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PC Bus Architecture


General information

Plug and Play

Card interfaces


Card standard which started from simple memory cards.

PC Card

PC Card is a new name for technology which evolved from PCMCIA to provide more advanced card functionality 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.


To address the limitations of conventional PC Cards, in 1995 the PCMCIA announced a new interface called CardBus, which is a 32-bit interface running at 33 MHz. The 32-bit, 33Mhz bus that allows blazing speed in applications from 100Mbit EtherNet to video conferencing. CardBus architechture implements PCI bus functionality to small PC Card size cards.

Miniature card

The newly adopted 60-pin memory card standard. Miniature Card was designed for low cost consumer needs. It is architected for a Linear Flash interface.


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 plus 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 adapter card that fully meets PCMCIA electrical and mechanical interface specifications. Conforming to PCMCIA ATA specification means that Compact Flash card behaves exacly like an IDE disk, so there is no need for any special drivers 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 a normal, 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.

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.


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-volatile memory device about the size of a postage stamp. SD cards are solid-state devices that provide quite 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 security functions 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.


ISA stands for Industry Standard Architecture. It has been the most common bus in the PC world. 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 still see ISA in many industrial computer 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 were originally based on information publishes in IBM AT Technical Reference and the ISA industry standard were written much later. 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.

The bus is not ever designed to be auto-configurable, but the Plug and Play standard had tried to add those functions later. ISA seems to be dying in normal desktop environment but will probably 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.


Micro Channel Architecture. proprietary standard established by IBM in 1987 to take over from ISA, and therefore incompatible with anything else. It comes in two versions, 16- and 32-bit and, in practical terms, is capable of transferring around 20 MBps. MCA is very well shielded which makes it immune to electrical noise. MCA is designed to eliminate the hassle associated with setting jumpers and DIP switches on adapter boards.

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.

General information


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. 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).

    • CompactPCI - PCI bus superset for industry computers
    • Compact PCI BUS Card Size
    • CompactPCI Short Form Specifications
    • CompactPCI Tutorial
    • PICMG - PICMG (PCI Industrial Computer Manufacturers Group) is a consortium of over 700 companies who collaboratively develop specifications that adapt PCI technology for use in industrial and telecommunications applications. PICMG specifications include CompactPCI® for Eurocard, rackmount applications and PCI/ISA for passive backplane, standard format cards.
    • PICMG Europe - The European PCI Industrial Computer Manufacturers Group (PICMG Europe) is an industry-independent, neutral, non-profit organisation of board-level manufacturers and end-users having common market interests. PICMG develops common specifications for non-office environments. PICMG Europe promotes the usage and supply of products based on the technical specifications as defined by the head office of PICMG in the USA, such as CompactPCI.

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.

Mini PCI (Peripheral Component Interconnect) expansion card technology provides a new industry standard for integrating communications devices into small form factor products, such as notebooks. 3Com, AMP, Compaq, Dell, Gateway, Hitachi, IBM, Micron, Mobility Electronics, and Toshiba founded a roundtable to put together a standard for a new notebook form factor. Their results were then turned over to the PCI Special Interest Group (PCI SIG) for evaluation and ratification. Mini PCI, the new form factor for communications peripherals in notebooks, offers several benefits over existing custom embedded solutions and PC cards.

PCI bus hardware development information

Extensions to PCI bus

    • 64-Bit PCI - In it's current implementation, the 32-bit PCI bus transfers data at up to 133MB/sec, less bus overhead. What will result from changing the bus to a 64-bit design running at 66MHz? How will these new 64-bit data transfers affect PCI bus cycles? What about the addition of 64-bit PCI bus extensions?
    • Electrical-design considerations for PCI-X and 66-MHz PCI cards - As PCI-X and conventional, 66-MHz PCI devices and systems increasingly appear in a range of industry-standard computer architectures, system designers confront new challenges in developing cost-effective products that comply with the PCI-electrical specifications.
    • PCI Hot-Plug Technology

PC/104 Bus

PC/104 is a small PC motherboard form factor used for industrial and embedded PC applications. PC/104 computer consists of motherboard and extension cards stacked on top of it to PC/104 bus. This bus is electrically same as ISA bus, but uses different connector.

Accelerated Graphics Port (AGP)

The accelerated graphics port (AGP) is an extension of the PCI bus. Intel developed AGP to addresses the limitations of PCI for handling large amounts of 3D graphics data. AGP is, in very simple terms, a direct connection between the graphics subsystem and system memory. It provides much higher data transfer rates than PCI, and was expressly designed to meet the demands of real-time 3D.


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.

  • I2C and SMBus framework - Information on I2C and SMBus.
  • Semtech SMBus information - Developed jointly by Intel and Duracell, the system management bus (SMBus) acts as the control bus for Smart Batteries. Through the SMBus, end-users can be apprised of present state and calculated battery info and command a battery recharge through a simple software interface.
  • 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.
  • SMBUS Project - Source code Visual Basic and Delphi for read SMBUS and PC sensors. Text in this page is in Italian.


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