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

34-6 Industrial Communication Systems
Moreover, some spare time is available in each microcycle for the transmission of asynchronous
traffic. This will allow the BA to consider additional requests for asynchronous traffic and allocate them
into a suitable time slot. If no request is received, the BA will transmit padding identifier (i.e., identifiers
that do not refer to any station) until the end of the elementary cycle, in order to indicate to the other
stations that it is still active.
Looking at Figure 34.5, we can observe that the schedule is repeated after a number of microcycles
(12 in the example considered). This interval is called macrocycle.
34.3.3 transmission of Asynchronous Traffic
Not all the information required by a distributed application can be distributed by the BA through the
cyclic scan table. In fact, asynchronous traffic can be produced by both sporadic variables (e.g., alarm
signals) and messages (e.g., configuration files). In these cases, as this traffic cannot be foreseen a priori
it cannot be scheduled. The solution adopted in WorldFip is to link this kind of traffic to cyclic traffic,
which instead can be scheduled. Both asynchronous variables and messages are transmitted by using
similar approaches, but with two important differences:
• Variables are shorter than messages, so it is easier for the BA to find a free space in the scan table
to allocate them.
• Variables never require to be confirmed, whereas messages can be confirmed.
The transmission of asynchronous traffic is performed in three steps:
1. In the first step, the producer communicates to the BA its need for additional bandwidth, setting
a field inside an RP_DAT frame.
2. The BA asks the producer the detailed list (amount) of the asynchronous traffic.
3. The BA sends the transmission authorization to the producer, according to the space available
inside its scheduling table.
34.4 application Layer
In the AL, a set of abstract objects called application objects are visible. The totality of these objects
makes up a virtual database that represents the distributed database. These objects can be manipulated
through some specialized services that are suitable for periodic/aperiodic data transfer in industrial
applications.
Services provided by the AL can be classified into three groups:
• Bus arbitrator application services (ABAS)
• Manufacturing periodical/aperiodical services (MPS)
• Subset of the services that are present in the Manufacturing Messaging Specification (subMMS)
The main module is MPS providing support to periodic traffic that represents the most common traffic
in WorldFIP. This module provides the user with the following services:
• Local read/write services
• Remote read/write services
• Variable transmission/reception
• Information concerning the freshness of the variables consumed
• Information concerning the spatial and temporal consistence of the variables consumed
The concepts of freshness and spatial/temporal consistence are of great importance in WorldFIP and
are associated to the ability of the protocol to provide a timely communication.
© 2011 by Taylor and Francis Group, LLC