wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

34-2 Industrial Communication Systems
Application
layer MPS SUB MMS
Layers 3–6
Data
link
layer
Physical
layer
Empty
Read/write/transfer of buffer
Request/transfer of ack/unack
TDMA protocol
Manchester II biphase coding
Power on the bus/intrinsec safety
Fieldbus
management
FIGURE 34.1
Layered architecture of WorldFip.
34.2 Physical Layer
PHL uses Manchester type encoding. Three versions have been defined: a first version, called S1, operates
at 50.kbps using twisted pairs; the second version, S2, operates at 1.Mbps; and the third version,
S3, operates at 2.5.Mbps. Both S2 and S3 versions use coax or fiber optics. Two interesting features of
PHL concern the use of intrinsic safety barriers for operations in hazardous environments and the
possibility to have power on the bus that can be used by field devices without the need of a dedicated
power line.
With reference to the topology adopted in WorldFIP, a minimum system is set up by a single trunk
with several stations, of which one has the role of bus arbitrator (BA) and manages the access to the
medium. Three kinds of devices can be connected to the bus:
• Junction boxes: These devices are multiport repeaters that connect a cluster of field devices to
the bus.
• Field concentrators: These devices are gateways that connect field devices that use the WorldFIP
protocol or different protocols. Moreover, they can be used to interconnect several independent
WorldFip trunks.
• Repeaters: These devices are used to extend the bus length by connecting two or more trunks.
34.3 Data Link Layer
DLL is split into a MAC and LLC sublayers. MAC is based on a time division multiple access protocol
(TDMA), where a special station called the BA makes the scheduling of all transmissions through the
distribution of a specific authorization. Any of the several WorldFIP stations active on the bus can operate
as the BA (this allows for redundancy in case of a fault in the current BA) but at each time instant
only one BA can be in the bus.
The presence of a centralized control for the access to the physical medium is an important feature
of the MAC protocol. The BA knows the transmission requirements of all the devices present in the
system and can distribute the bandwidth available in such a way so as to satisfy their time constraints.
This approach is particularly efficient in cases when most of the traffic is periodic since the BA can
authorize the transmission at the correct time. On the other hand, this approach is ineffective for
asynchronous traffic that cannot be known a priori and cannot be scheduled. For asynchronous traffic,
the protocol must use an approach that allows the BA to became aware of additional requests and
update its schedule.
© 2011 by Taylor and Francis Group, LLC