes per second (new 3.0 version allowes up to 66 megabytes/s speeds).

      • CompactFlash Association    Rate this link
      • CompactFlash Specification Download    Rate this link
      • Information about CompactFlash - general introduction    Rate this link
      • Using CF/PCMCIA cards on the IDE interface - This document outlines how to use CF/PCMCIA storage cards on a IDE interface. This is achieved by using a PCB (design not included), a MALE CF or PCMCIA connector to connect to the memory card, a male 40 pin IDE connector (to connect to the hd-cable) and a male 4 pin HD-power connector. The convertor will only work with CF/PCMCIA hard-disk-like storage devices (e.g. memory cards), and does not support any of the fancier PCMCIA/CF features such as LIVE disconnection and reconnection, reading or writing to configuration locations etcetera. (*some*? or *all*?) CF memory cards have a pin that when connected to the proper voltage at power-up selects the "True-IDE" mode of operation instead of the "PC-CARD-ATA" mode of operation. This is the mode used in the interface.    Rate this link

      Memory Stick

      The Memory Stick digital data storage is designed to become a standard storage and transfer media. It is smaller than a stick of chewing gum. . It is available in 4MB, 8MB, 16MB, 32MB, 64MB, and 128MB storage sizes and as a kit with a PC card adapter. Due to its compact design, it is best suited for use in small digital electronics products. It is highly reliable with a 10-pin connector, and an Erasure Prevention Switch that when set on "Lock" virtually eliminates the risk of accidentally erasing or recording over stored data. The memory is accessed using a special serial protocol (designed to be compatible with future models also). Memory Stick works in most new DV & Mini DV camcorders and digital cameras. The maximum read rate of Memory Stick is 2.45MB/second and write rate is 1.5MB/second. There has been recently relesed a new MemoryStick version called "MemoryStick Pro". This new version is same size as original one. The new version is colored to have gold color is case. This Pro version promises more capacity (up to 1 gigabyte) and more speed (enough for DVD video). This Pro version is going to be available at summer 2003.


      MultiMediaCard is a new very small thin flash card, sized from 2MByte to 16MByte, used in some mobile phones, and HPCs. As it is new into the market, whether will become commodity is unknown. This is a Flash card with a 7 contact interface that can be both MMcard bus, and SPI bus. MultiMediaCard products are currently only made by Sandisk.

      Secure Digital (SD)

      The SD (Secure Digital) Memory Card is a non-volatilememory device about the size of a postage stamp. SD cards are solid-state devices that providequite large storage capacity (32 MB & 64 MB in 2000, with the promise of up to 128 MB and 256 MB).SD cards are designed to hhave great flexibility and some securityfunctions built in.Secure Digital was introduced by Toshiba at year 2000.It was co-developed with SanDisk and Toshiba.SD will facilitate fast, simple, secure downloading of all types of digital files, like music, movies, photos, news etc.It is used to store data on portable devices, such as MP3 players, digital cameras, handheld computers and cell phones. In addition to memory, Secure Digital can also allow devices to add technologies, such as Bluetooth wireless connectivity or global positioning system (GPS).Secure Digital includes software that complies with Secure Digital Music Initiative standards protecting against unauthorized replication of copyrighted content. As a result, digital-audio files downloaded from music services, such as Napster, cannot be played on devices that use Secure Digital cards. Secure digital (SD) is available in two physical sizes: Original SD size is 32 x 24 x 2,1 mm and new MiniSD size is 21,5 x 20 x 1,4 mm.


