Computer bus information page

General Information

This pages describes bus technologies used in various computer based systems.


VMEbus is an industry-standard widely accepted 16/32-bit backplane interconnection bus. It was originally developed by Motorola Inc. but is now standardized by IEEE 1014 and IEC 821. VMEbus is used in a wide variety of applications like industrial control, military, aerospace, transportation, telecom and medical. VMEbus has enjoyed the largest market share (by sales) than any other microcomputer bus type (including ISA, PCI, cPCI and others) in 1998.

VMEbus is quite fast, especially in it's most recent manifestations, and this makes it fastest among the 'big commercial backplane buses' (more than 8 slots).VMEbus can be usd in both single master and multi master systems (up to 21 masters on one multimaster bus). VMEbus systems withstand shock, vibration, and extended temperatures better than the buses used in desktop computers, making them ideal for harsh environments.

The original VMEbus idea came from Motorola VERSAbus, which it had introduced for MC 68000 processor. At 1981 Mostek, Motorola and Philips/Signetics anounnced a common 16/32 bit bus, which became later known as VMEbus. The card format as well as the connectors used in VMEbus, conform to DIN 41494 ("Eurocard") and DIN 41612 ("Eurocard Connector"). The name VMEbus comes from "VERSA Module Europe bus".

The original VMEbus specification (IEEE-1014-1987) has a robust set of features: MASTER/SLAVE architecture, asyncronous bus, non-muliplexed bus, variable speed handshaking protocol, addressing range between 16 and 32-bits, data path widths of between 8 and 32-bits, bandwidths up to 40 Mbyte/second, multiprocessing capability and interrupt capability (7 levels with 8, 16 or 32-bit STATUS/ID interrupt vector). The VMEbus is made up of several sub buses: address, data, arbitration, priority interrupt and utility bus. Addressing lines are driven by a bus master and monitored by a bus slave. The data transfer bus consists of data lines and control lines. Data transfer bus id driven by master or slave depending on the needed operation (read or write). Some data transfer bus lines are used for more than one purpose. The data transfer arbitration bus is used by a bus master to request permission from the System Controller to use the bus. The priority interrupt bus consists of interrupt request lines and interrupt acknowledge lines. The utility bus contains system monitoring lines.

VMEbus uses bus type backplane where the main bus card connectors are are connected in parallel to the backplane bus. This bus architecture is used by both data and address lines. In addition to bus lines there are several daisy-chained lines. Bus terminations are used on all VMEbus signal lines except for the 'Daisy-Chained' lines. The terminations are used at each end of the VMEbus. The standard termination on 5V systems is is 330 ohms from signal line to +5V and 470 ohms from signal line to ground (this equals to 2.94 volts +/- 10% termination voltage with 194 ohm +/-5% thevenin resistance). The system uperating from 3.3V use 220 ohms from signal line to +3.3volts and 1.8K ohms from signal line to ground (the termination characteristics practically same as in 5V system). The bus itself uses standard TTL voltage levels (0 = 0..0.4V and 1 = 2.4-5V).

The VMEbus backplane may have between 1 and 21 slots. A VMEbus backplane can be up to 500 mm (19.68") in length. The backplanes built according VME / IEEE specifications are designed to have controlled impedance stripline design with 55-60 ohm effective impedance. A typical modern backplane uses 8 or more circuit board layers.

VME backplanes will always consist of a J1 backplane. The J1/P1 connector may be either a 3-row (x32) 96-pin or a 5-row (x32) 160-pin connector. The original VMEbus system was designed to use 3-row 96-pin connector. This J1/P1 connector has 24 address lines and 16 but data bus. VITA 1.1-1997, termed VME64x, is now using the 5 row (160 pin) connector, providing more ground/power pins needed for fast bus speeds. The 5 row connectors are only required for the 64x enhancements. The boards with old 3-row connector can bplug to new 5-row backplane connector. The VME backplane wiring is defined in the following way: Except for the daisy-chained lines all signals on rows A, B, and C of J1 must be bused.

