Electromagnetic Compatibility Considerations (EMC) for Single-Board RIO

Overview

Single-Board RIO controllers integrate into larger systems deployed to meet a variety of monitoring and control application needs. Because of this, these devices are not required to undergo EMC or safety testing. Instead, integrators must validate their overall products for compliance with the requirements of the countries or regions where they intend to sell. For example, OEMs must ensure that their end products meet European Union requirements to affix CE marks to the final products. To facilitate this task, NI designs Single-Board RIO controllers while keeping in mind that the end product needs to meet international/global EMC standards. The information in this white paper provides guidelines on how to integrate these devices as components in your overall product or application. Following these guidelines minimizes the impact Single-Board RIO controllers have on the overall product’s ability to meet international/global EMC requirements.

Contents

Design an Appropriate Enclosure

Use a Metallic Enclosure

Install Single-Board RIO controllers in a metallic enclosure to control emissions radiated from the PCB and all internal cabling. The ideal enclosure design should minimize seams, slots, or holes that could allow emissions to radiate out of or into the enclosure. Extruded or cast metals work best. Plastic and “folded” metals are not recommended because they do not provide adequate protection from emissions. You need to attach the different metallic components of the enclosure with screws sufficient to ensure the shielding effectiveness of the overall enclosure as required by the end application. Areas where metallic components are making contact should be free of paint or other nonconductive coatings to ensure a low-impedance connection.

 

Ensure Mounting Holes Connected to Digital or Chassis Ground Are Connected to the Enclosure

Once you have chosen or developed an appropriate enclosure, you need to ensure that all mounting holes connecting to digital or chassis ground are connected to the enclosure with a conductive, low-impedance connection. You can best accomplish this by using metal standoffs or cast/milled and threaded mounting supports. In addition, instead of relying on the screw threads, you should use a pan head screw for the low-impedance connection to ensure even contact between the PCB mounting holes and the chassis mounting supports. This is because you need conductive, metal-to-metal contact between the enclosure and the Single-Board RIO controller's mounting holes. Furthermore, you need to mask off the location of all the connections between the enclosure and mounting holes on the Single-Board RIO controller before painting to ensure that they are not covered with paint or other nonconductive coatings.

 

Manage External Connections to the Onboard Ports

When you have finished properly mounting the Single-Board RIO controller in the metal enclosure, you need to consider all the connections you can make to and from the ports on the product.

 

Power

Connect the power input, as specified in the user manual, to a power supply that provides an adequate level of power line filtering. Power ports may require you to place a ferrite bead (Fair-Rite Type 43 or 44 materials or equivalent) around the power cable close to the chassis to control emissions that could be radiated from an unshielded power cable.

 

Protective Earth Port and Ground Lug

You do not need to connect the protective earth port or ground lug on the Single-Board RIO controllers for proper operation. However, depending on the application, you may need to connect the protective earth port to the protective earth grounding system of the facility in which you install the product to enhance EMC performance. In addition, connecting this lug to earth ground usually results in less noise on the analog input measurements.

 

C Series

Many Single-Board RIO controllers allow you to add C Series modules to the system. You should connect these ports only to C Series modules. When integrating these modules into the overall system, you should follow the same installation recommendations discussed above for the chassis.

 

Analog and Digital I/O Headers

You should connect header I/O ports, such as the digital or analog I/O connectors, only to other PCBs within the same enclosure because no EMI protection or filtering is included on these ports. The enclosure can help prevent any undesired emissions from leaving or entering the system through these ports. If, however, you need to connect one or more of these connectors outside the enclosure, be careful to connect these ports first to a connector that supports an overall shield and that is mounted on the enclosure. Then use a shielded cable from the enclosure connector to the outside connection.

 

Communication Ports

With all remaining connectors that must be wired outside the enclosure such as the Ethernet or RS232 connectors, you need to use high-quality, shielded cables to minimize any undesirable interference.

 

Custom I/O

Any custom I/O you add externally to the Single-Board RIO controller should incorporate connectors that include termination for an integral cable shield (for example, coaxial, twinaxial, D-SUB, Ethernet, USB, IEEE 1394, and so on).

 

Select Proper Cabling

In general, you need to use high-quality, shielded cables for any connections that leave the metal enclosure. The use of discrete wires or unshielded cables for external connectivity through connector blocks and screw terminals is not recommended unless the shielding of the connected cabling is not necessary for the product’s required EMC performance.

As far as specific connector and cabling types, the following applies:

  1. Use coaxial connectors and cables only for enclosure-referenced signals such as non-isolated or single-ended signals including audio, TTL, CMOS, and most RF applications (RS232 is a single-ended interface, but it uses shielded, untwisted cables).
  2. Use connectors and cabling that support shielded, twisted pairs for isolated or differential signals such as RS422, RS485, most Ethernet, USB, IEEE 1394, and low-voltage differential signaling (LVDS).

 

EMC Testing Requirements

When you integrate your Single-Board RIO controller to meet all of these requirements, you should evaluate your overall product for EMC performance by conducting the necessary EMC testing for the markets in which you are selling the product. For test and measurement products, the primary requirements are those of the European Union. NI tests similar products according to IEC/EN 61326-1 to meet Class A emissions and industrial immunity requirements.

 

Additional Single-Board RIO Resources