The Foundation Fieldbus network

I’m sure you’ve heard of Foundation Fieldbus (FF), one of the many protocols available to connect your field devices. This system uses a digital, multidrop, serial two-way communication to connect your devices.

The old Fieldbus Foundation, now known as FieldComm Group, developed this protocol to replace our old friend analog. The group created it to work for all types of industries, and it has become popular in petrochemical, refining, nuclear, and other segments.

Image of Foundation Fieldbus
Image courtesy of fieldcommgroup.org

FF comes in two flavors, H1 and HSE (high-speed Ethernet), and they differ in physical media and communication speed. We’ll take a look at both in this article.

Today, FF has to fight for its spot in a new application against newer protocols that have begun popping up in the automation world. However, the protocol still brings a lot of benefits and unique features that can improve your process control.

Let’s find out how it works, how to use it, and its pros and cons.

What is Foundation Fieldbus?

This digital protocol enables two-way communication between your field devices and control system. It also simplifies startup and configuration and allows access to diagnostic data, which can save money and time.

Image of Foundation Fieldbus
Image courtesy of automation.com

So you can support a new application or even an entire plant using this technology. Let’s now take a look at how it works and its two types.

Peer-to-peer

The FF network uses the peer-to-peer concept, meaning that field devices can talk to each other without waiting for a command from a master. That means they can exchange measurements, diagnostics, alarms, and even control in the field. However, the network also allows temporary masters, such as handhelds, laptops, or tablets.

FF uses function blocks to configure the devices and all control logic. This  feature provides a lot of freedom with its use. Now let’s get into the types.

H1

FF H1 uses the IEC 61158-2 standard. The data transfer uses Manchester coding at a transfer rate of 31.25 kilobit per second (Kbps). You’ll use H1 on field devices that don’t require high-speed communication, as the protocol sends signal and voltage through the same pair of wires.

Furthermore, each device requires at least nine volts of juice and a power supply installed parallel to the trunk. If the device needs more energy, then it should have its own power supply outside the network.

To optimize your network, some software can calculate the voltage, current resistance, and supply voltage based on the network topology. You must make sure that the network consumes less current and voltage than the power supply provides.

Relevant rules of the physical layer:

  • FF devices can communicate with between two and 32 devices if you don’t need intrinsic safety and don’t have loop power. For an intrinsic, loop-powered application, you can have up to 16 devices. For loop power with no intrinsic safety, you can go from from one to 24 devices.
  • The length of the segment can’t exceed 1900 meters over trunk + spurs using type-A cable and baud rate of 31.25 Kbps.
  • You can’t have more than 4 repeaters on the network.
  • The FF network should continue working when you connect or disconnect a device in the network.
  • In a communication failure, it cannot affect the communication for more than one millisecond.
  • You must respect polarity to power the devices, and the devices can’t be sensitive to polarity inversion.
  • When you have a redundant system,  you can’t include a non-redundant segment between two redundant segments. The repeaters should have redundancy as well.

HSE

FF HSE, based on the Ethernet protocol, operates at 100 megabits per second (Mbit/s). Also, you can use Ethernet cable, optical fiber cable, or PoE (Power over Ethernet) technology. PoE has an interesting feature – natively HSE doesn’t provide energy for the device, but it can work with PoE.

Usually, you’ll see gateways, linking devices, host systems, input/output (I/O) systems and similar devices connected to HSE. It operates with a random carrier-sense multiple access (CSMA) bus access.

Furthermore, you can use Ethernet switches for a partial HSE network and create a more extensive network later.

How to connect H1 to HSE?

We call the connection between H1 and HSE a bridge; this application creates a standard network topology that you can see here:

Image of Foundation Fieldbus H1
Image courtesy of emersonexchange

The bridges connect the H1 network to the high-speed Ethernet, converting data telegrams and data transfer rates depending on the direction of the transmission.

The components of a Foundation Fieldbus network

Now let’s discuss the components you need in an FF network and describe the function of each.

H1 cards: These work as your interface with the FF devices. In a control system working with H1, you’ll have the H1 cards connecting your network at the controller or the I/O subsystem. Here you can have redundant cards if you need the backup.

Power supply: FF can power more than one H1 segment. You can find redundant power supplies, intrinsic models, and also models with signal conditioners and segment terminators built in.

Signal conditioner: To provide robust communication, you need to condition the power supplies. FF also adds inductance between the field devices and the power supply, protecting the signal from the low impedance of the power supplies and more.

Segment terminator: The network requires one segment terminator at each end of the trunk. The segment terminator matches the impedance of the cable and a balanced transmission line.  You need no more than two terminators on the network.

Wiring: The FieldComm Group provides specs for your wiring (FF-844), but primarily you should have a shielded and twisted-pair cable. This cable can maximize the network length and reduce signal reflections.

Device coupler or junction box: This unit will distribute the network to the spurs. It also operates as a point for maintenance, diagnostics, and circuit protection. Furthermore, you’ll install all devices in parallel in a junction box.

Foundation Fieldbus topology

Let’s take a look at some topologies and talk about their characteristics.

Image for Foundation Fieldbus topologies
Image courtesy of alchetron.com

Point-to-point or daisy-chain: This layout has all field devices connected to each in series. That means that one device will link to the next to the end of the network, using their own terminals to make these connections. (You won’t see this one much because its structure can cause network problems during maintenance.)

Bus with spurs: Here, a single bus directly connects to spurs and devices. Moreover, you can have many devices connected to each spur, and the spur length will depend on the number of devices connected and even the number of spurs. This table can help as a guideline, but you should not consider it absolute.

Image courtesy of FieldComm Group

Tree: The trunk of the network connects to several device couplers or junction boxes, and then each has many devices connected to them. You’ll sometimes hear this one called chicken-foot or star topology.

End-to-end: This topology gives you two scenarios. In the first, your field device connects directly to the H1 card. In the second, your device connects to the network, and then another device connects to this device in the network. For instance, you can have a pressure transmitter connect directly to its final control element.

Mixed: As the name says, here you can have a mix of some or all of these topologies. However, you must pay attention to the maximum length of the segment, based on the number of devices, length of spurs, consumption of current, and more.

Function blocks

FF has three types of blocks for data, functions, and more. Each device type will have a designation based on its function.

Image of Foundation Fieldbus Blocks
Image courtesy of fieldbus.org

Resource block: This block contains all the characteristics of the field device, such as serial number, device name, firmware version, and hardware.

Transducer block: This block expands your possibilities with data that can influence input and output parameters. It means you can use it to calibrate, linearize, convert units, and more.

Function block: This block designates what the device does and defines how to access it. Each device has at least one function block, and each block has a particular task for the input and output. This list names function blocks defined by the FieldComm Group:

  • AI: analog input
  • AO: analog output
  • B: bias
  • CS: control selector
  • DI: discrete input
  • DO: discrete output
  • ML: manual loader
  • PD: proportional/derivative
  • PID: proportional/integral/derivative
  • RA: ratio

What’s the difference between Foundation Fieldbus and PROFIBUS?

Both networks have a lot of similarities when it comes to FF H1 and PROFIBUS PA applied at the field instrumentation level. For both, you only use a twisted-pair cable to power and communicate with the device. They have similar architectures when you scale their networks, too.

But on the operational side, we found a big difference. The PROFIBUS PA network uses the master-slave concept while FF H1 goes peer to peer. This means that PROFIBUS PA field devices must wait for a master command. But in the FF H1 network, the devices can talk to each other freely. In theory, the PROFIBUS PA network has a simpler configuration because you have fewer blocks, unlike FF devices.

But which would work best for you? You’ll have to plug in your specs and see!

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