Data All Strung Out
by Tom Lecklider, Senior Technical Editor
Serial data buses have largely replaced parallel buses for high-speed data communications. Parallel buses are limited in speed by the amount of skew that can be accommodated. Also, signal integrity and EMC issues are much more easily handled for only a few wires than for large parallel buses.
As alternatives to traditional wiring harnesses in vehicles, serial buses are attractive because they save material, weight, and cost. In addition, a flexible mix of optional modules can be accommodated by software rather than necessitating multiple harness variants.
A wide range of bus protocols is available to match an application's requirements. Characteristics include determinism, latency, cost, reliability, accessibility, and fault tolerance. Providing one or more features that are key to an application may negatively impact others. For example, high reliability and fault tolerance generally drive up cost.
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Table 1. The Seven-Layer ISO OSI Model
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A bus protocol is distributed across the ISO Open Systems Interconnection (OSI) model's seven layers as shown in Table 1. The relative importance of bus characteristics within an application determines the complexity of the layers. For example, fault tolerance in a safety-critical system implies the capability to recognize a fault but continue operating in spite of it. Several OSI layers may be involved in error detection and mitigation.
Safety-critical avionics applications use many different serial buses, generally with high levels of redundancy. These buses are expensive, so it is only natural that a number of relatively robust COTS buses should be considered as possible lower-cost alternatives. In fact, the FAA recently compiled a set of criteria that airframe developers could use to evaluate COTS buses as well as more traditional avionics buses.
When the project began, the investigators almost immediately encountered difficulties in defining and categorizing bus attributes. The problem is complex because “the operation of a data network is so entangled with the avionics system it supports that it is not possible to make an evaluation of a data network on its own.”1
Eventually, an initial list of more than 200 questions was reduced to 36 criteria that were applied against seven types of networks. From this exercise, the criteria were further refined. The executive summary closed with an unsettling conclusion: “This application exercise confirmed the suspicion that there is a greater diversity in the ways that data networks can be designed than there is for microprocessors. This means that the decision for selecting a data network is even more complex than for a microprocessor.”1
Of course, the data bus operation in most consumer electronics products does not have a safety-critical aspect. Nevertheless, a great deal of insight is contained in the report, relating to the interactions associated with bus design decisions. A simple example is the trade-off between the improved error detection associated with a high-order cyclic redundancy code (CRC) and the increased likelihood of an error simply because more data bits are being transmitted.
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