      ExpressCard 1.0 is a new card standard for portable computers. PCMCIA developed the ExpressCard standard to carry forward the benefits of 'plug-in' I/O cards to the next generation of personal computing devices. The standard was developed by a large number of PCMCIA member companies including technology leaders, system manufacturers, card manufacturers, and representatives from all other parts of the PC Card industry. The ExpressCard standard supports both the USB 2.0 and PCI Express interfaces. The cards are available in two sizes:

      • ExpressCard/54: 54 x 75 x 5 mm
      • ExpressCard/32: 32 x 75 x 5 mm
      All ExpressCard modules are 5mm thick but the standard also allows for card developers to build longer 'extended' modules. These can have thicker portions which project beyond the envelope of the host system. Both cards use the same 26 pin special connector, so the cards are compatible with each other (small one plugs to large card connector). The cards support USB2 or PCI Express interface. Card manufacturers are able to choose whichever bus is appropriate to their application. The portable computer must support both USB2 or PCI Express interfaces. Mobile and Desktop host systems can provide ExpressCard/34 slots when space is at a premium, or the wider Universal ExpressCard slot to accommodate both module sizes. The Universal ExpressCard slot has a novel guidance feature to ensure that the ExpressCard/34 modules are always correctly inserted. This new card standard is stadardized by PCMCIA and PCI SIG. It is supported by Dell, HP, IBM, Intel, SMSC, Qualcomm and TI. The card connetor manufacturers include FCI, Hirose, Honda, ITT Cannon, JAE, Molex, Tyco and Yamaichi. The first products on this card standard are expected to be available at beginning of 2004. The first cards are expected to be cards that use USB interface.


    ISA stands for Industry Standard Architecture. It has been the most common bus in the PC world.The Industry Standard Architecture, or ISA, bus originated in the early 1980s at an IBM development lab in Boca Raton, Florida. The original IBM Personal Computer introduced in 1981 included the 8-bit subset of the ISA bus. In 1984, IBM introduced the PC-AT which was the first full 16-bit implementation of the ISA bus. The "AT bus", as IBM originally called it, was first documented in an IBM publication called the PC-AT Technical Reference. The Technical Reference included schematics and BIOS listings that made it easy for other companies like Compaq to produce IBM compatible clones. The companies producing IBM compatibles could not use the "AT bus" name however since IBM had protected it with a trademark. In response, the industry coined "ISA" as a new name for the bus that was eventually adopted by everyone including IBM. The AT version of the bus is upwardly compatible, which means that cardsdesigned to work on an XT bus will work on an AT bus. The ISA bus is still a mainstay in many applications, despite the fact that it is largely unchanged since it was expanded to 16 bits in 1984! Major desktop PCs have started dropping ISA out, but you can stillsee ISA in many industrial computer systems (for example PC/104 systems). The 8-bit version of ISA came on the original PC and the extensions that came with IBM AT made it 16-bit. The bus implementations of the cards wereoriginally based on information publishes in IBM AT Technical Reference and the ISA industry standard were written much later.Although the PC-AT Technical Reference included detailed schematics and BIOS listings, it did not include the rigorous timings, rules, and other requirements that would make it a good bus specification. As a result, the various implementations of ISA were not always compatible with each other. Over time various ISA bus specifications were produced in an attempt to alleviate the compatibility problems. But unfortunately these specifications did not always agree with each other, so no single specification for the ISA bus was ever developed. Several documents that include specifications for the ISA bus are as follows: EISA Specification, Version 3.12, IEEE Draft Standard P996 and PS/2 Technical Reference - AT Bus Systems.ISA bus has a maximum data transfer rate of about 8 megabits per second on 16 bit bus-master mode. The bus works typically at clock rate of about 8 MHz. Many expansion cards, even modern ones, are still only 8-bit cards.ISA bus is a parallel bus where there are separate pins for the address and data lines. In addition to those there are axtra controllines like DMA control lines, interrupt lines (IRQ), I/O read/write,memory read/write and reset lines. There are also available two clock signals: ISA bus system clock (typically around 8 MHz) and high speed clock (fixed accurate 70 ns / 14.31818Mhz). The high speed clock frequency (14.318 MHz) is four times the NTSC television colorburst frequency. The high speed clock is used as theaccurate timing source for some ISA cards.All signals are at standard TTL levels.Connector is a 62-pin edge connector with a secondary 36-pin edge connector. All cards connected to ISA bus are phycally connected in parallel tothe ISA bus signal lines. The address and control lines are generally controlled by the CPU (or ISA controller) and the data lines are controlled by the data source (CPU or expansion card). There is also possibility for a bus mater mode, where expansion card can take full control of the bus. Interrupt lines are always controlled by the expansion cards. An interrupt request is generated when an IRQ line is raised from low to high. The line must be held high until the microprocessor acknowledges the request through its interrupt service routine. Interrupt lines are prioritized in the following sequence: Highest IRQ 9(2),10,11,12,14,3,4,5,6,7.ISA bus is normally used in such way that the CPU or DMA controller will control the data transfer on the bus. In this configuration the ISA cards works as slave devices in the bus communication. An ISA device may take control of the bus (this is called bus masteroperation), but this must be done with caution. There are no safety mechanisms involved, and so it is easily possible to crash the entire system by incorrectly taking control of the bus. The ISA bus is not ever designed to beauto-configurable, but the Plug and Play standard had tried to add those fuctions later. ISA seems to be dying in normal desktop environment but will propably be around for many years in industrial applications.


