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 FIELDBUS FAQ

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 standard.  H1 Foundation 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.

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.

In practice, the Trunk or Homerun cable is protected and considered reliable enough to not require redundancy.

Where can I buy Foundation Fieldbus cable?

Relcom does not manufacture cable.  We stock fieldbus cable in small quantities for customer convenience.  For larger quantities to wire a process facility it is suggested that you contact Belden, Dekoron, or Northwire as domestic suppliers or Sumitomo, a Japanese supplier.

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 Supply.

Single point grounding is recommended to prevent ground loops which would likely cause noise.

Doesn't the ground on the Relcom terminator cause a ground loop?

The ground that is connected to the Terminator at the far end (field) of the segment DOES NOT directly connect to the shield of the fieldbus Trunk.  It connects to a surge protection device that has no DC connection to the shield.

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 Eaton (MTL Products).

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 Supply, a resistance measurement be made from the Trunk Shield at the Fieldbus Power Supply 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 on 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 Trunk shield (usually at the Fieldbus Power Supply). 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).

Can I use a standard 24VDC power supply to power a fieldbus segment?

In a word, NO.  A 24VDC power supply would absorb all of the fieldbus signals because of its low internal impedance.  Fieldbus Power Supplies are specifically designed to allow Fieldbus signals to exist on the Fieldbus cable.  They are tested to meet the Fieldbus Foundation's standards to ensure proper operation on a Fieldbus Segment.  A Fieldbus Power Supply is typically powered by a 24VDC power supply.

Do your wiring blocks filter out noise and ripple?

The Fieldbus Power Supplies are designed and tested to meet the requirements of a Fieldbus Segment and so do not generate noise or ripple that would adversely affect Fieldbus Communication.  Most noise comes from the field and can be cause by ground loops, VFDs, welding, etc.  Normally Fieldbus is fairly immune to noise but in extreme cases it can be affected.  There are no devices available to filter noise on a Fieldbus Segment.

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 F11-Labkit.  PC interfaces are available from National Instruments, Softing, and others.

Can I run two Fieldbus Power Supplies on one segment in case one fails?

This can only be done if the Fieldbus Power Supplies are designed to be connected in parallel.  Connecting two Fieldbus Power Supplies in parallel changes the impedance of the Fieldbus and if they are not designed for this, you will have communication problems.  Our Redundant Fieldbus Power Supplies provide this functionality in a convenient package.

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 supply equipment that Relcom makes will work identically for both types of busses.  The difference is in the protocol.  Therefore, we make two versions of the FBT-6 Fieldbus Monitor - one for Fieldbus and one for Profibus PA  (FBT-6-PA).  Please see our Links page for URLs to the standards organizations for more information on these busses.

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 Fieldcomm Group also publishes an engineering document for Fieldbus that contains such a procedure.