Developing and building systems using OpenVPX Profiles

OpenVPX (VITA 65) is built on existing base VPX specifications including VITA 46.x and VITA 48.x, largely based on serial fabrics implemented using pairs of differential point-to-point interconnects. In moving from a parallel bus-based scheme of interconnect to a serial fabric-based interconnect, the notion of one general backplane topology such as VME or cPCI is gone.

The industry has been using a set of backward-compatible standards for years that have allowed plug-compatible modules to be used with common backplanes.  For over 20 years, VME boards have been interoperable with little concern as to whether a board would operate when plugged into widely available standard backplanes. With the serial fabrics used in OpenVPX, board-to-board I/O is now point-to-point, and unique.

A central goal of the OpenVPX standard is to promote interoperability. Thus OpenVPX has introduced a language to describe and label interconnects required to implement specific system topology, and has also given us named profiles to identify unique topologies for interconnecting modules. In this article, we will look at how systems can be implemented using the standard Profiles defined by OpenVPX (Table 1below)

Developing and building systems using OpenVPX Profiles
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Table 1 – OpenVPX Profiles at a Glance

Before we get started, let’s look at some general items to consider when weighing OpenVPX versus other existing legacy system standards. For instance, when comparing VME or cPCI to VPX, the following points and features can be reviewed when comparing the technologies:

* Available form factor – 3U or 6U

* Convection- or conduction-cooled environment, supported by OpenVPX

* More available power in 3U and 6U form factors than VME and cPCI

* Higher bandwidth differential interconnects up to 6.25 Gbit/s

* Required system topology: Mesh, Star, Dual Star; the need for multiple signal planes

* Need for redundancy, for instance dual star architecture

* Legacy support for parallel VME is a hybrid system

* Single plane or multi-plane architecture: Control plane, Control and Data, need for Expansion plane

* Type of Fabric Protocol: PCI Express, Ethernet, SRIO, etc.

Planning Required When Selecting the Hardware

Determining the required architecture, backplanes and modules is often a daunting task when looking for an off-the-shelf solution using OpenVPX. In approaching a system implementation in VPX, a few additional steps are required to specify boards or modules for the system.

Table 1 below describes the types of profiles defined by OpenVPX. OpenVPX defines profile descriptors for modules to be used in a system, and then also defines slot profiles (Figure 1 below), which define the mapping of I/O onto connectors used in the backplane.

Developing and building systems using OpenVPX Profiles
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Figure 1. OpenVPX Slot Profiles from left to right: Payload, Peripheral, Switch and Storage.

The module profile is specific to the physical protocol to be used, such as PCI Express or 1000Base-T Ethernet, while the slot profile maps I/O onto a connector but is agnostic in terms of physical protocol.

The interconnect between the slots in a backplane is defined by the backplane profile (Figure 2 below). In that the backplane profile references slot profiles, the designer must identify the profiles associated with the standard boards that will be selected for the system.

Developing and building systems using OpenVPX Profiles
on image to enlarge.

Figure 2. OpenVPX Backplane interconnect profile

In order to do this, each board manufacturer must specify the module profile description of the OpenVPX board, to identify the physical interfaces of the module.

This would be a starting point in identifying interoperable modules to be used in a specific OpenVPX system topology. In addition, one must consider the system topology that will be required of the modules or boards to be used in the system.

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