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

35-2 Industrial Communication Systems
on schedule timetables. These can be generated online or off-line and are used by the link active scheduler
(LAS). The LAS manages all message transfers between different field devices on a bus and it is also
in charge of starting the execution of function blocks in each device. Due to its importance, in case of
failure of the device running the LAS, another previously assigned device will automatically assume
responsibility for message scheduling. The LAS is an important concept toward achieving a deterministic
real-time behavior and overcoming common deficiencies of fieldbus technologies, in relation to
timing constraints, high-processing load, and communication schedulability [FA00].
Fieldbus network traffic is periodic and a single interaction of a schedule in a link is called a macrocycle.
During the scheduled communications portion of the macrocycle, all the data related to process
control strategies are transmitted. One part of the macrocycle is used for periodic data (cyclical communications)
publishing (external links and control data) and another part is used for acyclic background
activities (supervision and network management messages). Periodic data are considered high priority
and their timing requirements must be respected during the execution of the control application.
Asynchronous requests (both initiated by devices or by external users) have lower priority and are only
transmitted between the transmissions of periodic data if and only if they do not delay the periodic
communications. Usually, the complete supervision information may require several macrocycles to
be communicated from the field devices to the supervision workstations. This kind of information is
nonperiodic.
The LAS selects between periodic data and acyclic messages using two types of tokens. Periodic data
are requested using message frames named compel data (CD), which are sent to the target publisher
nodes (scheduled token to periodic data) according to the schedule timetable. Once the publisher node
receives the CD, it places its data on the bus using a data transfer (DT) datagram and any devices on the
network can then read the message (publish–subscriber pattern).
In portions of the communications cycle occupied by neither the transmission of CD nor DT, the
LAS sends an unscheduled token called pass token (PT) to one of those devices included in a live list
(a list with all devices that are currently connected to the local link). Each unscheduled token has a
bounded interval, during which the receiving node can use the bus to transmit a pending message until
no further nonperiodic messages exist or the interval expires. This bounded interval precludes that the
nonperiodic message exceeds the gap before the next schedule transmission. When no waiting nonperiodic
message or unscheduled interval expires, the receiving device sends the token back to the LAS by
means of a return token (RT).
An FF application may include several process control strategies. One stand-alone function block or
many function blocks linked to other function blocks are used to implement each process control strategy
designed by the users. These function blocks consist of a set of data and algorithms that are intended
to provide a given functionality to the application. They may represent a control or I/O function block.
Examples are PID controllers, input and output blocks, transducer blocks, etc. Currently, there are commercially
available devices that can contain up to 19 function blocks. Each specific device runs the function
blocks at a specific speed, depending on its internal computing resources (processor, memories,
etc.). Consequently, the functionality provided by a given function block may have different execution
times depending on each device in which it is executed. This aspect should be taken into account by the
scheduler by reading this information from the DD file.
35.3 topology
The FF specification defines two levels of networks: H1, network for control with 31.25.kbps; and HSE, data
highway with 100.Mbps. H1 is a Manchester bus–powered and is usually applied at the field level and can
also used in ex-applications with safety barriers. The original specification includes a higher speed serial
communications protocol that was called H2 but it has never been developed. The high-speed Ethernet
(HSE) bus is adopted at higher levels, allowing an easier connection to Ethernet-based control networks.
© 2011 by Taylor and Francis Group, LLC