Ram Rajan, VP Engineering at Elma Electronic discusses the challenges involved in tackling I/O constrained enclosure designs
As systems get smaller, the challenge of bringing all the I/O out of the box has increased exponentially, but the good news is there are various methods by which I/O can be addressed in embedded systems applications. Even better news is these methods consider SWaP challenges and the reduction in the number of boards, all while still retaining the use of typical connectors.
As enclosure sizes decrease and I/O requirements increase, the obvious challenge in bringing I/O out of electronic packaging is the lack of real estate. The same powerful processors that enable smaller system footprints also support greater numbers and types of I/O from each card.
Thus, density is driven by both advanced silicon that handles more inputs and smaller form factors permitted by the more powerful processors.
With Rigid-Flex-Rigid Assemblies there are three sub-elements to an RFR interconnect structure: the main board, the flex PCB and the I/O board (Figure 1). The main board is where all the signals are generated, as in a backplane, for example.
The flex element is an impedance-controlled flexible circuit board integral to the two rigid boards. The I/O panel is the second rigid section mounted with I/O connectors. All three are fabricated at the same time.
There are several advantages of using the RFR approach. The RFR interconnect structure provides a constant impedance from point A to point B on a given signal path.
All the signal planes are surrounded by ground or power planes, which minimise crosstalk, and the structure behaves well under high vibration and shock environments.
This approach is usually the most compact and reduces labour costs required for separate cable assemblies, while eliminating the testing of these assemblies. Rear access to the connector pins at each end of the circuit allows accessible test point locations for high speed signal measurement.
Increasingly, systems are turning to rigid-flex-rigid cabling. This combination of a flex circuit for tight corners paired with a rigid PCB for the I/O connectors is ideal for space-constrained chassis or boxes. Figure 2 shows such an RFR assembly installed.
RFR design can be costly and the assemblies are more expensive because of specialised materials and processes used in their fabrication, so it is best for applications with higher volumes, where NRE (non-recurring engineering costs) can be distributed across many units.
The direct plug interconnect (DPI) is implemented with a direct connection between the main board and the I/O panel by means of a board-to-board connector. When attached at a right angle to one of the edges of the backplane, any family of typical backplane connectors can be utilised.
The I/O panel can also be located at a right angle to any one of the backplane’s four edges or even as a rear mezzanine by using stacking connectors. This flexibility, together with the varied mounting options for the subrack within the enclosure, allows the I/O panel to be placed on almost any desired face of the system enclosure. The important advantage of DPI over RFR interconnect is cost. Figure 3 shows a typical DPI interconnect.
This approach is typically more reliable than the other two cabled approaches discussed and provides the same reliability as any of card-to-backplane slot connection.
While more expensive than the sidewall I/O panels method which supports multiple connectors via cables, the connectorised DPI approach is significantly less expensive than the RFR approach. It offers a compact reliable solution that is easy to test and efficient to assemble.
Silicon will always move ahead of the rest of the system components in embedded computing architectures, and systems will struggle to keep up with new silicon capability.
At the present time there are a number of efforts within standards organisations to define both higher speed connector implementations and interface strategies for a number of new small form factor standards.
In time, new connectors will populate the backplane slots, I/O connectors will get newer, faster inserts and the fibre content of I/O cables will increase. Despite such inevitable advances, these five basic approaches to backplane I/O will likely remain for the foreseeable future.
Elma Electronic
