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techtrends C ble nd h rness EDA

mon pin. Wires are bundled together to form cables. If the cable is shielded, the designers must place termination pins to support grounding of the shield. De signers typically generate a number of variants for each design. Think of a ve hicle platform that will find use in a number of car brands, and each brand can have more than 100 options.You will then understand the complexity of nec essary design data management.And the process is truly global: For example, the manufacturers of the US Lincoln LS build the car on a British Jaguar plat form. Of course, designers need to keep track of all the parts a design uses and as sign a name and all appropriate attrib utes to each wire and cables. Most of these tasks are mechanically oriented with the goal of producing the logical map of the electrical system to be Fig built.You also need to ensure that

the design does not violate any electrical, physical , or manufacturing design. So you need a DRC tool that you can run at any time during this process. And, of course, the design must be associated with your pc board design at all times. The electrical system schematics now be comes an integral part of the design data base, associated with the electronic de sign schematics, so that connectors can be associated with logical signals and common properties can be seen in both designs.

After completing the design of the log ical schematic of the electrical system, the engineering work to prepare it for man ufacturing can begin. This phase of the process defines the harnesses, generates drawings to produce the fixtures used to build them, and a topological description of all the wires and cables mechanical en gineers use to assign their final attributes. First, engineers must produce an archi tectural layout, a 2 D rendition of the electrical system showing its complete topology and attributes. Engineers place a symbol for each electronic module connected to the electrical system and route wires and cables using the logical netlist they previously generated. They must assign wire gauges, estimate the length of each wire and cable, and iden tify where connectors and fuses should be. The result is a more precise but still logical representation of the electrical system. They then partition the electrical system into harnesses. This step is time

LOGICAL

SCHEMATIC

ARCHITECTURAL

DESIGN

LAYOUT

DESIGN

COMPONENT DEFINITIONS

ENGINEERING

GENERATE PHYSICAL SCHEMATICS

MANUFACTURING

3 D SOLID HARNESS MODEL

MANUFACTURING PREPARATION TOOLS

ELECTRICAL

MECHANICAL

MANUFACTURING BILL OF MATERIALS

ure 2 Designers follow a complex flow to complete a cable design (courtesy Innoveda).

consuming when done manually. For each harness, the designers must gener ate a 2 D design that includes manufac turing information, such as cable cover ing and retention devices, such as brackets and grommets. At this point in the process the design is not to scale but provides early information to manufac turing. Manufacturing begins to have an idea of how many harnesses it will have to produce, their degree of com plexity, and the approximate amount of each part that manufacturing needs to order.

The design is now ready for the me chanical engineers. Each of the cabling design systems from Innoveda, Mentor, and Zuken interfaces with at least one mechanical design package. Two me chanical design packages share this mar ket. The French company Dassault Sys temes developed CATIA, and IBM also distributes this product. SDRC (Struc tural Dynamic Research Corporation) developed a product called I DEAS, al though people often refer to it as SDRC. Both of these products take the logical netlist and the physical attributes devel oped using EDA tools and allow a true 3 D rendition of wires and cables, as they

planning and routing of a harness . The designers then export the information about the electrical system to the EDA tool. Engineers update the drawings for each harness and the electrical system bill of materials and produce the draw ings and specifications for the form boards that are used to assemble each harness.

Once you design the system, you must ensure that it will operate safely, reliably, and according to the specification. Phys ical routing can have an impact on sys tem interconnect performance. The fail ure to account for voltage drops through the wires during the design phase can re sult in costly changes. In the automotive and aerospace environment, 12V dc at high currents is prevalent, and wire loss es are not negligible. Engineers must monitor and analyze the effects of rout ing on their original design and provide routing rules to avoid the problem by, for example, restricting the gauges used for wires or setting a maximum cable length.

Electrical sneak paths can cause un wanted functions to occur or prevent de sired functions from performing cor rectly even when all components are

will be finally positioned in the finished product. At this stage, engineers deter mine the precise length of each wire and the location of the connectors and fas teners. Figure 3 shows an example of the

working properly. Sneak paths are usual ly the result of external asynchronous in puts that a circuit designer either could not foresee or could not control. To iden tify a sneak path, you must analyze the

108 edn | March 29, 2001

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