    EISA stands for Extended Industry Standard Architecture. Unlike ISA, here the name is not indicative of reality, for the EISA bus never became widely used and cannot by any stretch of the imagination be considered an industry standard.EISA began as Compaq's answer to IBM's MCA bus, and followed a similar path of development--with very similar results.An evolution of ISA and (theoretically) backward compatible with it. Increased data throughput is mainly due to the bus doubling in size-but you must use EISA expansion cards. Cards are configured using configuration software. EISA-based systems have today been mostly relegated to a specialty role; they are sometimes found in network fileservers. The EISA bus is virtually non-existent on desktop systems for several reasons. First, EISA-based systems tend to be much more expensive than other types of systems. Second, there are few EISA-based cards available. EISA is not totally dead as a platform.


    MCA had a great deal of potential. Unfortunately, IBM made two decisions that would doom MCA to utter failure in the marketplace. First, they made MCA incompatible with ISA; this means ISA cards will not work at all in an MCA system, one of the few categories of PCs for which this is true. Second, IBM decided to make the MCA bus proprietary. These two factors, combined with the increased cost of MCA systems, led to the demise of the MCA bus. With the PS/2 now discontinued, MCA is dead on the PC platform, though it is still used by IBM on some of its RISC 6000 UNIX servers


    PCI is currently far the most popular local I/O bus. Peripheral Component Interconnect (PCI) bus was developed by Intel and introduced in 1993. PCI is a mezzanine bus giving some independence of the CPU. PCI bus is time multiplexed, meaning that address and data lines share connections. PCI has its own burst mode that allows 1 address cycle to be followed by as many data cycles as system overheads allow. PCI bus can operate up to 33 MHz or 66 MHz (with PCI 2 specification). PCI is part of the plug and play standard so it allows auto configuring. The connector may vary according to the voltage the card uses (3.3 or 5v; some cards can cope with both). At 33MHz-32bit PCI bus theoretical speed is 33x4 = 132Mbytes/sec and at 66MHz-32bit PCI bus theoretical speed is 66x4 = 264Mbytes/sec. In practice the transmission speeds are lower because of overheads. In top of the grade components the practical maximum is 122Mbytes/sec on a 33MHz bus. But the realistic speed in a PIII PC is around 40Mbytes/sec for one device. It could be faster, but it depends on the design of the chipsets. In an embedded system on 33MHz, used in a real life application, around 100Mbyte/sec is a more realistic speed.


