Frequently Asked Questions

What is the CompactPCI bus?
CompactPCI is a very high performance industrial bus based on the standard PCI electrical specification in rugged 3U or 6U Eurocard packaging. Unlike its desktop cousin, the CompactPCI board uses a high quality 2 mm metric pin and socket connector that meets IEC and Bellcore standards. CompactPCI boards are inserted from the front of the chassis, and I/O can break out either to the front or through the rear.
Where does the CompactPCI specification come from?
The CompactPCI effort was initiated in 1994 by Ziatech Corporation under the auspices of the PCI Industrial Computer Manufacturer's group (PICMG). The CompactPCI specification is the result of a concerted effort of the CompactPCI subcommittee composed of the following companies: Digital Equipment, GESPAC, I-Bus, Pro-Log, Teknor, Hybricon, and Ziatech.
What is the PICMG?
The PCI Industrial Computers Manufacturer's Group is a consortium of more than 350 industrial computer product vendors. PICMG's charter is to develop specifications for PCI-based systems and boards for use in industrial computing applications. Member companies include industry leaders such as IBM, Motorola, Texas Microsystems, HP, Compaq, Industrial Computer Source, DEC, Force Computers, GESPAC, Pro-Log, Teknor and Ziatech. Membership in PICMG is open to any organization or individual with a legitimate interest in helping to extend the PCI standard in the industrial marketplace. The PICMG can be contacted at (781) 224-1100.
What other specifications has the PICMG issued?
The first effort of the PICMG was to publish a specification for passive backplane computers using both PC style (card edge connector) ISA and PCI bus. This was completed in early 1995, and is generally known as the "PCI-ISA Passive Backplane" specification. The PICMG has also issued a specification for PCI to PCI bridges which allow passive backplane computers to extend the number of PCI slots. Both of these specifications are explained elsewhere on this web site. The CompactPCI project began in 1994, with the first formal specification approved in November of 1995.
What applications are targeted by CompactPCI?
CompactPCI is intended as an industrial bus for application in telecommunications, computer telephony, real-time machine control, industrial automation, real-time data acquisition, instrumentation, military systems or any other application requiring high speed computing, modular and robust packaging design, and long term manufacturer's support. Because of its extremely high bandwidth, the CompactPCI bus is particularly well suited for many high speed data communication applications such as servers, routers, converters and switches. Although as yet incomplete, a hot swap feature has been planned for in the CompactPCI specification which will be particularly well suited for the telecommunication industry.
What are the unique features and benefits of CompactPCI?
Compared to standard desktop PCI, CompactPCI supports twice as many PCI slots (8 versus 4) and offers a packaging scheme that is much better suited for use in industrial applications. For example, Compact PCI cards are designed for front loading and removal from a card cage. The cards are firmly held in position by their connector, card guides on both sides, and a face plate which solidly screws into the card cage. Cards are mounted vertically allowing for natural or forced air convection for cooling. Finally, the pin-and-socket connector of the CompactPCI card is significantly more reliable and has better shock and vibration characteristics than the card edge connector of the standard PCI cards. The power and signal pins on the CompactPCI connector are staged so as to allow the specification in the future to support hot swapping, a feature that is very important for fault tolerant systems and which is not possible on standard PCI. Also, 6U CompactPCI supports 3 additional 2mm connectors with a total of 315 pins. These can be used for secondary buses (like SCSA or MVIP telephony buses), bridges to other buses like VME or SCSI, or for user I/O. User I/O can be routed out the back of a 6U card and out the back of the chassis, a practice popular in the telecommunications industry.
Can I have more than 8 slots in a CompactPCI system?
Yes. Each CompactPCI bus is limited to eight slots for electrical loading reasons. This can be easily expanded with PCI-PCI bridge chips, available from a number of manufacturers. The bridge chip acts as a sort of "super buffer" chip. Interrupts, plug-and-play information, and data are easily and generally automatically transferred across the bridge. A bridge chip usually exacts a one clock penalty ( generally about 30 nanoseconds) per transaction. If the data transaction is a burst mode type - transferring hundreds or thousands of bytes at a time - this overhead is extremely small. One advantage of bridge chips is that each side of the bridge can be performing data transfers to cards on its side of the bridge simultaneously.
What processors can be implemented on CompactPCI?
Although PCI has gained most of its recognition as a local bus for 80x86 based PCs, PCI is at the core of all modern microprocessor designs. PowerPC and DEC's Alpha, for example, are supported with chip sets with PCI interfaces and can be easily implemented on CompactPCI. In fact, CompactPCI is the industrial bus that does the most justice to these very high performance new chips, giving them a system bus with all the bandwidth that these chips are capable of.
Are CompactPCI products prone to early obsolescence?
No. Unlike the desktop PC market which is driven by volume and fast changing consumer demand, CompactPCI is driven by professional customers who value product stability and long term availability. All major CompactPCI manufacturers have at least 10 years of experience each serving the OEM marketplace and have established reputations for protecting their customers from the dangers of early obsolescence. This is achieved by a careful selection of components and their suppliers, and even, in many cases, by stocking several years worth of demand of key components.
