VXS Backplane Design: A Closer Look The VME Switched Serial (VXS) specification is becoming increasingly popular. More and more vendors are developing products and customers have been prototyping various configurations. This relatively new specification is an important step in the continuing evolution of VME and its integration with switched fabrics. The VMEbus has several product niches. VME64x architectures are the most popular with its wealth of I/O pins, ruggedness, and backwards compatibility. But, not only the latest and greatest development of a technology like VME serves the market. In fact, standard 3 row 6U VME is still very commonly designed in new applications. Many don't need the extra I/O pins and functionality of VME64x (along with the higher costs). Even 3U standard VME has its niche in many new designs. VXS will also have its niche as a backwards-compatible VME architecture with switched fabric performance. Not all applications will need the performance of switched fabrics. Many industrial, control, military and other markets do not always need the speed and bandwidth. Legacy VME will still be strong for many years to come. However, some applications need the control, tightly coupled multi-processing, stability, and wealth of products of VME with higher bandwidth. For implementing switched fabrics with VME, backwards compatibility will be a critical deciding factor. Backwards compatibility One important consideration for VME as it has evolved over the years is its backwards compatibility. From three row 16 bit, 40 Mbytes/sec to 32- bit (3U) and 64-bit (6U) (80 Mbytes/sec) to five row VME64x (160Mbytes/sec), VME has always had increased performance along with compatibility to previous specifications. VXS is no exception. The VXS design starts with a standard VME64x backplane design and implements a high speed fabric by replacing the existing P0 connector with the Multi-Gig 7 Row connector and adding hub slots fully populated with the new connector. However, the backplane is backwards compatible to VME64x/VME, allowing standard VME and VME64x cards (without the P0 connector) to be used in the system. Backwards compatibility is an immensely important issue. Here are some of the key reasons to maintain it: · Preservation
of investment in a technology VITA 41 vs. VITA 46 - let's clear up some of the confusion Although V41 (VXS) is quickly gaining more and more popularity, some in the industry have been waiting to see what happens with V46 (VPX). This is probably more out of confusion than anything else, and the VME community has not provided a clear enough message. First, V46 is not a progression from V41, it is a different path for different needs. The two specifications might not compete often, and in some cases hybrids will be a good fit. At times, V41 will fit a niche better and other times V46 will fit another niche better. Either architecture achieves many of the same goals as the other. For example, switched fabric performance.... both technologies can implement Single Star, Dual Star, or Mesh. (For an example of a "Mesh" VXS backplane, see Elma Bustronic's switchless Mesh in Diagram 1, discussed further below) Diagram 1 The 5-slot Switchless VXS backplane from Bustronic does not require switch cards for prototyping and development of VXS systems. You can do a larger Mesh topology on a V46, but mesh topologies are at times more conducive to smaller segments anyway. Also, with a hybrid V41, you can have a Single Star or Dual Star segment on a VXS backplane next to a Mesh segment. This arrangement allows mesh performance to be combined with backwards-compatible VME64x legacy cards. Further, a multi-segmented Mesh would allow multiple sets of Mesh segments to be combined. Having multiple meshed bus segments managed and supported in a single system is an efficient use of system resources. How about I/O pins? You can have more I/O pins on a backplane/cards with a Mesh segment with either architecture. Or, this can be achieved with a V41/V46 hybrid. It should be clarified that in this case, the Mesh segment slots would need to resemble switch card slots - with MultiGig connectors consuming the entire slot. (See concept in Diagram #2). Conduction cooling can also be implemented on both. Diagram #2 The "dummy" concept above can be done in a hybrid VXS configuration with separate segments or in a VITA 41/46 hybrid. One of the main differences from the backplane standpoint with V46 is the option of the 3U form factor and the support of a larger Mesh configuration. These requirements are only a certain niche in the market. Also, V46 is not backwards compatible (except in hybrids). So, waiting to see what happens with V46 is not necessary, as the two specifications will likely serve different niches. As you can see, with creative implementations, there are not that many differences. Those who plan to implement V46 when it becomes available can start with V41 implementation today and would have the flexibility to move to a hybrid solution in the future. Whether using V41, V46, or other architectures with high-performance, characterizing the interconnect path is increasingly important. With a new specification, Elma Bustronic decided to develop its first VXS backplane (the 12-slot Dual Star) with a high-grade laminate material and with more (but thinner) layers. Without distinct SI modeling to provide the parameters for characterization of the backplane, we designed the first backplane to "safely have excellent signal integrity". It's too early in product development for this new technology to do full characterization. But, we can look at one of the most basic elements to measure - the impedance. Let's take a look at the actual impedance of the Nelco4000 13-SI and see if we were on track. Signal Integrity At higher clock speeds, the PCB requires cleaner signal transmission without compromising the stability of the system. Signal integrity issues such as reflections, cross talk, frequency dependent transmission line loss and dispersion can significantly lead to poorer system performance propagating through the interconnect. As VXS is a relatively new specification, we are first measuring the impedance of the PCB. Depending on the configuration, routing a VXS backplane with superior performance can be challenging. In the higher slot sizes, the number and length of the traces can have an effect on the signal integrity. Particularly with larger backplane the number of traces and lack of physical space, it takes creative and intelligent routing schemes from an experienced designer. Avoiding undesirable