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

WorldFip 34-7
Write service
Read service
Layer 7
Layer 2
Layer 1
V S
Variable
status
7 V S
Producer
2 Consumer
1
FIGURE 34.6
Transfer of a variable from the producer to the consumer.
Information timeliness is one main feature of WorldFip protocol, and to this aim when a user
receives a variable he or she can also receive information regarding its freshness. This is obtained
through a Boolean flag that can be bound to each variable to be consumed by the application
processes.
Figure 34.6 shows how the timeliness information is processed and transferred from the producer
process to the consumer. A producer process (in the left side of Figure 34.6) uses a write
service to store the variable value inside a suitable buffer, and a refreshment status check process
evaluates the timeliness of this value with reference to a theoretical refresh period (TRP). Each time
a new value is stored in the buffer, the status flag is set “true” and a timer is started with a duration
equal to the TRP. If the timer expires before the variable is refreshed with a new sample, the status
flag is reset to “false.” In this way, when the consumer process receives the variable, it can know if
its value has been correctly refreshed or not.
34.5 timing Properties of WorldFIP Networks
As stated earlier, timeliness is one fundamental property that strongly features the WorldFIP protocol
and differentiates it from other fieldbuses. In this section, we will analyze and evaluate some important
timing properties of WorldFIP with reference to the construction of the scan table and the transmission
of asynchronous traffic.
34.5.1 Concept of Producer/Distributor/Consumer
As stated earlier, in WorldFIP, the exchange of identified variables services are based on a producer/
distributor/consumer (PDC) model, which relates producers and consumers within the distributed
system. In this model, for each process variable there is one, and only one producer, and several
consumers. In order to manage transactions associated with a single variable, a unique identifier
is associated with each variable. The WorldFIP DLL is made up of a set of produced and consumed
buffers, which can be locally accessed (through AL services) or remotely accessed (through network
services).
The AL provides two basic services to access the DLL buffers: L _PUT.req, to write a value in a local
produced buffer, and L_GET.req, to obtain a value from the local consumed buffer. None of these
services generate activity on the bus. Produced and consumed buffers can be also remotely accessed
through a network transfer service (also known as buffer transfer). The BA broadcasts a question frame
ID _DAT, which includes the identifier of a specific variable. The DLL of the station that has the corresponding
produced buffer responds using a response frame RP _DAT. The DLL of the station that contains
the produced buffer then sends an indication to the AL (L_SENT.ind). The DLL of the station(s)
that has the consumed buffers accepts the value contained in the RP _DAT, overwriting the previous
value and notifying the local AL with a L _RECEIVED.ind.
© 2011 by Taylor and Francis Group, LLC