Virtual IO Module Network Cable Shielding and Grounding

The purpose of this technical note is to specify the network cabling and grounding standards for the M-Series and S-Series Virtual IO Modules (VIMs).  

The information contained in this article references material found in DeltaV Knowledge Base Article NA-300-0022.  For more information regarding NA-300-0022 or to access the article, please contact Emerson Global Support at 1-800-833-8314.

Network Cable

While the term 'shielded' category 5 twisted pair cable is applied to DeltaV, the twisted pair cables only require a single outside foil surrounding all the twisted pairs.  This type of cable is actually refered to as "screened" twisted pair or FTP (Foil Twisted Pair) within the networking industry.  Also, within the networking industry, "shielded" twisted pair has an outer foil and foil that surrounds each twisted pair.  This is not required for DeltaV installations.  See the figure below for further clarification.

RJ45 Connector Shielding and Grounding

The Ethernet specification for a grounded segment states that if a segment is grounded, it will only be grounded at a single point.  In all DeltaV systems, this ground point will be at the central network device, whether it is an Ethernet Hub or Switch.  In a typical configuration, the internal foil shield of the cable will be connected to the shielded RJ45 connectors.  To avoid multiple grounds, it is recommended to use cables with one shielded connector and one unshielded connector for most network devices.

This is not the case for DeltaV Controllers and the VIMs.  Shielded connectors must be used on both RJ45 connector ends.  The VIMs are internally very similar to DeltaV Controllers.  Both the VIMs and the DeltaV Controllers have Faraday shields on the the circuit board to extend the twisted pair cable shield surrounding the Ethernet circuitry.  The Faraday shields surround the RJ45 connectors, the Ethernet controllers, and the supporting circuitry.

There is no DC ground connection from this shield to the remaining circuitry.  The shielded RJ45 connectors will only make contact with the Faraday Shields.  The RJ45 connectors are isolated; therefore, the connectors are still floating and the single point of ground is still made at the central network device.  In this case, the shielded RJ45 connector is not being used to connect to earth ground at the VIM side, but to connect the Faraday Shields to earth ground at the network switch end.

This Faraday Shield design has been implemented in MD+, MX, SD, and SX controllers as well as in both M-Series and S-Series VIMs.