What is FOUNDATION Fieldbus
FOUNDATION Fieldbus is a Networked Serial Bus System designed to replace the standard 4 to 20mA control system in the process industry. The transmission technology for the system was defined in 1994 with the publication of the international standard IEC 61158-2 (later integrated into the European standards as EN 61158-2). This same standard serves as the transmission technology for both FOUNDATION fieldbus and PROFIBUS-PA, although the logical implementation of these two networks is significantly different. One of the key benefits of FOUNDATION fieldbus, as with network systems in general, is the dramatic reduction in wiring. The FOUNDATION fieldbus H1 system carries data and power for all devices on a single pair of wires, as opposed to the traditional need for a separate wire pair for each device. The physical wiring and electrical signal specification is also suitable for use in hazardous (classified) areas when appropriate energy limiting technology is incorporated. FOUNDATION fieldbus also supports a second physical communication specification, referred to as High Speed Ethernet (HSE). The HSE system is typically used as a higher-level communication network, acting as a backbone to link several H1 segments together through "linking devices" (referred to as LDs).
FOUNDATION fieldbus is unique among the popular open industrial networks in that it uses a true distributed control structure. I/O devices in the field communicate process variable and feedback information directly to each other and contain the necessary function blocks for the control scheme to operate. This makes it especially suitable for large scale process applications where reliability and continuous operation are more important than speed.
Another major benefit of FOUNDATION fieldbus over a traditional hard wired system is the reduction in installed material (wire). While the hard wired system requires that a pair of wires be pulled for each instrument in the field, the FOUNDATION fieldbus system uses a single pair of wires for all the devices in a particular segment. This results in significant savings on the cost of the wire itself, and more importantly the cost of installing the wire. Additionally, the system is able to be installed faster, resulting in the plant being commissioned sooner.
• Lower field wiring cost
• IS capability reduces cost of hazardous area installs
• Control loops performed by devices in the field
• Time stamp applied to data in the field
• High level of process information
• Standardized control function blocks
• 1900 m length (120 m spurs)
• Wide manufacturer support
FOUNDATION fieldbus supports up to 16 or 32 devices on a segment (depending on the host system), but in practice most segments are much smaller than this. Common practice is to keep essential control loops on separate segments from each other in order to enhance the security of the entire process (if a segment is lost the system may still continue to function).
The DPC-System (Diagnostic Power Conditioner System) is a power supply system for the installation of FOUNDATION fieldbus H1 segments. It provides comprehensive diagnostic functions for the monitoring of FOUNDATION fieldbus segments and supports asset management for the entire system.
A DPC system consists of one or more module racks (DPC-49-MB-RC) each with up to eight power supply modules (DPC-49-IPS) and one diagnostic module (DPC-49-ADU). Up to four H1 segments for each module rack can be operated and monitored redundantly. The diagnostic data from the H1 segments are transmitted via the HSE interface module (DPC-49-HSEFD/24VDC) to the higher level asset management system.
The diagnostic module (DPC-49-ADU) is used as a communication and diagnostic interface between the H1 segments and the power supply module. The diagnostics module monitors the electrical parameters and the communication parameters of the H1 segments. Operation without diagnostic module is possible. In this configuration, simple diagnostics are provided locally.
The diagnostic information is collected in the device and transmitted via the HSE interface module to the higher fieldbus level (e.g. to the host) as diagnostic and alarm data. The diagnostic module can be plugged and unplugged during operation (hot swapable).
Fieldbus - The dynamic asset
Information concerning the components of the control system and field devices are typically stored and monitored by that system. Information on assets that make up the communication infrastructure (physical layer components) have been simply stored in an asset management system. With the DPC system, the physical layer components are continuously monitored providing virtually instantaneous information regarding the quality and the status of the communication link.
This aspect of the system is the key to achieving the main objective of asset management to minimize maintenance and lower system operating costs.
TURCK has drastically improved on existing physical layer components for use in FOUNDATION fieldbus applications. The introduction of this system allows the continuous monitoring of every physical layer component, thus treating the entire physical layer as an asset and providing the means for it to be managed as such.
The DPC System detects errors that may develop over an extended period of time or through typical failure modes. These changes can occur due to many factors, such as environmental changes, deterioration of components over time, and any other factors that may affect the physical components of a fieldbus segment. Some of these factors may appear as changes in jitter, hum, noise levels etc. Alarm strategies may be employed that will warn of typical asset errors, potential errors or failures. Preventive measures can be implemented well in advance of a potential system failure. Most common failures can be completely avoided when a preventive maintenance schedule is implemented. The DPC system also supports the set-up of fieldbus assets by using expedient localization of error sources, as well as documentation indicating a "good condition" of the segment structure.
The DPC system provides an option for redundant segment supplies. The system, fully loaded, can accomodate up to
16 fully redundant FOUNDATION fieldbus segments each with an output of 800 mA and 30 VDC. Diagnostic date is available via a DTM, standard FOUNDATION fieldbus function block libraries or an embedded web server in the HSE field device.