What are the software implications of PCI and CompactPCI?
The PCI architecture, developed by Intel, has been carefully planned to simplify the software integration of a peripheral device. For example, all PCI or CompactPCI device have a set of 256 registers which contain information on the device identity, as well as a great deal of software programmable parameters such as address maps, or interrupt types and levels. As a result, the system CPU can automatically detect and identify a device on the bus and configure it without the need for jumpers on the peripheral. PCI is a key element of the "Plug and Play" concept. CompactPCI is truly a "systems level" bus, with configuration (plug and play) and hardware abstraction layers. This permits a high level of software portability, common in the desktop PC world but much more rare in embedded systems.
What are the system implications of CompactPCI?
Every modern computer architecture has an internal PCI bus, whether or not it supports PCI add-on slots (which it usually does). This is the case for nearly all Pentium PCs, Alpha workstations, and PowerPC systems based on the PREP or CHRP reference platform standard. CompactPCI makes it possible to build any computer compliant with these hardware system designs. As a result, CompactPCI systems can be built using standard components and can run practically any operating system and thousands of application software packages without modification.
Which Operating Systems can CompactPCI computers run?
Pentium based CompactPCI computers can run all operating systems that have ever been ported to the PC, including MS-DOS, Windows 3.11, Windows 95, Windows NT, VxWorks, OS/2, SCO and BSD UNIX, LINUX, Novell Netware, IntranetWare, OS-9000 and QNX. PowerPC based CompactPCI computers will be able to run AIX, SOLARIS, Windows NT, Mac OS and OS-9.
Will CompactPCI system run real-time operating systems well?
The performance of CompactPCI is particularly well tuned to real-time application, from machine control to machine vision, fast data acquisition and data acquisition. Operating systems like OS-9, PSOS, VxWorks run very well on CompactPCI hardware and will be instrumental in the expected success of CompactPCI in these markets.
What products are available on CompactPCI?
CompactPCI Pentium and PentiumPro systems are available from a variety of suppliers in a variety of packages. System level add-on function such as 100 Mbit/s Ethernet, fast SCSI, accelerated VGA, image acquisition, and analog and digital I/O, ATM, FireWire and FibreChannel communications, motion control, and PMC and IndustryPack carriers are currently available. Other companies have announced plans for PowerPC CPU boards, fast fiber optics networks, image acquisition and processing, and a host of other I/O.
What other functions could benefit from CompactPCI?
CompactPCI can benefit all applications requiring very high data transfer rates. Data communication interfaces such as ATM and broadband ISDN are good examples. In the field of high energy physics research, very fast multi-channel data acquisition cards will benefit from CompactPCI. Many of the most exciting applications are probably yet to be invented, but if history is any indication, the sophistication of systems will increase to use all available computing bandwidth that CompactPCI computers have to offer.
What about Hot Swap?
The staged (multi-length) pins in the CompactPCI connector cause some connections to be made before others when inserting a card. The reverse occurs when a card is removed. This, in principle, allows CompactPCI products to be hot swapped. This is a complex issue, however, and significant efforts are underway by a number of PICMG member companies to develop a quality solution. There are some significant obstacles to be overcome. First, special circuitry must be developed so that a board can be inserted and removed from a live, operating PCI bus. Second, DC power to boards being hot swapped must generally be ramped up and down to avoid "glitching" the system's DC bus. Thirdly, applications software and operating systems must be developed that recognize when a board is removed and another inserted. This will be required to re-initialize complex I/O chips like graphics adaptors or network interfaces. Despite these obstacles, however, hot swap is a very desirable feature and a great deal of energy is being devoted to developing viable techniques.
Are CompactPCI products expensive?
No. Even though they offer unprecedented performance, they are based on broadly available silicon implementation produced in very high volume. This high volume silicon has the highest performance/price ratio available because of the economies of scale. Additionally, because the PCI bus is not terminated, no external bus drivers are required to interface a PCI peripheral. An Ethernet controller, for example, connects directly from the leads of the controller chip on to the PCI connector. CompactPCI products are typically priced below equivalent VME product but slightly above desktop PCI products.
Where can I learn more about PCI technology in general?
There is a great deal of information on the web published by the PCI SIG, which is the organization responsible for desktop PCI standards. You can learn more about the SIG's activities by contacting
What future developments are likely for CompactPCI?
As of April 1996, several technical subcommittees have been formed to expand the definition of the bus. They include:
Also, a "Pin Registry" is being developed to detail recommended practices for using the upper three connectors on 6U boards for I/O. For example, PICMG has developed recommended practiced for IndustryPack and PMC pinouts on the additional connectors.
How can I get a copy of the CompactPCI Specification?
You can get a copy by calling the PICMG at 781-246-9318. PICMG members receive free copies of the specifications and updates. A short form is available on this web site. You can get a membership application by calling the same number.
Special Thanks to Cosma Pabouctsidis, GESPAC Corp., for these FAQs Updated by Joe Pavlat, December, 1996
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