Foundation fieldbus high-speed Ethernet just the facts

Tutorial / Control Fundamentals

Foundation fieldbus (FF) comes in two flavors. H1 (Hunk 1) is a lowcost bus for devices at the lower level, operating at lower speeds like sensors, actuators, and transducers. High-speed Ethernet (HSE) is for interconnecting subsets of low-level devices.

It is a protocol that allows a controller to recognize a rich set of configuration and parameter information (device description) from devices that have plugged in to the bus.

FF HSE encapsulates the H1 protocol inside of TCP/IP, in much the same way Modbus/TCP encapsulates Modbus protocol and transmits via Ethernet.

WHAT'S THE DIFFERENCE?

Hi segments use standard 4-20 mA wiring and operate at 31.25 kilobits per second. H1 is a digital replacement for the old 4-20 mA standard. HSE operates on standard Ethernet links, typically at 100 megabits per second (Mbps). 10-Mbps HSE devices are available as well.

FF Hi links local islands of transducers and actuators, while HSE links controllers and individual H1 networks, transmitting high-level information over large distances.

FF caters to process control systems in which large amounts of data transmit at relatively low speed. Installations are often very large campuses with thousands of nodes. There is typically a great deal of analog data, and hazardous conditions often apply.

The protocol is sophisticated and even allows a device to transmit parameters relating to the estimated reliability of a particular piece of data. It uses a scheduler to guarantee the delivery of messages to address the issues of determinism and repeatability. Each segment of the network contains one scheduler.

HOW DOES HSE CONNECT TO H1?

The Fieldbus Foundation (www.fieldbus.org) defines a linking device for this purpose.There are four such types of devices in the specification:

* Class 42a is for configuration. It allows TCP/IP client access to extend from the HSE network down to the H1 device and allows the H1 (server) response to travel back up the H1 link to the HSE client.

* Class 42b is the same but also transmits events, not just configuration data.

* Class 42c is the same as 42b but adds publisher/subscriber messages,which can pass among devices across multiple H1 networks and between H1 and HSE.

* Class 42d is the same as 42c but adds extended services for flexible function blocks in H1 devices.

Physically, a linking device looks sort of like an Ethernet gateway with multiple channels that manage H1 subnetworks.

WHAT'S THE PHYSICAL LAYER DIFFERENCE?

There is no difference between HSE and Ethernet physical layers. Any Ethernet gear that meets IEEE specifications and the physical demands of the environment works.

Indeed, FF HSE actually operates on TCP/IP, which means that in theory, any physical layer that supports TCP/IP transmission will work. However, the Fieldbus Foundation specification calls for 802.3u Fast Ethernet as a physical layer.

In demanding environments where equipment faces corrosive chemicals or washdowns, be sure to select cables with insulation that withstands those chemicals. A good choice is polyurethane. Otherwise, chemicals can be absorbed by the insulation and degrade the transmission properties of the cable.

If cables will need to move or flex, specify a cable designed with this in mind. Also, most plastics (especially PVC) become brittle at low temperatures, so be aware of temperature ratings of the cable.

WHAT ARE THE TRANSMISSION PROPERTIES?

Category 1 cable works for telephone applications, while Category 3 works up to 16 Mbps and is quite common in existing installations. Category 5 works up to 100 Mbps and is the most popular kind of cable computer vendors sell.

Category 5E supports Gigabit Ethernet and is preferable if there is ever a possibility of system upgrade to 1,000 Mbps.This is unlikely in a control system, but it's something to consider.

One can use shielded twisted pair (STP), unshielded twisted pair (UTP), or multimode or single-mode fiber-optic cable. UTP is much more popular than STP.The shielded version will show improved performance only if you also use shielded RJ-45 jacks.

When selecting a specific grade of cable, all components and interconnects on the network must also be equal to that quality level. At 10 Mbps, cable quality is not much of an issue. However, if migrating to 100 Mbps is a possibility, then use either Category 5 or 5e, which are specific to Fast Ethernet.

A single-mode optical fiber cable will have an inner core diameter of 10 micrometers ((mu)m) or less. The most popular multimode fiber-optic cable is 62.5/125(mu)m. However, 50/125(mu)m is also available. At 10 Mbps, multimode fiber is the most prevalent. At 100 Mbps, both single-mode and multimode fiber are popular.

WHAT PRECAUTIONS SHOULD ONE TAKE?

When selecting HSE hubs, switches, and devices, one needs to take care. Process control plants have a lot at stake. An Ethernet hub or switch is the nerve center for information in an environment where mistakes are very expensive at minimum and can be catastrophic.

Downtime, personnel safety, and correct operation of one's equipment are all at risk. If the equipment is pampered by air-conditioned equipment closets, if your factory has no grease, oil or water, moving,or vibrating equipment and if there are never problems with power quality or availability, then office-grade hubs and switches may do just fine.

But if it's a typical setting with hot, dusty panels, washdowns, dirt, dust, temperature extremes, irregular power, motors, and vibration, then there will be a need for the added reliability that industrial-grade switches, hubs, and routers provide.

George Thomas is president of Contemporary Controls (www. ccontrol.com). Write him at intech@ccontrols.com, and ask for his complimentary Ethernet Slide Chart and book, 10 Issues to Consider before Installing Industrial Ethernet. Perry Marshall is the author of The Industrial Ethernet Pocket Guide (ISA Press, 2002). Write him at perry@perrymarshall.com.

Copyright Instrument Society of America Mar 2003
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