In a traditional control system I/O devices in the field are individually wired to a central controller, which is responsible for all control function processing in the system. This type of system typically consumes a lot of physical space (due to the amount of wire and the number of I/O cards in the PLC or DCS) and requires a lot of design and labor to install. Additionally, finding errors in this kind of system can be very time consuming because of the number of possible error points (each physical wire termination).
In the fieldbus system the I/O devices are wired to a trunk line (segment) using tee connectors or multi-drop boxes. Rather than separate pairs of wires carrying data to and from each I/O device, the devices use a common pair of wires for communication, with each having a turn to "talk" on the network. Instead of performing all the control functions in the host, the FOUNDATION fieldbus system allows for control blocks to be executed in the field devices themselves, creating an efficient, high integrity system. One device on the network is responsible for scheduling communication between the various devices on the system. This is called the Link Active Scheduler (LAS). It can be the host interface or a device in the field. In most FOUNDATION fieldbus systems at least one backup LAS is defined as well. This allows communication and control to continue in case the original LAS device fails. Most FOUNDATION fieldbus devices are powered completely from the network supply. In some cases a device may draw enough current to make it impractical to power it from the network. In these cases the device is typically powered from a separate (auxiliary) supply.
Another key benefit of using FOUNDATION fieldbus is the ease of adding I/O devices to the system in the future. Because it is a serial bus where all devices use the same wires for communication, a device can be added by simply splicing it onto the network. This eliminates the need to pull a new wire pair all the way back to the controller.
FOUNDATION fieldbus devices also typically include a multitude of parameters and diagnostic information, all accessible over the network. Advanced diagnostics and maintenance scheduling are made much easier with this feature.
The FOUNDATION fieldbus H1 communication signal is a square waveform superimposed on a DC carrier. The frequency of the signal is 31.25 Khz. Although it is not a requirement, most devices derive their supply power from the fieldbus communications cable. The fieldbus specification states that devices must not be polarity sensitive. However, it is good electrical practice to have all devices wired with the same polarities. The voltage range allowed for proper operation is 9 to 32 VDC. A typical fieldbus device will consume 20 mA of current.
Fieldbus Cable Specifications
The specifications for fieldbus H1 physical media are defined by IEC 61158-2 and the ISA-S50.02 Part 2 Physical Layer Standards. The same standard is also listed in the FOUNDATION fieldbus specifications under 31.25 Kbps Physical Layer Profile FF-816-1.4. There are essentially four types of cable designations for fieldbus. Type A cable preferred for new installations, because it allows for the most versatile lengths. The other cable types are for installations where cable already exists from 4-20 mA systems. See table below.
|Type||Cable Description||Conductor Size||Maximum Length|
|Type A||Shielded, Twisted Pair||18 AWG||1900 m (6232 ft.)|
|Type B||Shielded, Multi Twisted Pair||22 AWG||1200 m (3936 ft.)|
|Type C||Unshielded, Multi Twisted Pair||26 AWG||400 m (1312 ft.)|
|Type D||Shielded, Untwisted Pair||16 AWG||200 m (656 ft.)|
TURCK offers type A cables with both two conductors and three conductors, with the third conductor available for a centralized ground of devices if needed. TURCK cables meet or exceed the specifications of ANSI/ISA-SP50.02-1992, the fieldbus standard for use in industrial control systems.
The maximum spur length is determined by the number of devices in the segment.
|Cable||Number of Devices||Maximum Spur Length|
|Trunk||25-32||0 m (0 ft.)|
|19-24||30 m (98 ft.)|
|15-18||60 m (197 ft.)|
|1900 meters||15-18||60 m (197 ft.)|
|13-14||90 m (295 ft.)|
|2-12||120 m (394 ft.)|
The FOUNDATION fieldbus communication signal requires that each end of the system be terminated with a 1 µF capacitor in series with a 100 Ω resistor across the communication lines. This termination must be installed at each extreme end of the network segment. Do not use more than two terminators on a communication segment.
Hazardous Area Usage
FOUNDATION fieldbus networks may be used in hazardous areas as long as required energy limitations for the specific area are observed. One way to achieve this is to use the "entity" concept, which requires the network designer to calculate the voltage and current requirements for each device and determine the system limitations.
A simpler option is to use the Fieldbus Intrinsic Safety Concept (FISCO) or Fieldbus Non-Incendive Concept (FNICO). These concepts define the limitations required for devices on a network system to be used in a hazardous area (Class I, Div 1 for FISCO and Class I, Div 2 for FNICO). Many newer FOUNDATION fieldbus devices are rated to meet the requirements of FISCO and/or FNICO. As long as the devices used and the power supply are marked with FISCO or FNICO they may be connected together in the appropriate hazardous area. It is important to note that the cabling used must still meet the defined parameters.