A VME backplane may also have a J2/P2 backplane in addition to J1. The J2/P2 backplane may be a separate backplane circuit board located just below the J1 backplane or be a monolithic backplane containing both J1 and J2. All signals on row B of J2 must be bused. The use of P1 and P2 connectors allows used of 32-bit addresses and 32-bit data bus. The connector types for J2 connector are the same as used for J1 connector.

VMEbus system can use a wide variety of mechanical hardware based on the IEEE 1101 standard. The mechanical height of the rack sizes 3U and 6U (9U optional). The 3U racks use 132.5 mm slot height while 6U system uses 265.9 mm slots. The chassis size and accepted Eurocards defines the type of backplane and its connectors. A Eurocard is a European designed circuit board that uses a 96-pin plug. There are three sizes: 3U which is 4 x 6 inches, 6U which is 6 x 12 inches, and 9U which is 14 x 18 inches. 3U cards support 8- and 16-bit data paths and 6U cards support 32-bit data paths.

VME32 backplanes are typically installed in Low Profile 3U chassis. VME64 backplanes are installed in High Profile 6U chassis and offer extra I/O capacity. Standard 3U Eurocard/VME size fir the card circuit board 100 x 160 x 1.6mm (3.93" x 6.30" x 0.062"). Standard 6U cards have size of 233 x 160 mm x 1.6 mm (9.19" x 6.30" x 0.062"). There are also larger card sizes defined for special applications, for example ANSI/VITA 1.3-1997 defined VME64x 9U x 400mm oards, backplanes and subracks.

The VMEbus system is a master/slave type system shared by master and slave cards. Because the VMEbus is a shared resource, however, a mechanism must exist to allow boards to arbitrate for and be granted the bus. This mechanism is called arbitration and is carried out by the VMEbus system controller. VMEbus Controller controls access to the bus and takes care of the interrupt handling. Only one Controller may reside on the VME bus. The VMEbus Master Reads and Writes data to or from a Slave board. A VMEbus Slave interface simply monitors the Address and Data bus for Reads or Writes sent to it. When master needs to access the bus, it sends "Bus Request" to the bus controller and wait to get "Bus Grant" from the bus controller before attempting to control the bus lines. The arbitrator on the system controller can be set up in one of three arbitration modes: priority (PRI), roundrobin (RR), or single level (SGL). The arbitration mode selected is part of the system initialization process and is static during the life of the system. The choice between priority mode and round robin mode is application dependent and is left to the discretion of the system designer. PRI mode is usually best for a system that must service events in a hierarchical fashion.

Transfer mechanism is asynchronous, with both multiplexed and non-multiplexed bus cycles. There is no central synchronization clock. The bus supports 16, 24, 32, 40 or 64-bit addressing range and 8, 16, 24, 32 or 64-bit data widths. Both address and data width are selected dynamically.

There are two main types of Data transfers on the bus; single cycle, or block transfer. A single cycle consists of the Master performing an Address cycle followed by a single Data transfer cycle that transfer one data byte/word. A block transfer cycle consists of the Master performing a single Address cycle followed by up to 256 data cycles (allows transferring up to 256 bytes of data per transfer cycle). Block transfer reduces bu overhead when large amounts of data is transferred though VMEbus.

The Controller also handles Interrupts on the bus in the following way: When an Interrupt is received on one of the IRQ lines the Controller will process that Interrupt by accessing the Interrupting card, and acknowledge the Interrupt.

VME versions:

  • IEEE 1014-1987: Original specification which defined two 3 row P1/P2 connectors, providing a 32 bit Data transfer rate of 40MBytes/second.
  • IEEE 1014 Revision C: This revision added 64 bit block Data transfers, with the upper 32 data bits being multiplexed onto the address bus once the address cycle was complete
  • ANSI/VITA 1-1994: This revision is often called VME64. The data bus width is increased to to 64 bits providing transfer speed of 80MBps. VME64 is a mechanical and electrical 'superset' of the original IEEE 1014-1987 standard. All of the enhancements under VME64 are optional and new products work in conjunction with older 'legacy' boards.
  • VITA 1.1-1997: This revision is termed VME64x. VME64x uses the 5 row (160 pin) connector, providing more ground/power pins (3.3 V power supply pins and more 5V pins); an increase in speed of up to 160Mbps is achieved. All of the enhancements under VME64x are optional and new products work in conjunction with older 'legacy' boards. VME64x supports live insertion / hot-swap capability.
  • VME320: VME320 a modified VMEbus architecture that increases the bus speed to 320 MBps on the back-plane with 3-row connectors. This system is patented by Arizona Digital, and licensed by Bustronic Corp. The VME320 backplane uses a 'star' interconnection method to speed up the VMEbus backplane itself. All of the interconnections on the backplane are connected together at the middle slot of the backplane. The VME320 concept uses a new bus protocol called 2eSST.

