Backplane Routing Topology for Gigabit Copper and Fiber Networks

August 20, 2019 Zachariah Peterson

Ethernet ports on a PCB
You’ll need a backplane to stack multiple ports in a rack-mount unit

Many systems require some kind of copper or fiber network interface, including gigabit-capable network cards for PCs and high performance embedded systems, data centers, the military, and aerospace. Greater use of fiber optic modules and expanding networking capabilities in a variety of systems will require many PCB designers to understand backplane design in order to accommodate multiple transceivers in a single unit.

If your next design requires a large number of fiber or copper inputs and transceivers, then you’ll need a backplane to house these modules in an organized manner. This important piece of equipment is critical for maintaining electrical signal integrity, ensuring transceivers remain mechanically stable, and for providing a compact form factor that provides an interface with a copper or fiber network.


Some Backplane Design Requirements

There are some basic design aspects that must be considered for any backplane, but the exact layout and construction of a backplane, including an optical backplane, will depend on the transceivers used in your product. Different transceivers provide different connections to fiber, copper, and the backplane; your backplane will need to be designed around these possible connections. Additionally, your design should consider the proposed application; applications in military and aerospace that require embedded computing will carry different performance requirements and must accommodate different form factors.

Newer optical transceivers integrate many of the important optical and electrical components required for high speed networking into a single device. These normally mount to the board using a BGA, thus there will be some escape routing required to send electrical signals to the PHY and onward to the MAC. Thankfully, these BGAs still have rather low pin count, even in transceivers that use multi-mode fiber. The pin count from these integrated transceivers will determine which connectors you can reliably use to connect your transceiver daughterboard to your backplane.

BGA on a green PCB
Your newer transceiver will likely mount to the board with a BGA

The logic family used in transceivers will also determine the remainder of your backplane design. TTL and LVTTL were the basic standards for a significant period of time for transfer up to 100 Mbps, but 1 Gbps and higher protocols will use ECL for data transfer and processing. One reason this is a better option in general is that it supports either single-ended or differential routing, providing designers some flexibility in routing. Some transceivers and supporting components for backplane design are compatible with LVDS routing.

One option for use in a transceiver backplane is Gunning transceiver logic plus (GTLP), which was specifically designed for use in backplane architectures. GTLP transceivers are supplied with Vcc = 1.5 V, thus they have a lower voltage swing during switching. The non-backplane side of a GTLP device is still compatible with standard TTL and low voltage CMOS components. These qualities, as well as availability of PLDs that use or are compatible with GTLP (e.g., ASICs and FPGAs that support differential routing) make it more desirable than ECL for use in backplanes.


Optical Backplane Routing Topology

Placing an individual transceiver in a given layout is no trivial matter, although the guidelines are standardized and can be found in a variety of locations. The backplane for connecting multiple transceivers should be designed with a particular topology, depending on the logic family and signalling standard you use. No matter which logic family you use, you will need to determine the right routing topology for your backplane.

ECL and GTLP are the obvious choices for extremely high speed backplanes. The three principle topologies used for backplanes are point-to-point, multipoint, and multidrop. The choice of topology depends on the particular application. Although there are notable variations among these configurations, the most significant difference is between single-ended and differential topologies. From a noise suppression standpoint, differential routing is preferable as it offers immunity to common-mode noise.

Single ended and differential point-to-point topology becomes less useful as the number of transceivers increases, making multipoint and multidrop desirable from a layout standpoint. However, point-to-point routing is best in terms of signal integrity and impedance management. In contrast, multipoint and multidrop require precise impedance matching for each transceiver on a bus. Impedance mismatch can arise from stubs, connectors, or the transceiver itself. The figure below shows multidrop and multipoint topologies for drivers and receivers on a bus.

Multidrop and multipoint optical transceivers
Two differential topologies for multiple transceivers in a backplane

There are plenty of other high speed design issues to consider in extremely fast boards, and some design choices will cross over into RF territory. Examples include grounding, shielding, impedance control, and termination (see the above figure). We’ll look at some of these design issues in greater depth in future articles.

If you’re in the business of designing backplanes and other designs for interfacing with copper or fiber networks, look to Altium Designer® for a full suite of design and simulation tools for any high speed device. The design and simulation tools are ideal for applications in high speed networking, embedded computing, and other RF devices. The data management and documentation tools are ideal for quickly preparing your designs for prototyping and manufacturing.

Contact us or download a free trial if you’re interested in learning more about Altium Designer. You’ll have access to the industry’s best layout, simulation, and data management tools in a single program. Talk to an Altium expert today to learn more.

About the Author

Zachariah Peterson

Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.

His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies.

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