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With the design flexibility of VPX, there are many configurations available for all types of applications. For certain I/O requirements, achieving the intended design goals could be challenging. Using custom designed Rear Transition Modules (RTMs) for VPX is one solution. But for development and deployed systems, an RTM solution is not practical. It would be ideal to be able to directly connect one VPX slot to another and pull out I/O signals to connectors such as RJ45, 38999 circular connectors, SMA contacts and more.
When the VPX VITA 46 specification was being drafted, the developers were looking for an architecture that provided a high-bandwidth solution in 3U and 6U height and offered plenty of I/O options. So, the specification was designed for a high degree of I/O flexibility. As VPX has morphed into the OpenVPX VITA 65 specification (largely due to too much flexibility in VITA 46), bringing out the I/O signals is as important as ever. Custom RTMs are always a possibility. Plus, there are “universal RTMs” in the market for VPX that have many of the common connector interfaces (Figure 1). A typical configuration has connectors installed to bring out J0, J2, J3 and J4 signals.
This Universal VPX RTM Breakout Board can allow a test engineer to access I/O signals on custom built VPX boards.
The board would not be intended for high-speed signals but would typically be used to bring out single-ended TTL signals that might be part of a customer’s custom I/O board. Boards like the universal RTM can be helpful tools in VPX design. However, it would be ideal to also directly connect slots via cabling. Plus, RTM solutions are not always practical for all applications, so alternatives would be useful.
Responding to market demand, a VPX cabling system was created to give engineers more options in design solutions and prototyping/testing. This system was designed and based on input and suggestions from several engineers in the embedded industry from different fields. The goal was to gather the input from the industry and provide a durable, flexible, cost-effective cabling solution.
The direct cabling solution enables the testing of VPX systems. There are test backplanes that have signals routed from the P1 section of the backplane to SMA and SATA connectors (Figure 2). With direct VPX cabling, engineers can now directly plug into the backplane, for example, with a SMA connector off the cables into a TDR or other test device. This makes for a simpler and more efficient test setup. Using VPX cabling, the engineer can use a wide range of terminations. All of the termination options have the ability to transform into single, double, triple, or quad stacked assemblies, while supporting multiple protocols, such as InfiniBand, Serial Rapid I/O, Ethernet and PCIe.
The test backplane (a) has SMA and SATA connectors for connecting to external test devices. Using VPX cables, the simpler test backplane (b) could be created.
For Today’s Applications
VPX has been ideal for the Mil/Aero market with its high-performance, 3U and 6U size options, I/O flexibility, expanding ecosystem, rugged design and more. Using VPX cabling systems would benefit many of the Mil/Aero applications. This includes ATR applications bringing certain I/O signals to the front of the enclosure. Rather than customizing an I/O module, the cables can handle the routing simply and securely. This could be true in any UAV, cargo or fighter aircraft, as well as in sea and land-based applications. There may also be a need to connect signals across the backplane from one slot to another.
Ground-based communications systems for signal relays and processing are also using VPX. These are often in a rackmount application, where cabling could be used to bring out I/O signals or could connect multiple chassis in the rack. Despite the application, the cabling systems will be very helpful in prototyping/testing the systems. This will allow the designer to experiment with different configuration options during prototyping. The cables can also help the engineer do various tests for a working prototype while waiting for some pluggable modules to be available.
In VPX systems, there are various types of signal clusters, such as ultra thin pipes (one link: 1 Tx pair + 1 Rx pair), thin pipes (2 links), and fat pipes (4 links). VPX cabling wafers can be snapped together to create the signal channel that is required (Figure 3). One VPX wafer comprises of 4 coax wire lines, so both differential pairs are terminated. Each wafer therefore consists of an ultra thin pipe. To create a thin pipe, the engineer can simply snap together two wafers. For a fat pipe, snap together four wafers. The wafer modules plug directly into backplane slots and can be securely latched with optional locking bars. For rugged deployed applications, ensuring that the cabling solution can withstand shock and vibration is critical. Therefore, locking bars are supported in a metal shell, which outlines the Multi-gig connector. The shell is mounted down with screws using the same type of implementation that has been used in other Mil/Aero designs for decades.
This close-up of VPX wafers shows they can start and end with either an odd row or even row signal.
Rear Cabling Options
The cabling system can be used on the front or rear of the backplane. However, supporting the weight of the cables from the rear RTM connectors can be challenging. But, it is possible to use the larger diameter holes from the front guide pins. This would allow large enough screws to support the weight. Locking rails can be attached at the four mounting points on each shroud to secure the cable assemblies and provide a smooth surface with slots for tie-wraps. The tie-wraps also provide strain relief.
VPX cabling can also be used to make connections to a SerDes Test Device easy. The device is a simple “health monitor” allowing fast and efficient BER testing for VPX backplanes, boards, or the full interconnect path. The wait queues for a full room of test equipment (let alone a specialized SI engineer) can be staggering. With a simple-to-use device, BER testing and pattern generation can be done with a laptop, downloadable software, and a small space for the SerDes Test Unit and hardware. With a wafer-to-SMA cable connection, the testing of a VPX backplane or board is quick and direct, without the use of probes.
VPX Cabling in Test Applications
Open VPX backplane requirements are laid out in the VITA 46 and VITA 65 standard. With the accelerated design to market cycles always challenging deadlines, efficiency in testing of the product is crucial. With the versatility of a cabling solution, it allows the engineers to emulate the system without the time and expense of a large PCB solution. Typical applications are to test cross talk between rows, measure signal integrity from slot to slot, and allow easy access to check skew and Tx quality. Some examples of second end terminations that have been instrumental in testing applications include SMA, CX4, DVI, ESATA, USB and VGA connectors along with a variety of other options. Figure 4 shows multiple connector options/types with wafers plugged into the backplane.
This figure shows multiple connector options/types with wafers plugged into the backplane.
With the point to point architecture flexibility that VPX cabling allows, the possibilities for moving single or multiple signals has opened up many benefits for the designers. With the need for rapid development it is not always possible to finalize the required selection until further information is known. VPX cabling provides the ability to utilize cabling solutions and to keep the costs low in terms of time and custom board designs. Engineers can use the cabling to adapt a standard backplane for rapid deployment, also providing the option of transmitting I/O through external connectors.
A Variety of Options
Using several available cable wire types, engineers have the option of taking double-ended wafer assemblies, cutting to length and terminating to a large variety of off-the-shelf connectors. There is also a variety of existing configurations with popular connectors terminated to the second end available. This provides a quick and flexible method of meeting their specific requirements for their application. This is a cost-effective and time saving alternative to re-spinning the backplane.
VPX cabling enables system designers to disregard the normal developmental time frames and use a highly flexible option in prototyping and deployment. They also make reliable testing of VPX systems simpler with a direct plugging mechanism from SerDes test devices, TDRs, oscilloscopes and other test equipment.
Elma Electronic Systems