      Compact peripheral component interconnect (CPCI) is an adaptation of the peripheral component interconnect (PCI) specification for industrial computer applications requiring a smaller, more robust mechanical form factor than the one defined for the desktop. CompactPCI is an open standard supported by the PCI Industrial Computer Manufacturer?s Group (PICMG). CompactPCI is best suited for small, high-speed industrial computing applications where transfers occur between a number of high-speed cards. At the heart of CompactPCI is a gas tight, high density pin-and-socket connector which meets the IEC-1076 international standard (IEC 1076-4-101 family of 2 millimeter hard metric connectors). Its low inductance and controlled impedance make it ideal for PCI signaling. This 2 millimeter "Hard Metric" connector has 47 rows of 5 pins per row, with a total of 220 pins (15 pins are lost to the keying area). An additional external metal shield is also used. This connector's controlled impedance minimizes unwanted signal reflections and enables CompactPCI systems to have eight slots, as compared to the desktop PC's four. This can easily be expanded with PCI bridge chips. 3U CompactPCI processor boards use a single 220 pin connector for all power, ground, and all 32 and 64 bit PCI signals. Twenty pins are reserved for future use. 6U boards can have up to three additional connectors with a total of 315 pins. These are also 2mm style. These optional connectors can be used for a variety of purposes. They can be used as a bridge to other buses in hybrid backplanes. The optional connectors can also be used for rear panel I/O (IEEE 1101.11 draft standard for rear panel I/O provides a standard method for doing this, and works well with CompactPCI). The mechanical characterisitcs of CompactPCI systems are defined in IEEE 1101.1 (Eurocard) PCI form factor and IEC 60297-3-xxx standards (Eurocard equipment practice).

      Mini PCI

      Mobile systems have different requirements for expansion capability than those of desktops. Although mobile systems have the electrical equivalent of a desktop PCI bus, desktop PCI cards are too large and require too much power to be used on a portable computer. As a result, portable computer makers wishing to implement integrated devices such as network interface controllers (NICs) and modems must have earlier used proprietary form factors connected to the legacy Industry Standard Architecture (ISA) bus.


      PCI-X is a compatible extension of the existing PCI Bus to allow higher bus speeds. The 64-bit PCI-X architecture runs at speeds up to 133 MHz, providing burst transfer rates above 1gigabyte per second. The PCI-X Specification is an open industry standard and available from the PCI Special Interest Group (SIG) as an addendum to the PCI Local Bus 2.2 Specification. The history that lead to PCI-X specification tell it's story on the technolgy: PCI's developers introduced the specification more than 10 years ago as a high-performance alternative to the ISA bus for desktop computers. The initial bus frequency was 33 MHz with a 32-bit-wide parallel datapath, yielding a 133-Mbyte/sec data rate. In 1995, the PCI-SIG released Version 2.1 of the specification, which increased the bus frequency to 66 MHz and path widths to 64 bits and maintained backward compatibility with earlier hardware and software. In the late 1990s, the developers added PCI-X to the mix to initially boost clock frequencies to 133 MHz and then finally to as much as 533 MHz. Although every generation is interoperable, each device on the bus must operate at the speed of the slowest installed adapter. In addition, buses in some higher frequency PCI-X configurations have only a single slot.