NOTE: All VMEbus modules that conform to the IEEE-1014-1987 are now considered to be VME64 compliant (regardless of their data transfer capability).

NOTE2: There are a number of other specifications which increase the data through-put even higher by using the "user defined" P2 pins, the P0 connector or by bringing the data out the front panel.

The VMEbus standards themselves have little to say about software. However, there have been an increased emphasis on software standards within the VMEbus community. VMEbus has the largest software base of any computer architecture. That's quite a claim, but it is supported by the fact that there are over 103 known, commercial operating systems running on VMEbus.


VXIbus is the most popular open standard platform for commercial ATE (Automated Test Equipments) systems in the world today. The VXIbus architecture has become the de-facto standard for ATE Systems around the world. VXI stands for VME eXtensions for Instrumentation. It is an open standard first developed by five leading Test and Measurement companies in 1987 to standardize a backplane capable of developing open, interchangeable instrument modules that could be used to build Automatic Test Systems (ATS). The standard has been ratified by the IEEE and has grown to be a significant part of the Test and Measurement market with over eighty companies producing VXI products.

  • VXIbus - The VXIbus Consortium is a not-for-profit corporation made up of the leading companies in the Test & Measurement industry. It's goals are to develop, support, and promote the VXIbus.    Rate this link


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.

The CompactPCI connector is a high-density pin-and-socket connector that meets the IEC-1076 international standard for the PCI interconnect between add-in boards and the backplane. Backplanes use male (pin) connectors and plug-in boards use female (socket) connectors. Standardization guarantees the connectors are compatible regardless of manufacturer. Also stipulated in the IEC 1076-4-101 standard is coding. Coding is necessary to insure proper placement of plug-in modules. For I/O's out of the electronic units, shrouding is used in many cases. In addition to providing power, ground, 32 and 64-bit PCI signals, these 2 mm Hard Metric connectors can be used as bridges to other buses such as VME or ISA in hybrid backplanes. The connectors can also be used for rear panel I/O.

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).


PXI stands for CompactPCI eXtensions for Instruments and was formed in 1997-1998. PXI combines the high-speed PCI bus with integrated timing and triggering designed specifically for measurement and automation applications. PXI is built on the modular and scalable CompactPCI specification and the high-speed PCI bus architecture. The PXI specification defines how PC technology can successfully be applied to measurement and automation. By leveraging off of CompactPCI, Microsoft Windows, and VXI, the PXI specification brings together different technologies for PC-based test and measurement, instrumentation, and industrial automation.


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.


Industry Standard Architecture. The 8-bit version 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. The bus is not ever designed to be auto-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 PC applications which use passive bus cards and/or PC/104 components.

Mezzanine technologies

Today there is a clear tendency to buy hardware instead of making it. There is no best mezzanine technology, the different technologies are complementary. They target different applications. Mezzanine technology may help in solving this problem. First decide on the processor to be used, then find a mezzanine carrier board with or without system bus interface and finally, personalise the board by adding one or more mezzanines. By buying the carrier and the mezzanine boards or by making some mezzanine boards yourself you will be quicker in the market with a new design and that is vitally important today.

CompactPCI (cPCI) and VMEbus (VME) manufactures producing a comprehensive range of products based on the 680x0, and x86 (Pentium and K6) families of CPUs. In addition there is extensive support for IndustryPack and PMC mezzanine modules and PMC carriers.


