Backplanes are circuit boards that contain sockets or expansion slots for connections to other circuit boards.
There are two types of backplanes: active and passive.
Active backplanes contain bus control and motherboard circuitry; however, they do not contain processor complex components such as the central processing unit (CPU), chipset, or cache.
Passive backplanes contain circuitry for bus connectors and, in some cases, buses and drivers. Buses are devices that transmit data from one part of a computer to another. Drivers interface hardware devices to an operating system or other software.
Both active and passive industrial backplanes may also include bridges that interface two buses of the same type or of different types.
Backplanes differ in terms of bus type.
- Industry standard architecture (ISA) buses are I/O devices that can handle 16-bit data transfers at a clock speed of 8 MHz. Extended ISA (EISA), an enhanced version of the ISA bus, is capable of 32-bit data transfers.
- Peripheral component interconnect (PCI), a local bus system for high-end computers, can transfer 32 or 64 bits of data at a clock speed of 33 MHz.
- Compact PCI (cPCI) uses the electrical standards of the PCI bus, but is packaged in a Versa Module Eurocard (VME) bus.
- The VME bus (VMEbus) is a rugged, 32-bit device used in industrial, commercial and military applications.
- Other bus types for backplanes include VME extensions for instrumentation (VXI), multisystem extension interface (MXI), embedded technology extended (ETX), and embedded PCI extended (ePCI-X).
The form factor determines whether industrial backplanes support AT, ATX, terminal block, or 3.3 V power connectors. There are many form factors for backplanes.
- Advanced technology (AT) is the original IBM motherboard design for personal computers (PCs). Full-size AT boards are up to 12” wide by 13.8” deep. By contrast, small or “baby” AT boards are only 8.57” wide by 13.04” deep.
- Low profile (LPX) backplanes mount expansion slots on a bus riser that connects to the motherboard.
- ATX combines features from the AT and LPX designs. Related technologies include MiniATX and microATX.
- NLX, another form factor for industrial backplanes, supports both current processors and technologies such as accelerated graphics port (AGP), universal serial bus (USB), and dual in-line memory module (DIMM). NLX cards are up to 9” wide and 13.6” deep.
- Other common form factors for backplanes include Size A (100 mm x 160 mm), Size B (233 x 160 mm), Size C (233 x 340 mm), and Size D cards (360 mm x 340 mm).
Important specifications for backplanes include the number of segments in the card and the number of expansion slots. Typically, active backplanes include a slot for plugging in the CPU. The bus type determines whether the backplane contains slots for ISA, PCI, cPCI, PXI, VME, VXI, or MXI slots. Some backplanes include a connector for a keyboard. Others include on-board terminators that minimize reflections and interference on the bus.
Backplanes must adhere to certain standards to ensure proper design and function.
ARINC 659 - This document defines the electrical characteristics for a controlled impedance backplane bus for use with Integrated Modular Avionics (IMA). It defines the necessary characteristics of the bus that will ensure electrical compatibility at the interface between the avionics module and the IMA cabinet. ARINC Specification 659 defines the protocol, timing, bit rate and power requirements for the physical backplane. It defines the physical medium, physical layer and parts of the Media Access Control (MAC) sublayer referred to in the OSI Reference Model.
DIS-MISC-81063 - Backplane component implementation plan.
EIA-616 - This standard is applicable to modular two-part connectors for printed wiring boards associated with equipment for telecommunication or electronic devices using similar techniques. The standard covers high-density connectors providing modular contact arrangements with signal contacts, power contacts, and options for special contacts (high current/coaxial/optical). Both fixed and free board mounted connectors have module sizes in multiples of 12 mm. Both are end-to-end stackable without loss of contact positions.