Plug-and-play connectorization has been standard practice for many years in industries ranging from appliance manufacturers to industrial sensors. These industries have found it necessary to compete in a business climate where speed and consistency of connection is king. Connectorization is the perfect complement to fieldbus systems. The concepts and goals are identical: reduce installation time, reduce troubleshooting and easy expansion. The fieldbus system minimizes point-to-point wiring that can be time consuming and difficult to troubleshoot. Connectorization takes that one step further, almost completely eliminating troubleshooting. Plants that have implemented plug-and-play connectorization claim up to a 75% reduction in start-up. This directly translates into real cost savings.
The initial capital cost is the major factor in selecting a method of connecting devices. These costs include material and installation. The cost of incorporating plug-and-play connectivity will be 10 to 60 percent less. Actual savings will depend on the size and complexity of the installation.
Other cost saving factors include reduced design cost, reduced maintenance cost, reduced troubleshooting cost and reduced expansion costs. Some of these cost savings are difficult to determine until the condition exists. However, these costs can quickly change from potential cost savings to real cost savings when the installation begins.
Design Cost Savings
Most projects begin with a rough definition to develop the capital scope and then progress to detailed development. Development of the capital scope is often expressed in terms of segments, transmitters and tanks. The cabling can be expressed in the same way. Each transmitter requires one device gland and one cordset. Each tank requires one tee, one drop cordset and typically one brick. The home run or trunk cable can run in either conduit or cable tray, so either a field wireable tee or a conduit adapter is required at each tank. A terminating resistor is needed at the beginning and end of the network. A simple estimated bill of materials can be developed as follows:
For: 4 segments, 50 transmitters, 10 tank process
|Device Glands||RSFV 49-0.3m/14.5||50|
|Cordset||RSV RKV 490-6M||60 (50 Transmitters + 10drops)|
|Field Wireable Tee||SPTT1-A49||10|
|Terminating Resistor||RSV 49 TR||8|
|Bulk Cable||CABLE, 490-300M||1|
Often for estimating purposes, an average length of cordset and segment length is assumed. In this example 6 m (20 ft.)
cordsets and four 75 m (250 ft.) segments are estimated.
cordsets and four 75 m (250 ft.) segments are estimated.
The cost and time of coping with continuous changes during the engineering design phase can be very expensive. However, with this model the changes are limited to the length of the cordset and spool of bulk cable. Design changes can even wait until all the transmitters are mounted. Simply taking physical measurements is as valid as any other design method.
The cost of cordsets and bricks will be slightly lower than the cost of termination in enclosures. The plug-and-play junction bricks are IP 67 rated (equivalent NEMA 4X). This means they can be mounted indoors or outdoors without any secondary enclosures. A NEMA 4X enclosure can cost anywhere from $75 for steel to $275 for stainless steel. The cost can increase by another $40 to $60 for the design and installation time required to put mounting holes in the enclosure and installing cable glands. A cage clamp style termination block costs $200 to $450 depending on whether is has short circuit protection. The plug-and-play bricks cost only $322 and $486 depending on whether they have short circuit protection. A set of six cordsets costs only $264 (RSV RKV 490-1M)*.
The material cost comparison for a stainless steel installation is as follows:
NEMA 4X Box (Hoffman or equivalent) $275
Cage Clamp Termination Block $450
Installation of block in box $50
Bulk cable (6 meters) $12
Device gland (1/2 NPT fitting - $8) $48
Junction brick (JBBS 49SC-M613) $486
Cordsets (six RSV RKV 490-1M - $44.00) $264
Device gland (RSFV 49-0.3M/14.5 - $30.30) $181
Total $ 931
A junction brick system that is completely encapsulated for use indoors or outdoors is equivalent to or approximately
10% more expensive than a termination block mounted in an enclosure. The real savings are in the speed and ease of installation.
* Costs given are examples only, and are subject to change.
The cost of installing a plug-and-play connector system can be 90% lower than terminating in cage clamps. The time required to make a plug-and-play connection is less than 30 seconds per connection. The time required to strip the jacket, prepare the conductors, feed the cable through a gland, insert the wires into the terminals and tighten the cable gland is 5 to 10 minutes per connection. This is further complicated when the installation is in a physically demanding location. At a labor cost of $28/hour per NECA labor units, this adds up fast. Terminating this many connections on just a 6-port junction brick means a difference of $38.73 as compared to $2.80. Further savings are often hidden since the wiring errors are eliminated.
Install 6 connectors to brick (1/2 min. ea) $1.40
Install 6 cables to device (10 min. ea) $28.00
Termination check of system (20 min.) $9.33
Total $ 38.73
Install 6 connectors to brick (1/2 min. ea) $1.40
Install 6 connectors to brick (1/2 min. ea) $1.40
Termination check of system (n/a) -----
Total $ 2.80
One of the most important features with plug-and-play connectors is that transmitters can be taken on and off the bus very quickly - with little or no disruptions to operations and no disruptions to the bus. A device can be added to a junction brick spare port much more easily than any terminal strip. Any conceivable change required in the process can be made simply by planning a couple of spare ports.