PCB Design


Active Backplanes



See also:

Custom Backplanes

Standard Backplanes

Boards Design

Schematic Design

Components and Library Management

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Why Active Backplanes?

Although the Active Backplanes can be considered a subcategory of the Custom defined Backplanes we consider them a distinct and very well defined category by itself. This is because, unlike the traditional backplanes, the active backplanes host active circuitry that cannot, or it is not desired to, be hosted on the circuit boards that plug into the backplane. The active circuitry on the backplane will perform various functions required by the application such as PCI-to-PCI or PCI-to-PCIExpress bridging, PCIExpress switching, Ethernet switching, transceivers, magnetics, DC-DC converters or voltage regulators, I2C and IPMI processors and the examples can continue.

Traditionally it was considered that a backplane must be a passive component of the system in order to yield large MTBF and basically reduced or no need to repair or replace. Any active component on the backplane would have only reduced the MTBF. In addition, most of the backplanes were based on standard bus architectures, such as VME, VME64x and cPCI, making a backplane usable in various applications with little or no change at all. Same backplane in the same system could be used over and over again with new generations of circuit boards based on a standard bus architecture. It is also obvious that a backplane can be removed from a system but only with significant effort and time.

Lately, however, a number of advances in the technologies led to a change in this philosophy. The MTBF of the active components greatly increased over the years. The bus architectures, while still in use and sometimes required as a base physical layer, are increasingly superseded by newer serial architectures such as VXS, VPX and PCIExpress that by themselves increase the complexity of the backplane. The connector's pin and signal density increased with the speed of the signals. The needs of an application, given the superior performance and processing power of the new circuit boards greatly increased and diversified. All these created the foundation for a new generation of backplanes that provide additional functionality specific to an application, working with a specific set of boards and ultimately performing functions that traditionally belong to the circuit boards.

Such a solution comes in handy when, for example, the processing units that are required to interface with all the boards in the system will require a great number of high speed differential signals that from far exceeds the number of the available pins of the planned circuit board connector's set. Clearly not all the processing units can be placed on a single board. Moving the processing units on the backplane and run all the differential pairs to the appropriate slots will solve the problem and will increase the signal integrity by eliminating connector pairs from the signal's path. Further more, you can put on the backplane your IP contained in the proprietary circuitry, allowing you to use off the shelf I/O boards, no longer concerned with how to manage the signals. You can sell your proprietary backplane and system to customers for them to use with off the shelf boards.

Although such an active backplane comes with an increased price tag and a new thermal approach for the chassis, the benefits will dwarf the traditional disadvantages.

Custom defined active MicroTCA backplane




Configurable dual-mode dual cPCI active backplane


This is a 6U, 7"-wide cPCI based backplane with two cPCI bus sections of 7 slots working at 33MHz on 32 bits. The first section is in the P4/P5 zone and the second section in the P1/P2 zone. The requirement for this specific application was to have the ability to easily switch between two functional modes.

In the first mode the two cPCI sections are separate and equipped with own CPU board to control the cPCI bus. In the second mode the two cPCI bus sections form a unique bus through a PCI-to-PCI bridge. This unique cPCI bus is controlled by the CPU board seated in the first section. The switch between the two modes should be done when shut down or during a reset, by a toggle switch on the backplane or an optional switch mounted on a chassis panel.

We proposed a solution that utilizes a 32-bit, 33MHz PCI-to-PCI bridge in BGA package and CMOS Wide Bandwidth Quad 2:1 Muxes in Chip Scale Package to do the switching of the cPCI signals (see pictures) . All the circuitry is placed on the backplane utilizing the available space on the secondary side. The primary side of the bridge is hard wired to the first bus and the control signals of the system slot in the first section.

In the first mode each system slot controls its own bus section. The bridge is idle, with output clocks and controlling signals in the secondary side suspended and the bus lines downstream the bridge are separated from the second bus by the mux circuits. In the second mode there is only one CPU equipped in the first system slot, the bridge is running, the downstream bus lines are connected to the secondary bus and the clock and control signals are generated by the bridge for all seven slots of the second bus. All muxes, bridge reset and clock buffer are controlled by a single signal generated by the toggle switch status. LEDs show the mode and verify the actual mux status. We added latch circuitry triggered by the Reset signal to secure the toggle switch status and buffers for the Clock signals.


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ATCA based PCIExpress switch active backplane


This is a 10-slot ATCA based backplane. The application required a processing board equipped with six PCIe Gen2, 5GT/s 48-lane, 12-port PCIe switches, two 16-port managed packet processors with the corresponding transceivers and magnetics, to provide the PCIExpress links and Gigabit Ethernet base interface to all 10 slots. The challenge came from the fact that such a board had to provide six X16, one X8 and six X4 PCIExpress Gen2 links and 20 Gigabit Ethernet links across the backplane to all 10 boards seated in the ATCA slots. Given the limited pin's number on a fully populated ATCA slot such a board could not be built.

Elinktron Technology proposed and designed a solution that solves both the pin's real estate issue and the signal integrity issues by placing all the PCIe and Ethernet circuitry on the backplane. Thus, all the PCIe links were routed directly from the switches to the appropriate slots, eliminating a connector's pair from the signal path. Same for the Gigabit Ethernet signals. We added to the mix DC-DC converters, voltage regulators, IPMI control plane to monitor the local voltages and temperatures all seamlessly integrated on the backplane. Special attention was given to the cooling by providing heat sinks where necessary and providing the 3D models (shown) of the backplane to the chassis designers for thermal simulations.


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