Submitted by MAC Panel

Before choosing cables and connector’s for your next PXI based system, it’s important to clearly understand the real-world worst-case environment the test station will reside.  For example, many environments are conducive to electromagnetic interference through radiation or conductivity, which can have an adverse effect on measurement quality.  Another example environment could be an off-shore high humidity region where the factory manager tells you they are at room-temperature; but then never cleans filters or conveys that the AC works only one-half the time.   In this later example, moister creeps into everything causing debug nightmares.  Cables and interconnect often get little attention verses software and instrument selection.  This article focuses upon cabling and interconnect and is a reminder that the weakest link in your test system build is often not obvious.  Designers need not forget to place emphasis on the paths between their PXI instrumentation and the external world. 

Wires verses Cables

You’ll often read the terms “cable assembly” and “wire harness” used interchangeably. Many also mistakenly use the terms “cable” and “wire” synonymously.  In truth, these components couldn’t be more different.  While they’re both used in electrical and communication fields to route electricity between different locations, that’s where the similarities end.  Each has a distinct purpose, which means the success of your PXI system depends on selecting the right one based upon the application need.

Wire Harness:  Open up a wire harness and you’ll find multiple wires, each with its own single layer of exterior covering.  Each single conductor, often made of copper or aluminum, are of low resistance, low cost and measured by “gauge”. The smaller the gauge, the thicker the wire.  The wire covering is typically made from a thermoset or thermoplastic material, and serves two primary purposes.  First (1) to protect outside stimuli, like environmental materials or technicians, from harm caused by the flow of electricity.  Secondly (2), to help with the design and maintenance, while ensuring the PXI system is organized and easy to navigate. 

The jacketing material that covers each wire often does little to protect the wire itself, which is why this solution is typically employed in applications where moisture, dust, temperature and friction are not a concern.  To add organization and manageability, many custom wire harness builds will include spiral-wraps to allow breakout of single/multiple wires.  Another option may include the use of corrugated tubing which often has a slit down its side to allow for simplified bundling.

Cable Assembly: A cable assembly is made up of cables that often include two or more conductors wrapped in a single jacket. Most cables consist of a positive wire that carries the electrical current, a neutral wire that completes the electrical loop and a grounding wire.  Like wire harnesses, cable assemblies are groups of cable that also have an exterior covering.  However, the entire bundle is encased in an extremely durable sheath made of material like shrink-wrapped thermoplastic, rubber, or vinyl.

Cable assemblies serve three main purposes.  First (1) to protect outside stimuli from harm caused by the flow of electricity. Second (2) to ensure test systems are organized and easy for technicians to navigate. Third (3) to protect the cable against wear and tear or other harsh environmental conditions.   Cable assemblies are often custom-designed to address logistical challenges such as size constraints, temperature exposure, moisture, dust, friction, flexibility and more.