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What is a Backplane: A Simple Guide

Printed circuit boards (PCBs) are the backbone of modern electronics. Today’s computers and servers would not exist if it were not for PCBs. The backplane is an excellent example of this. Essentially, it would not exist without the invention of the PCB. But what is backplane, and why are they so important? This guide will answer these questions.

What is a Backplane?

Backplanes contain a collection of electrical connections sitting in parallel to each other. The individual pins of each connector sit in such a way that it creates a connection system. The technical term for this system is a data or computer bus. We use backplanes to develop computer systems. We do this by connecting different PCB-based components to them. Serial backplane technology sends information using a low-voltage signal.

Manufacturers usually create backplanes using several types of PCBs as a base. Yet, wire-wrapped versions are still available. Although, they are pretty rare. Fabricating wire-wrapped backplanes can be expensive – especially in mass. As such, we usually use these versions in miniaturized computers. Additionally, you can find them in industrial applications that use high-reliability parts.

A fine example of this is the Altair 8800. It is a microcomputer that uses a backplane and various plug-in cards.

Illustration of Altair 8800

Backplane vs. Motherboard

Backplanes (or backplane systems) are similar in structure and use to motherboards. One key difference is that backplanes use a cable-free management system. As such, people often get them mixed up.

Furthermore, they do not have onboard support (sockets) for processors. Instead, backplanes use expansion or plug-in cards to add processing and storage. Thus, you can tell the difference between backplanes and motherboards by looking at the connections and sockets. 

Motherboards have more diverse connectors than backplanes. Usually, backplanes have one bus type. This can be limiting. Yet, you can add storage to the backplane by connecting a single-board-computer (SBC) or system host board (SHB).

Backplane connected to an SBC

Backplane connectors are more reliable than cables. Backplane-based systems tend to last longer than cable systems (motherboards). When you add a new component, you must change the positioning of the thread, causing it to bend. Accordingly, these actions damage the line, resulting in mechanical failures over time.

Passive vs. Active Backplanes

There are two types of backplanes – passive and active. True passive backplanes have no active bus driving circuitry. Daughter expansion cards handle signal routing, processing, or logic. Contrastingly, active backplanes feature chips that buffer the various signals to the slots.

The difference between passive and active backplanes isn’t straightforwards clear. We use passive backplanes for systems with a single point-of-failure (SPOF). At the same time, active back-planes have a non-zero risk of malfunction because of their complexity. Yet, it may become an issue for computer systems that have many points of failure.

Even so, maintenance and board-swapping can disrupt both types of boards. Removing and inserting panels can damage the connect/pins on the backplane. This may cause a complete outage. Thus, manufacturers have begun introducing new systems using high-speed, redundant connectivity. They interconnect system boards point-to-point. As such, they have no SPOF anywhere in the system.

Backplane Connection/Bus Types

The most common backplane bus and connection types are:

  • VME eXtensions for Instrumentation (VXI)
  • Multisystem Extension Interface (MXI)
  • Embedded Technology Extended (ETX)
  • Embedded PCI eXtended (ePCI-X).

Backplanes have become more complex since ISA and S-100 form factors were released. Where a common bus handled all the connections between the connectors, you can find both passive and active today. This fact is due to the limitations of the PCI specification.

Backplane Form Factors

Much like motherboards, backplanes have design form factors. These form factors affect the size of the chassis/enclosure that houses the backplane. Additionally, it will dictate what power connection the backplane can use. The most common form factors are as follows:

  • Advanced (AT): They come in two types – complete and small. Full-sized backplanes are up to 31 cm wide and 35 cm deep. Contrastingly, tiny (AKA baby) backplanes can be up to 22 cm wide and 33 cm deep. With this form factor, IBM originally designed backplanes for personal computers (PC).
  • Low Profile eXtension (LPX): These are 23 cm wide and 33 cm deep. Western Digital developed this form factor in the late 80s as an alternative to the AT form factor. It’s notable for its use of a riser card.
  • Advanced Technology Extended (ATX): The successor to the AT form factor. It combines both the features of AT and LPX form factors. It comes in five different sizes and configurations. They include standard, micro, mini, nano, and pico.
  • Nex Low-Profile Extended (NLX): Found in industrial applications due to its ruggedness. It’s an alternative low-profile form factor, and many consider it LPX’s successor. As such, it has similar measurements to LPX.

You can also find backplanes in the following size and form-factor specifications:

  • Size A (10 cm x 16 cm)
  • Size B (23,3 cm x 16 cm)
  • Size C (23,3 cm x 34 cm)
  • Size D cards (36 cm x 34 cm)

Backplane Specifications

How do you specify and differentiate between backplanes of the same size and form factor? You can judge backplanes by the following criteria:

  • Card Segments
  • Expansion Slots
  • CPU Slot (For Active Backplanes)
  • Bus Type (Type of slots)
  • Onboard terminators

FAQs

What is a backplane in a server?

Backplanes contain several connectors. Hence, we use them in servers to connect multiple hard drives, for instance, in a network-attached storage system or server.

Backup disks in a data center

Backup disks in a data center

What is the backplane in a router?

Some routers use miniaturized backplanes as opposed to a motherboard. The functional principle is the same. The backplane provides connectivity for modules.

What is a backplane of an SBC?

Encountering a single board computer (SBC) that uses a backplane as its base is rare. Instead, they use motherboards. Yet, we often connect these motherboard-based SBCs to backplanes. This action allows us to extend their purpose and functionality.

What is a backplane in networking?

We typically use backplane systems to construct modular repeaters for network applications. Repeaters allow you to add modules to them. Hence, we use them to create completely customized network configurations. 

Conclusion

Backplanes are modular alternatives to motherboards. As such, they’re more suitable for industrial and commercial purposes. They are especially ideal for servers with hot-swappable drives. 

Even so, you can find backplanes of all shapes and sizes. They use PCBs with varying thicknesses. Accordingly, the thickness of the PCB will determine the application of the backplane. Nevertheless, the above guide explored backplanes and their uses. If you have reached this point, you should now be able to tell the difference between motherboards and backplanes.

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Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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