      PCI Express

      PCI Express, the latest upgrade to the aging PCI bus, promises to pump new life into legacy PC components and millions of lines of software that run on it.PCI Express combines scalable, high-bandwidth datapaths, packetized data protocols, and compatibility with PCI hardware and driver software. The PCI Express specification is maintained by PCI-SIG (PCI Special Interest Group). The base specification defines PCI Express as having a transaction layer, a data-link layer, and a physical layer. Formerly known as 3GIO (Third Generation Input Output), PCI Express is based on LVDS (low-voltage differential signaling) for maximum bandwidth between nodes. The basic PCI Express link comprises two signal paths that use small differential voltage swings and constant-current line drivers to communicate at 2.5 Gbps in each direction (data rate is expected to rise up to 10 Gpbs in the future). The standard way increase the bandwidth of a PCI Express link to over 2.5 Gbps today is by simply adding signal pairs, or "lanes," until you reach the desired performance level. The PCI Express specification supports ?1-, ?2-, ?4-, ?8-, ?16-, and ?32-lane widths. The basic 2.5-Gbps direction-transfer rate plus encoding overhead results in a single-lane performance of approximately 200 Mbytes/sec. A typical PCI Express system comprises a CPU, a host bridge, memory, and multiple I/O devices. The switch replaces the conventional shared bus and provides communications channels between the CPU/memory and individual I/O-device endpoints. On the transmitting side, the transaction and data-link layers divide information into packets and append addressing, error-checking, and sequence information to ensure proper transfer through the interconnecting datapath. The physical layer includes parallel-to-serial conversion, path drivers, and impedance-matching circuitry. If an interconnection link has more than one lane width, the physical layer is responsible for distributing data evenly across all paths. On the receiving side, the three layers decompose packets to match the original data. The basic premise of PCI Express is that the host PCI software remains compatible with and can talk to and receive data from an endpoint device without new drivers or operating-system software. During initialization, the operating system must be able to discover all of the endpoint devices and their characteristics with the same protocol a shared-bus system uses. During runtime, the PCI Express layers must automatically packetize the information, send it over the serial link, and reconstruct it on the other end without the need to modify the original software or the endpoint-adapter hardware. This transparency allows designers to upgrade high-data-rate sections of their systems for improved performance and retain other low-cost but adequate I/O adapters. You can mechanically configure PCI Express hardware into form factors that look similar to those of PCI I/O adapters. A general-purpose computer motherboard could have standard PCI slots alongside PCI Express slots. The PCI Express slots has 36 pins for a ?1 lane width and an increase in the number of pins, depending on the lane width. The PCI-SIG is working on a PCI Express Mini Card specification to produce a successor to the conventional Mini PCI for wired- and wireless-communication peripherals for notebook and mobile computers. Because PCI Express is in the early stages of development, its designers are interested in off-the-shelf test and analysis equipment.

    Accelerated Graphics Port (AGP)

    The Accelerated Graphics Port (AGP) is a high performance, component level interconnect targeted at 3D graphical display applications. AGP is a high-performance interconnect between the core-logic chipset and the graphics controller for enhanced graphics performance for 3D applications. AGP has 32 lines for multiplexed address and data. There are an additional 8 lines for sideband addressing. AGP relieves the graphics bottleneck by adding a dedicated high-speed interface directly between the motherboard chipset and the graphics controller. This removes bandwidth-intensive 3D and video traffic from the constraints of the PCI bus and allows the ability to use more textures than can typically fit into local memory for high detailed images and texture maps. Segments of system memory can be dynamically reserved by the OS for use by the graphics controller. This memory is termed AGP memory or non-local video memory. The net result is that the graphics controller is required to keep fewer texture maps in local memory.


    The Video Electronics Standards Association (VESA) formed a committee in June 1992 to study existing and certainly emerging, local bus video systems in terms of connector layout, signal and data structures. In September 1992 they finalised the VESA Local Bus standard. The local bus is one more directly suited to the CPU; it's next door (hence local), has the same bandwidth and runs at the same speed. This VLB system was very popular for connecting high performance graphics cards to PCs that used 486 processor.?The VLB is a 32-bit bus which is in a way a direct extension of the 486 processor/memory bus. A VLB slot is a 16-bit ISA slot with third and fourth slot connectors added on the end. The VLB normally runs at 33 MHz, although higher speeds are possible on some systems. While VLB was extremely popular during the reign of the 486, with the introduction of the Pentium and its PCI local bus in 1994, wholesale abandonment of the VLB began in earnest. When Pentium processors and PCI bus became more popular, the use of VLB has practically stopped, because design was strongly based on the 486 processor and tricky electrically.Today VLB is obsolete for new systems.

    PC control bus systems

    Here are links to information on bus systems used for controlling and management of PC hardware (in some PC systems). Most of the bus systems here are some accessory and control busses used for low-speed control applications.

    • SMBus - SMBus is the System Management Bus defined by Intel? Corporation and Duracell in 1995. It is used in mobile and desktop personal computers for low-speed communication in the system. The System Management Bus is a two-wire interface through which simple power-related chips can communicate with rest of the system. It uses I2C as its backbone. SMBus allows the simple connection of a large array of smart batteries and other devices, like EEPROMs, temperature sensors, digital potentiometers, etc.    Rate this link

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