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Application Note

Foundation

™

Fieldbus:

System and Diagnostic Basics

From the Fluke Digital Library @ www.fluke.com/library

The current trend in factory automation is to replace

traditional control schemes in which each device has its

own control wiring with bus systems that link a number of

devices via the same cable. One benefit of bus networks

is that they require far fewer cables and wires to connect

devices to controllers. One of the most popular and widely

used of these bus systems is Foundation Fieldbus.

Figure 1: Basic structure of a Fieldbus set-up.

Developed and administered by

the Fieldbus Foundation, which

was formed by a group of manu-

facturers of factory automation

equipment, sensors and actuators,

Fieldbus includes two different

protocols to meet different needs

within the factory automation

environment. The two use differ-

ent physical media and commu-

nication speeds.

The first protocol is H1, which

operates at 31.25 kb/s and gen-

erally connects to field devices

– sensors, actuators, valves,

control lights, I/O devices, etc.

– and allows for two-way com-

munication between devices and

a controller. H1 provides both

communications and power over

a two-wire system. Standard,

shielded twisted-pair wiring is

recommended to reduce noise

interference on the network.

The second protocol is HSE

(High-speed Ethernet) protocol.

It operates at 100 Mb/s and

typically connects high-speed

controllers such as PLCs, multiple

H1 subsystems (through a linking

device), data servers and work-

stations. This application note

focuses on the H1 protocol.

Network structure

The basic structure of a H1

Fieldbus network is shown in

Figure 1.

The network comprises the

main network cable, which

interconnects a series of junction

boxes or couplers. The couplers

allow the devices and the con-

troller to be connected to the

main cable or trunk. In general,

the shorter cables between junc-

tion boxes and device are called

spurs.

Junction boxes can be built to

connect single or multiple devices

to the trunk. If each device has a

dedicated junction box the topol-

ogy is called a spur topology. If

multiple devices are connected

to the same junction box, the

arrangement is typically called

a chicken foot or a tree topol-

ogy. Most common are mixed

networks with both spur and tree

topologies, as in Figure 1.

While it is theoretically pos-

sible to route the trunk directly

from device to device without

using junction boxes, the foun-

dation recommends against it.

Such a topology (called a daisy

chain) requires an interruption

of the trunk every time a device

is removed or added to the net-

work.

Fieldbus’ technology imposes

limitations on the size of a net

2 wires + shielding

terminator

Fieldbus

Controller

Device 1 Device 2 Device n-1 Device n

DC Power

Source

Fieldbus

Power

Supply

Junction

Box

Junction

Box

Junction

Box

Junction

Box

Control Room

Factory Floor

terminator

100

1µ

Sh Sh

100

1µ

WN AG

Summary of content (4 pages)

PAGE 1
Foundation™ Fieldbus: System and Diagnostic Basics Application Note The current trend in factory automation is to replace traditional control schemes in which each device has its own control wiring with bus systems that link a number of devices via the same cable. One benefit of bus networks is that they require far fewer cables and wires to connect devices to controllers. One of the most popular and widely used of these bus systems is Foundation Fieldbus.
PAGE 2
work. The maximum length of all the wiring in a trunk and its spurs added together is 1900 m (approx. 6250 ft) per section. If more length is required, one can add a section using a repeater. A repeater takes the place of a device, but adding it allows for another 1900 m of cable. A network may use as many as four repeaters for a total length of 9500 m (31000 ft.). Note that the shielding is connected to earth-ground at only one point in the entire system, and that is important.
PAGE 3
As noted, an H1 Fieldbus network operates at a speed of 31.25 kb/s, which equals a clock cycle of 32 µs. So, when there is a cable anomaly, one should expect to see reflections of pulses delayed by an amount of time up to 19 µs. The actual time of the reflection depends upon the distance between the pulse source and the anomaly. While a complete short circuit will cause a full-amplitude reflection, any disruption or deviation from the homogeneous nature of the line will generate a reflection.
PAGE 4
Electrical signal generation If one were to open up the cable at any point along the trunk and access the two wires, he or she would be looking into two sections of cable, going in opposite directions away from the access point. Electrically, those two sections would be seen as connected in parallel. The impedance at any point along the line therefore is equal to the impedance of two sections of cable in parallel.