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INDUSTRIAL LAN | WIRING COMPONENTS AND TESTERS

Fieldbus Frequently Asked Questions (FAQ)

Please contact us with your questions, comments, and suggestions!

Where can I learn more about Foundation Fieldbus HSE (High Speed Ethernet)? 
Visit the Fieldbus Foundation's web site, www.fieldbus.org. Not only can you learn about the technology, but there are links to manufacturers who have FF approved devices.

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I would like to run two homerun cables in parallel for redundancy. Is this OK? 
If you mean that these two cables are to be connected together electrically to provide some form of redundancy, then this is SPECIFICALLY NOT ALLOWED by the S50.02 Physical Layer Standard. H1 Fieldbus is a balanced transmission line that must be terminated on each end and may NOT be connected in parallel. Redundancy is not supported by H1. It is supported by Fieldbus HSE.

Of course, you may run a duplicate homerun cable for future use but it may not be connected in parallel. If a failure occurs because of physical damage, you need to disconnect the damaged homerun and connect the undamaged one. If you choose to run a duplicate cable as protection from physical damage, it should not be routed within the same cable tray.

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Where can I buy Foundation Fieldbus cable? 
Relcom does not manufacture cable. We stock fieldbus cable in small quantities, sufficient to wire a demo or staging. To wire a process facility, it is suggested that you contact Belden, Dekoron, or Northwire as domestic suppliers or Sumitomo, a Japanese supplier.

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Where can I buy Relcom Fieldbus products? 
Relcom Fieldbus products are available worldwide through MTL Instruments. Visit www.mtl-fieldbus.com (or in the U.S. call 1-888-746-4685) to locate your local representative.

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Should I ground the shield of the cable at each fieldbus device? 
Each cable that connects to a fieldbus device (transmitter) MUST NOT have the shield of that cable connected to ground at the device. The shield of the cable at the device (transmitter) is trimmed and covered with insulating tape or heat shrinkable tubing. This is to prevent ground loop currents from occurring within the segment. The only "ground" that occurs on the segment is the one that is chosen by the system architect, usually at the control room Fieldbus Power Conditioner.

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Why do you say not to have more than one ground point, yet the terminator in the field is grounded? 
The ground that is connected to the Terminator at the far end (field) of the segment DOES NOT connect to the shields of the fieldbus. It provides a high frequency path for AC but blocks the DC and low frequency AC path.

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How can you protect against lightning strikes? 
The Relcom wiring blocks cannot protect against a direct lightning strike to the fieldbus wiring. It is possible, however, to protect against voltage spikes induced from a near lightning strike. Our Terminators have a spark gap arrestor which will fire and clamp the shields in the event of a near lightning strike. For locations where lightning strikes are common, we recommend additional surge protection devices that are supplied by MTL.

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How do I confirm that my ground system is OK before I power up my segment? 
It is recommended that when the segment is wired, before the control room ground is attached to the Fieldbus Power Conditioner, a resistance measurement be made from the ground bolt on the Power Conditioner to the earth ground connection point. This measurement should be very high resistance - in the Meg ohms. If it is not, then one or more points within the segment are already attached to ground, probably at the devices (transmitters). All of these ground points MUST BE found and removed before the segment is made operational. Once the shields of the fieldbus read a high resistance with respect to ground, you may connect the earth protective ground to the Power Conditioner ground bolt. After this ground connection is made, measure the resistance from the cable shields on the Terminator at the far end (field) of the segment to a nearby earth ground point. This measurement should be very low resistance, in the low ohms. If it is not, then one or more points within the segment do not have the shields attached to the wiring blocks. Find these areas and correct them by making certain all shields are attached at each wiring block, and NOT grounded at the devices (transmitters).

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How do your power supplying devices react to (recover from) short circuit conditions? 
The answer to this depends on the exact Power Conditioner model that you are using. All of our Power Conditioners have an internal electronic fuse that opens to remove power from the segment under over-current or short circuit conditions. The "fuse" automatically resets when the fault condition is removed. See the individual Product Specifications to determine the exact behavior of the device.

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Can I use a "standard" power supply to power a fieldbus segment? 
In a word, NO. A "standard" power supply would absorb all of the fieldbus signals because of its low internal impedance. A "standard" power supply may be used to provide power to a fieldbus power conditioning device, or power supply impedance, as long as the requirements of the Power Conditioner are met.

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Do your wiring blocks filter out noise and ripple? 
The Fieldbus Power Conditioners do NOT do any power regulation, nor do they filter any of the noise or ripple that may be present from the input power supply. They are the electronic equivalent of a 5 mH inductor in series with a 50 ohm resistor. Therefore, any ripple or noise that is present on the power supply will also be present on the fieldbus. The Power Conditioners must be powered from a "quiet" power source in order to have a "quiet" fieldbus.

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Do you have a power supply I can use for fieldbus? 
We do not manufacture devices to convert AC Mains power to 24VDC. All of our Fieldbus Power Conditioners require a nominal 24VDC power supply. MTL does manufacture a Class I Division 2 power supply. Please see their web site at www.mtl-fieldbus.com

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Do you have an "all in one" fieldbus package? 
Relcom specializes in the "physical layer" aspects of fieldbus. We do not manufacture fieldbus devices or computer interface cards. We have a bundle of the following: Fieldbus Power Hub with wall power supply, 2 each 1.5 meter cables with molded DB-9 male connector on one end and tinned wires on the other, and 2 each 1 meter flexible cable prepared for attaching from the Power Hub to a Fieldbus Device. The DB-9 connectors attach to a PC based fieldbus controller or analyzer card. The Power Hub provides power for up to ten fieldbus devices, and includes two internal terminators. This bundle is the FCS-Labkit. PC interfaces are available from National Instruments.

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Can I use two power supplies and a couple of diodes to power up your Power Conditioners? 
Our Redundant and Dual Power Conditioners already supply this feature for you. There are two pairs of terminals that allow you to connect two Power Supplies to the unit. This provides redundant power feeds to the Power Conditioners.

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Can I run two Power Conditioners on one segment in case one fails? 
This can only be done if the Power Conditioners are designed to be connected in parallel. Connecting two Power Conditioners in parallel changes the impedance of the Fieldbus and if they are not designed for this, you will have communication problems. Our Redundant Power Conditioners provide this functionality in a convenient package.

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How is Profibus PA different from Foundation Fieldbus? 
Both Profibus PA and Foundation Fieldbus share the same Physical layer definitions. Therefore the connection and power equipment that Relcom makes will work identically for both types of busses. The difference is in the protocol. Therefore, our FBT-3 Fieldbus Monitor will not work with Profibus PA as it was only designed for Foundation Fieldbus. Please see our Links page for URLs to the standards organizations for more information on these busses.

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What are the most common Fieldbus wiring errors? 
There are two very common wiring errors that will cause operational difficulty if not remedied. The first is a short from the shield to one of the Fieldbus conductors. Although this may not immediately cause communication errors, it ground references (since the shield is grounded) the Fieldbus pair and makes it much more susceptible to electrical interference. The second most common problem is an incorrect number of Terminators. There should be two and only two Terminators on each Fieldbus segment. Since the Fieldbus installation will likely be installed by contractors who are unfamiliar with Fieldbus, we recommend a Segment Checkout Procedure BEFORE powering up or commissioning the segment. Many host suppliers have such procedures. The Fieldbus Foundation also publishes an engineering document for Fieldbus that contains such a procedure.

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