Bused embedded compute platforms based on VMEbus and CompactPCI have served the industry well and have built up extensive ecosystems of complementary and competing products. But although the bus interconnects on these platforms will be employed well into the next decade, new high-speed switched serial interconnects have emerged that hold the promise of performance beyond the limits of the current parallel multidrop buses all the way to the upper edge of copper physics.
One way of protecting the investments made in the current bused interconnects and allow migration into switched serial interconnects is through hybrid architectures. They can simultaneously accommodate both a traditional parallel bus and a switched serial interconnect, providing a seamless bridge from today to the future. One such emerging hybrid standard is VMEbus Switched Serial (VXS), which is being defined by the VMEBus Standards Organization under the VITA 41 working group.
VXS is an emerging standard that extends the capabilities of the traditional VMEbus Eurocard form factor of 6U high by 160 mm deep with a high-speed switched serial fabric. Essentially, VXS adds new capabilities to VMEbus by adding a high-speed P0 connector to VMEbus cards (now called payload cards) between the existing P1 and P2 connectors. This new high-speed connector will allow payload cards to communicate at unidirectional speeds approaching 10 Gbits/second per serial pair, or bidirectional speeds of 20 Gbits/s per serial pair, which is believed to be the upper limit of copper interconnect.
The VXS specification allows for four differential serial pairs per direction per link and supports up to two such ports on each VMEbus card. Initially, VXS has subspecs for Infiniband 4x links and Serial RapidIO 4x links. The VXS documents are structured so that a new link technology can be added without changing the base specification, which is where all the mechanicals are defined.
The VXS switch card is the hub of the switched interconnect. The card does not have the traditional P1 and P2 connectors found on a VXS payload card because the VXS card does not have to maintain backward compatibility with the existing VMEbus ecosystem.
VXS will require a new backplane to implement the high-speed switched serial interconnect. Though the VXS specification details the full characteristics of a VXS payload card and a VXS switch card, with regard to the backplane the specification describes only a payload slot and a switch slot. This is so that backplanes of various slot capacities and topologies can be constructed while still being compliant with the standard.
While VXS requires a new backplane, the framers of VXS strived to achieve the maximum compatibility with existing VME-bus products. For example, existing VMEbus cards should plug into a VXS payload slot if the card does not have an existing P0 connector or some other obstruction in the P0 area. As well, VXS payload cards should plug into a traditional backplane slot under those same conditions. Note that in this scenario the payload card will not be able to make use of the switched serial interconnect, nor should it attempt to draw more power than can be supplied by the backplane.
The VXS switched serial interconnect is an optional interconnect over and above the always present parallel VMEbus. Applications that do not need VXS speeds do not have to connect to the fabric; they can confine their references to the traditional VME parallel bus. While most VXS implementations are expected to have two switch slots for redundancy, it's also possible to use only one. Both scenarios would be compliant with the standard.
The P0 connector of every payload slot is wired to the switch slot. In addition, the VMEbus is also wired to every slot, with the exception of the switch slot.
An example using eight payload cards can illustrate the three different ways that a payload card may interact with the backplane. Payload Cards 1, 2, 3, 5 and 6 attach to both the VMEbus and the high-speed switched fabric. Such a payload card is making use of both the VMEbus and the high-speed switched serial fabric.
Payload Card 4 references only the VMEbus. This scenario would be used for an existing VMEbus card plugging into a VXS backplane. Payload Cards 7 and 8 attach only to the high-speed switched fabric. This scenario is called "pure VXS," meaning that the cards do not reference the parallel bus.
Note that Cards 7 and 8 would still have to have the traditional P1 and P2 connectors on them because this is how power and other noninterconnect services are delivered to payload slots.
Each of the various payload slots on the backplane is wired in exactly the same way-the payload cards discussed in this example could be shuffled into different payload slots without consequence. So the interconnect that a payload card chooses to use, parallel, serial or both, is a function of the card, not of the backplane.
The VXS draft specifications are available on the Web at www.motorola.com/computer/VXS.
