Digital fieldbus networks

It's not so easy to deploy digital fieldbus networks in existing process plants. Working to upgrade from discrete and analog connections to digital fieldbus networks presents a host of options and challenges.

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Dan Hebert, PEBy Dan Hebert, PE, Senior Technical Editor

The benefits of digital networks designed to connect field devices to host controllers and monitoring systems are well-known. Most process plant end users know that they can save money on installation costs, reduce maintenance costs and perform better diagnostics with digital field networks.

These benefits have caused most every new process plant constructed in the last five years to deploy one or more digital fieldbus technologies successfully.

But it’s not so easy to deploy digital fieldbus networks in existing process plants. Working around and with legacy HMIs, controllers and field devices to upgrade from discrete and analog connections to digital fieldbus networks presents a host of challenges.

For this article, we’ll define a digital fieldbus network as any one that transmits data in a digital format from field devices to host devices. This encompasses a wide range of protocols, from lower-level networks such as  DeviceNet and CAN, to higher-level networks, such as Foundation Fieldbus, Profibus PA and various flavors of Ethernet.

Let’s start with the host control and HMI system, which is probably a legacy distributed control system (DCS). Older DCSs were designed to accept a great variety of discrete and analog inputs. Modifying these systems to accept digital fieldbus signals can be very expensive or even impossible.

In such cases, the engineer has basically two alternatives. The first is to bypass the DCS and go directly to a different host. This can make sense if the data to be collected from the field devices is not needed by the DCS for regulatory control.

For example, if data collected from digital field devices is going to be used by maintenance-management software running on a PC, then the data should be brought directly to that PC. Every digital fieldbus network has inexpensive hardware available to connect field devices to PCs, so this is a simple and inexpensive option for connecting signals to the host.

Even if the first preference was to bring data into the DCS for analysis and display, it may be simpler and cheaper to first bring the data to a PC. PCs are wonderful data handlers, and there are wealth of options for converting data gathered by a PC in to a format recognizable by a DCS.

The alternative is to interface directly to the legacy DCS. It may be able to accommodate at least some of the available digital fieldbus protocols. If so, this can drive your choice of fieldbus technologies and be your best solution.

If this is not the case, you must find a way to convert digital fieldbus signals to a format recognizable by your DCS. Let’s say that you want to modify a remote area of your plant and increase the instrument count from one to nine. You have only one twisted-pair wire running from this remote area to the DCS.

The obvious solution is to use this twisted pair to carry a digital fieldbus signal that would multiplex digital data from each of the nine instruments. The problem is that your DCS does not accept digital fieldbus signals.

Moore Industries suggests a solution. “When there is only one twisted pair available to transmit multiple signals, a peer-to-peer solution is the answer,” says Gary Mathur, applications engineer at Moore Industries International.

“This solution uses a high-density input brick installed in the field and an output brick installed in the control room. The input brick receives multiple 4-20mA signals and transmits these signals digitally on a single pair to its peer output brick. The output brick then converts the digital signal back to the original analog 4-20mA signals, and these signals are wired directly into the DCS’s analog cards,” he adds.

What if your DCS does not have extra analog inputs, but does work with some older digital fieldbus protocols? Moore Industries and others make products that connect to existing analog and digital instrumentation and use existing two-wire cabling to transmit these remote signals to a DCS in various digital fieldbus protocols.

Another wrinkle occurs when existing process plants have many instruments with HART capability and a DCS that is not HART-compatible. In this case, a HART concentrator can be employed.  HART concentrators are typically installed in the control room. They can poll smart HART instruments and provide multiple analog and discrete output signals to a DCS.

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