News | February 9, 1999

I/O Cabling In Compact PCI Systems

According to the specification, there are four ways to do it.

by Steve Cooper, One Stop Systems


•I/O Cabling Options
•Compatibility of Off-the-Shelf Products

As designers begin to design systems based on the Compact PCI architecture, they quickly run into one of the most intriguing, yet sometimes confusing, aspects of the bus structure - how to route their I/O cabling.

The intrigue and confusion both occur because the Compact PCI specification defines several methods to route I/O. Having extra flexibility in terms of I/O cabling is like a double-edged sword. It enables some very nice design capabilities, but also requires that the designer understand the alternatives and the system design implications of each.

I/O Cabling Options

There are four basic ways to cable out I/O in a Compact PCI system:

  1. Face Plate I/O: This is where the I/O connectors come out the faceplate of a board. This is the most common form of I/O cabling. Since the face plate design includes EMI shielding, it is the internal/external barrier in the chassis. This means that coming out the face plate works well for I/O that is needed outside of the enclosure (Ethernet, for example), but it is not well suited for I/O that is needed inside the enclosure (like an IDE hard disk interface).

  2. On-board I/O Headers: This is where I/O signals are brought to header pins or connectors in the middle of a board. A flat ribbon cable is then connected to these headers and cabled over to an internal device (such as a disk drive), or over to a bulkhead connector to exit the enclosure. This alternative is the most inexpensive, but causes several problems. First, attaching a cable to the middle of a board that goes in and out of a backplane is awkward. The cable can be difficult to install, and often "catches" on the board next to it. Second, having a ribbon cable running between two boards and then over the card rails can significantly interfere with the airflow that is needed to adequately cool the boards. For these reasons, this technique is primarily only used in 3U systems to cable out the disk drive interfaces.

  3. Rear I/O With Transition Modules: The Compact PCI specification defines 5 backplane connectors (known as P1-P5 on the backplane or J1-J5 on the add-in boards). Of these 5 connectors, the Compact PCI bus occupies P1/J1 for 32-bits and P2/J2 for a full 64-bit implementation. The other connectors are available to route out user I/O. The routing out of this I/O is facilitated by a special backplane that has connectors on both sides. Thus, signals are available on the backside of a Compact PCI backplane. The connector type used for Compact PCI is a very large and dense connector, so a special "transition module" is needed to breakout the I/O signals into individual I/O connectors. The transition module is a small board (80 mm deep by 3U or 6U tall) that slides in on rails from the back of an enclosure and connects to the back side of the backplane. These modules contain a face plate through which I/O connectors are mounted. Some transition modules also have connectors in the middle of the module for use with I/O that is needed inside the enclosure (such as IDE). The mechanical definitions of transition modules are included in the Compact PCI specification, and various companies supply enclosures that include the required mechanical supports. Using rear I/O transition modules has the advantage of eliminating I/O cabling from the front face plate. This makes the insertion and removal of boards more convenient since you do not have to add or remove any cables. This would be of particular interest in systems that are designed for hot swap of the I/O boards or in systems where minimizing the mean time to repair (MTTR) is critical. Unfortunately, this technique also adds significant cost to a system. This extra cost derives from:
    • The special backplane with connectors out the back ($150 to $250 extra per system)
    • The mechanical supports for the transition modules ($300 to $500 extra per system)
    • The transition modules themselves ($150 to $300 per board that uses rear I/O)

  4. Rear I/O Routed through the Backplane: Another way to route rear I/O is through the backplane itself to specific I/O connectors mounted on the backplane. This technique works well in systems where the only board that uses rear I/O is the CPU board. By routing the I/O through the backplane, a system can be designed without the high cost of transition modules and the associated enclosure support structure required. This technique is restrictive, however, because the backplane must be specifically designed for a particular manufacturer's CPU board. Several manufacturers have begun discussions about standardizing pin-outs for J3-J5, but thus far, each manufacturer has implemented their unique pin-outs. Special backplanes to support rear I/O routed through the backplane are now becoming available.

Click here to see sidebar article "New Cooper-Arnold Backplanes Simplify Compact PCI I/O Cabling

The following table summarizes the various alternatives for Compact PCI cabling. Each of these techniques is being used today in Compact PCI systems.

Where Used



Face Plate I/O

Connectors contained on face plate of add-in board, used for external I/O connections

Simple, self-contained, low cost

Not effective for internal I/O, must connect cables to boards after they are installed

On-board Headers

Connectors or headers contained on add-in board. Used on 3U CPU boards to route disk interfaces

Simple, self-contained, low cost

Difficult to add and remove boards with cables attached, cables can interfere with air flow

Rear I/O with Transition Modules

I/O is routed to rear I/O transition modules for cabling internally or externally. Preferred in some telecommunications systems, and systems using hot-swap

Can install and remove boards without having to disconnect cabling

Requires transition modules and enclosure support, making this the most expensive system solution. Requires access to rear of enclosure, cannot use with wall-mount enclosures.

Rear I/O Routed through the Backplane

CPU board's I/O is routed through the backplane to specific I/O connectors

Allows cabling out of CPU I/O without the need for transition modules

Requires a backplane that is specific to the CPU board family being used

Compatibility of Off-the-Shelf Products:

Standard off-the-shelf products typically only support one type of I/O cabling, so designers need to be aware of the I/O cabling technique used in each element of the system. There are implications as to the feasibility of many off-the-shelf products. For example,

  • Most I/O boards only support faceplate I/O
  • Some 6U telephony boards only support rear I/O with transition modules. These same boards often require a special telephony bus (known as H.110) to be routed on the backplane across the P4 connectors, which also requires a special backplane.
  • Most 3U CPU boards have some I/O on the face plate and route disk interfaces to on-board headers
  • Most 6U CPU boards have some I/O on the face plate, and route most other I/O (including disk interfaces) through J3-J5

Using a backplane and enclosure that support rear I/O transition modules is the most universal approach to building a Compact PCI system. However, since the cost goes up significantly to support this technique, many designers are choosing to forego the conveniences of rear I/O and solve their I/O cabling with more cost-effective techniques.

Contributed by: One Stop Systems, 1317 Simpson Way, Suite A, Escondido, CA 92029. Tel: (760) 745-9883; Fax: (760) 745-9824.