One type of design that has been used in some systems over the years is the backplane system. These systems do not have a motherboard in the true sense of the word. In a backplane system, the components typically found on a motherboard are located instead on an expansion adapter card plugged into a slot.

In these systems, the board with the slots is called a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems. Backplane systems come in two main types—passive and active.

A passive backplane means the main backplane board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane.

Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard is essentially a complete motherboard designed to plug into a slot in the passive backplane.

The passive backplane/mothercard concept enables the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high-function mothercard, this type of system design is rarely found in standard PC systems today, although it was once favored by a few early 286/386 vendors such as Zenith Data Systems.

The passive backplane design does enjoy popularity in industrial systems, which are often rack-mounted. Some high-end fileservers also feature this design. Figure below shows a typical Pentium III single-board computer used in passive backplane systems.

Passive backplane systems with mothercards (often called single-board computers or SBCs) are by far the most popular backplane design. They are used in industrial or laboratory-type systems and are rack-mountable. They usually have a large number of slots and extremely heavy-duty power supplies.

They also feature high-capacity, reverse flow cooling designed to pressurize the chassis with cool, filtered air. Many passive backplane systems, such as the one pictured in Figure below, adhere to the PCI/ISA passive backplane and CompactPCI form factor standards set forth by the PCI Industrial Computer Manufacturers Group (PICMG). You can get more information about these standards from PICMG.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active backplane systems contain all the circuitry found on a typical motherboard except for what is then called the processor complex.

The processor complex is the name of the circuit board that contains the main system processor and any other circuitry directly related to it, such as clock control, cache, and so forth. The processor's complex design enables the user to easily upgrade the system later to a new processor type by changing one card.

In effect, it amounts to a modular motherboard with a replaceable processor section. Many large PC manufacturers have built systems with an active backplane/processor complex. Both IBM and Compaq, for example, have used this type of design in some of their high-end (server class) systems.

ALR once made a series of desktop and server PCs that also featured this design. This allows an easier and generally more affordable upgrade than the passive backplane/mothercard design because the processor complex board is usually much cheaper than a mothercard.

Unfortunately, because no standards exist for the processor complex interface to the system, these boards are proprietary and can be purchased only from the system manufacturer. This limited market and availability causes the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have advantages and disadvantages. Most original PCs were designed as backplanes in the late 1970s.

Apple and IBM shifted the market to the now traditional motherboard with a slot-type design because this kind of system generally is cheaper to mass-produce than one with the backplane design. In the late 1980s, Zenith Data manufactured a line of backplane-based 8088, 286, and 386-based systems but later abandoned this for a standard motherboard design similar to other vendors.

The theoretical advantage of a backplane system, however, is that you can easily upgrade it to a new processor and level of performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design system, you often must change the motherboard, a seemingly more formidable task.

Unfortunately, the reality of the situation is that a backplane design is frequently much more expensive to upgrade. For example, because the bus remains fixed on the backplane, the backplane design precludes more comprehensive upgrades that involve adding local bus slots.

Another nail in the coffin of backplane designs is the upgradeable processor. Starting with the 486, Intel and AMD began standardizing the sockets or slots in which processors were to be installed, allowing a single motherboard to support a wider variety of processors and system speeds.

Because board designs could be made more flexible, changing only the processor chip for a faster standard OEM type (not one of the kludgy "overdrive" chips) is the easiest and most cost-effective way to upgrade without changing the entire motherboard.

Because of the limited availability of the processor-complex boards or mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry-standard form factor.

The bottom line is that unless you have a requirement for a large-capacity industrial or laboratory-type system, especially one that would be rack-mounted, you are better off sticking with standard ATX form factor PCs. They will certainly be far less expensive.