Page 247 - Instrumentation Reference Book 3E

Fieldbus function and benefits 231
or eight if a trapezoidal FISC0 (Fieldbus Intrin- devices related to several loops 011 a single pair
sic Safety Concept) barrier such as the Smar of wires. However. this is no different from
DF47 is used. The above-mentioned barriers have connecting devices related to several loops to
an important feature in that they both have the same I/O module in a traditional architec-
repeaters built in on the safe side, allowing them ture. A module failure or accidental removal
to be multidropped on a safe side network seg- would affect all loops. To increase availability
ment such that a total network of 16 devices can for so-called critical loops, users typically
still be achieved per interface port keeping the spread the devices related to these on separate
interface quantity and system cost down. segments, such that each segment affects only a
Because of the relatively low speed no special single critical loop, the rest of the devices on a
wiring or connectors are required for fieldbus. segment being non-critical control or nionitor-
Normal shielded twisted-pair instrument cable ing. This is particularly comnion in the petro-
can be used for installation. Use of 0.8mm' chemical and chemical industries where safety
(AWG# I. 8) gauge wire is recommended in order barriers are used which conveniently divide
to achieve a full 1.9 km overall bus length for a the fieldbus network up in several isolated seg-
fully populated network. For shorter distances or ments.
fewer devices thinner wires such as in multi-core Because FOUNDATION'" Fieldbus H1 and
cables can be used. The latter is ideal for the Profibus-PA have identical physical layers, the
"homerun" cable from the field junction box to networking accessories. such as repeaters, power
the control room. Distance can be increased using supply impedances. safety barriers. and termin-
up to four repeaters in series. ators. are the same. However, the data link and
The fieldbus network segment must be termin- application layers are different which gives them
ated in each end. In the far field-end this is done different characteristics.
using a terminator such as the Smar BT302. In
the host-en'd the terminator should preferably
already b'e built into the network linking device 13.12.1. I FOUNDATIONT-" Fieldbus HI
or power supply subsystem. Power for the field
FOUNDATIONTM Fieldbus allows multiple link
instruments is provided from the field end. For masters to be connected on the network. At any
non-intrinsically safe installations the power one time only one of them acts as a Link
comes from a regular 24V d.c. power supply Active Schedule (LAS). The LAS controls the
through the power supply impedance module, communication ensuring that messages are sent
such as the Smar DF53, whose purpose is to pre-
at the exact right time. This ensures that control
vent the d.c. power supply from short-circuiting variables are transmitted in a precise interval
the a.c. communication signal. Two DF53 units eliminating jitter. FOUNDATIONTM Fieldbus
may be connected in parallel to provide redun- synchronizes the function block execution with
dant power for high availability. There is no limit- the communication minimizing delays even
ation for the current that can be drawn by a further. Unscheduled traffic does not affect
field device, but selecting devices consuming scheduled traffic. FOUNDATIONTM Fieldbus
12mA or less is required to achieve long wire
runs and to put several devices on safety barrier. provides for three types of communication rela-
tionships:
In older :systems fitted with fieldbus the power
supply subsystem is implemented using external Client-server: One device, called client. requests
components. a value such as a tuning parameter requested by
The purpose of a linking device is to bridge the host from a positioner on the network, called
communication between one or more field-level server, which responds with the value.
networks up to the host level network, e.g., Ptiblisher- subscriber^ Initiated by the LAS
from H1 io HSE (100Mbit/s on Ethernet) schedule one device, called publisher, broadcasts
FOUNDATIONTM Fieldbus or from Profibus a message such as a process variable from a
PA to DP (up to 12 Mbit/s on RS485). A linking transmitter on the network. All devices that need
device has the intelligence to buffer messages the variable, called subscribers, such as a valve
allowing them to be transmitted on the two net- positioner with a controller in it take it. Thus
work levels at different speeds. A simpler device several devices can receive the same variable in a
called a coupler does not provide this buffering, single communication. Direct peer-to-peer con-
thus forcing the host level network to operate at a munication eliminates the need for a request by a
much lower speed close to that of the field level centralized device and subsequent retransmission
network. The host level network in turn connects to each individual user after the response. This
the linking devices to the workstations. reduces the communication load. The publisher-
At firs: glance there may be some concern subscriber functionality is a key enabling control
regarding the availability of multidropping in the field devices.