    PMC is short for PCI Mezzanine Card. It is a postcard size small module which can be plugged to industrial PC motherboard PMC interface connector or to external carrier card. PMC card carry PCI bus signals in tiny connectors.

    If you need high performance and/or intelligent I/O then PMC-modules are a good choise. PMC modules use PCI components which are becomign incresingly popular. If you need high performance and/or intelligent I/O then PMC-modules are a good choise.

    PC MIP

    The PC MIP module bus to the carrier board is largely based on the PCI standard. The electrical and logical layers are the same as those defined by the PCI standard. The PC MIP standard itself is still under development by the ANSI/VITA task group VITA-29.

    Th PC MIP exists in two widths: single and double. The single form factor is 47 x 90 mm (42.3 cm2), and the double is 94 x 90 mm (84.6 cm2). It comes in two flavours: Type I (without front bezel I/O) and Type II (which is 9 mm longer and with front bezel I/O). The module can have components on both sides, and the manufacturer may choose on which side to put the highest components. The I/O connector to the carrier board is a 50 pin one.

    PC MIP tries to combine best of the two worlds: it has almost the same surface as the IP-module and has high performance PCI bus in it. PC MIP consolidates the presently used PCI. PC MIP is more standardised than PMC, and has a smaller surface (lower production cost) it will become a major competitor with PMC.


    The M-Module Specification was approved on May 20, 1997 as an American National Standard and will be known as ANSI/VITA 12-1996, M-Module Specification. M-Modules exist in three sizes: single, double and even triple size. The single size measures 148.3 x 52.9 mm (78.5 cm2), the double 148.3 x 106.2 mm (157.5 cm2), and the triple 148.3 x 159.6 mm (236.5 cm2). Components on M-Modules can have a maximum height of 10.5 mm (for about 60% of surface) and 5.25 mm (40%). The single size M-Modules uses a 40-pin (optional 60-pin: for MA-Modules) connector for interfacing with the carrier board, and a 24 pin I/O connector (I/O via the carrier board). M-Modules may also have front bezel I/O.

    The M-Module bus simple so M-module development is also easy and cheap. The PCB surface, although larger than the IP, is still small. An advantage over IP modules, is the larger potential component surface and height. There exist an option for interconnect bus between different M-modules. M-Modules are well suited for non-intelligent I/O. Nevertheless, M-Modules are less standardised and less available than IP modules.

    IP (IndustryPack)

    IP is the abbreviation of "Industry Pack". The standard was prepared by VSO (VITA Standards Organisation) and SBS GreenSpring Modular I/O. The single size IP-module uses two 50-pin connectors: one for interfacing with the carrier board and one to interface with the external world (I/O via the carrier board). The single size module measures 1.8 x 3.9 inch (45.3 cm2), the double 3.6 x 3.9 inch (90.6 cm2). Components on an IP-module can have a maximum height of 0.29 inch (7.4 mm). IP modules can be connected to VME or ISA bus using special IP module carrier cards.

    IP module bus is simple, so it is easy and cheap to develop an IP module. As the PCB surface is small, also the production costs are limited. IP modules have a low performance interface, and as IP has only slave possibilities it is intended to be used more as a simple I/O extension. IP-module is well suited for a large number of non-intelligent I/O applications, which do not require high performance. They are cheap and widely available.

Other stackable cards systems

    PC/104 Bus

    PC/104 is a small PC motherboard form factor used for industrial and embedded PC applications. PC/104 conmputer consists of motherboard and extension cards stacked on top of it to PC/104 bus.

    The PC/104 bus is an adaptation of the ISA bus for embedded computing use. It uses the same signals as ISA, but uses a smaller connector and cards that are stackable, which eliminates the need for a backplane. The name PC/104 comes from the fact that the bus was invented for the PC, and has 104 pins. This means that PC/104 bus is electrically same as ISA bus, but just uses different connector.


    PC/104 is an embedded PC hardware system where the whole computer system is built by stacking postcard size PC motherboard and interface cards on top of each other. Original PC/104 standard supports ISA bus architechture for the cards. Newer PC/104+ standard has added PCI bus support for newer PC/104+ systems.

    Mini PCI

    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.

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