stubs for upper layer backplane traces presents some tough choices. One option would be to have these worse case vias back-drilled -- a costly fabrication process which removes the unused portion of the plated via structure below the layer at which the signal is terminated. Another possibility is to minimize the length of via stubs by choosing a laminate with a lower dielectric constant as Bustronic did with its 12-slot Dual Star Backplane. To provide an illustration, let's look at two VXS backplanes - the 5-slot switchless Mesh (where the Mesh spans 3 of the slots) and the 12-slot Dual Star. The smaller backplane with traces that spanned 3 slots was done using FR-4. The larger backplane with traces spanning 8 slots was built using Nelco4000-13SI - a high-grade material. To ensure a clean signal, it is necessary to understand and control impedance in the transmission environment through which the signals travel. Impedance mismatches (due to vias and connectors) and variations can cause reflections that decrease signal quality as a whole. Time Domain Reflectometry (TDR) measures the reflections that result from a signal traveling through a transmission environment like-a circuit board trace, cable, connector. The impedance values of typical transmission lines as a function of the trace geometry and the dielectric constant of the surrounding environment. The VXS backplanes being analyzed had the following features. The 12-slot Dual Star VXS has an 18-layer controlled impedance stripline design. To ensure the highest possible results in early development, the initial version was fabricated using Nelco 4000-13SI material, a laminate with a lower dielectric constant. The material also has a significantly lower loss tangent value than FR-4. Therefore, the backplane has superior signal integrity and stronger overall performance, but would not be necessary for all designs and requirements. Elma Bustronic SI engineers are looking at using FR-4 for new 12-slot designs and are confident that with intelligent routing strategies and HSPICE simulation studies, the backplane would still have more than adequate performance while keeping the layer count low. But, this is a topic for a future discussion. The 5-slot Switchless backplane has a 10-layer controlled impedance stripline design in standard FR-4. Both the 5-slot and 12-slot were designed in a 7U height to allow extra power bugs below the card cage for high current options and for easy cabling. The connector used is the Multi-Gig Rt-2 7 row from Tyco Electronics. The company gives an approximate capability of 6.4 Gpbs. With a unique wafer design, the connector does not have a typical pin and socket interconnection. Signal Impedance (VXS J0 connector) 12 Slot Only the longest trace connection paths of the 12-slot Dual Star were tested. (Based on experience, we are assuming that in this design these will be the worst-case paths. However, we will verify this with our simulation tools in the near future.) The expected value of differential trace line is 65+/-10% ohm. Diagram 3: (Layer_INT04_Slot06_J1_G10 Impedance waveform). The measured average value of differential trace line for the 12-slot VXS (6380.2 mil) is 68.7 ohm. Diagram 4: (Layer_INT03_Slot07_J0_A15_ Impedance waveform) The expected value of differential trace line is 100+/-10% ohm. The measured average value of differential trace line is (5232.5 mil) 106 ohm Diagram 5: (Layer2_Signal_Slot03_J0_CD12_ Impedance waveform) The expected value of differential trace line is 100+/-10% ohm. The measured average value of differential trace line for the 5-slot VXS is (2892.6 mil) 99.8 ohm Measuring the impedance is just the first step. For both the 5-slot backplane with shorter traces and the 12-slot with longer traces, the impedance measurements were very strong. In the near future, Elma Bustronic will be posting our signal integrity studies on VXS backplanes. This includes model extraction, HSPICE simulation, and in depth backplane characterization. The signal integrity is just one element of VXS design. Another important issue in the success of VXS will be making prototyping and development easy and cost-effective. One facet that is needed is a backplane that is convenient in size and configuration to perform this task. Easy Development New products are coming out to make the development of a VXS system easier. As switch cards for VXS are not yet readily available, it would be helpful to have a platform to allow early development. One solution is a switchless system that serves as platform for developing and testing new VXS cards and is a cost-effective for smaller applications and demonstration systems. Developed by Elma Bustronic in cooperation with Pentek, the 5-slot switchless VXS backplane (see Diagram #2) allows the direct connection of up to 3 node cards without the requirement for a switch card. Pentek's Model 6821,12-bit 215 MHz A/D and Dual Virtex-II Pro FPGA VME/VXS board, is the first in a family of VXS products supported with this new switchless VXS system. Elma and Elma Bustronic have also been working with TransTech DSP (recently acquired by Vmetro) on systems that combine Transtech's multi-processor / FPGA products with Elma's high-performance VXS backplanes and enclosures. The unique switchless VXS backplane could be used to develop and test communication between end-point cards, support small system needs and allow the demonstration of VXS node cards prior to the availability of switch silicon. As a VXS switch card is inherently compliant with a particular data transport protocol, this backplane can be used to test multiple data transport protocol end-point designs without the need to invest in multiple switch card types. Compliant to the VITA 41.x specification, the five-slot system combines three payload slots plus two conventional VME64x slots and allows design engineers to take full advantage of the VXS technology immediately. Conclusion There are many
parameters to VXS backplane design. There is a wealth of configurations
to utilize. Many forget that Mesh segments can be implemented as
stand-alone backplanes or in hybrids. This adds to the versatility
of VXS. Various laminate materials are another factor. In special
cases, a high-grade dielectric material can provide higher performance
for the signals. Other times, FR-4 is more than adequate in maintaining
strong signal integrity for demanding applications. Regardless,
the backplane designer and manufacturer must use creative and well
thought-out design strategies for superior performance. |
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