JUDUL:
Real-Time Fieldbus
Communications Using Profibus
Networks
PENGARANG:
Eduardo
Tovar (member)
PENERBIT:
IEEE
RESUME:
This paper provides a comprehensive study on how to use Profibus fieldbus
networks to support real-time industrial communications, that is, on how
to ensure the transmission of real-time messages within a maximum bound time. Profibus is based on a simplified Timed Token (TT) protocol,
which is a well-proved solution for real-time communication systems. We propose
two approaches to guarantee the real-time
behaviour of the Profibus protocol: an Unconstrained Low Priority Traffic profile and a Constrained Low
Priority Traffic profile. The proposed analysis will show that the first
profile is a suitable approach for more responsive systems (tighter deadlines), whilst the second allows
for increased non-real-time traffic throughput.
Within industrial communication systems, fieldbus
networks are specially
intended for the interconnection of process controllers, sensors and
actuators, at the lower levels of the factory
automation hierarchy. These hierarchical levels
have dissimilar message flows, in terms of required response times, amount of
information to be transferred, required reliability and message rates (how
frequently messages must be transferred).
Kita mempertimbangkan membatasi
waktu atau deadlines as the maximum allowable time span
between a message transfer request and its transmission on the bus. In fact,
the total message cycle delay
results from multiple
factors, such as the access
and queuing delays, the
transmission time (frame length / transmission rate) and the protocol
processing time. As we are dealing with real-time communication across a shared
transmission medium, we focus our analysis on the access delay and queuing delay factors.
The Profibus MAC
mechanism is based
on a token
passing procedure used
by master stations to grant the
bus access to each one of them, and a master-slave procedure used by master
stations to communicate with slave stations. The Profibus token passing procedure uses a
simplified version of the Timed Token protocol. An
important Profibus concept is the message
cycle. A message cycle consists of a master's action frame (request or
send/request frame) and the associated responder's acknowledgement or response
frame. User data may be transmitted in the action frame or in the response
frame.
One of the main functions of the Profibus
MAC is the control of the token cycle time. After
receiving the token, the measurement of the token
rotation time begins.
This measurement expires at
the next token arrival and results in the real token rotation time (TRR). A target token rotation time (TTR) must be defined
in a Profibus network. The value of
this parameter is common to all masters, and is used as
follows.
Profibus supports four data transmission
services: Send Data with No acknowledge (SDN); Send Data with Acknowledge
(SDA); Request Data with Reply (RDR) and Send and Request Data (SRD). The
SDN is an unacknowledged service used for broadcasts from a master station to
all other stations on the bus. An important characteristic of these
services is that they
are immediately answered, with a response or an acknowledgement. This feature,
also called "immediate-response", is particularly important for the
real-time bus operation.
Compared to the Timed Token protocol [9], the main difference of the
Profibus token passing consists in the absence of
synchronous bandwidth allocation (Hi). In the Timed Token
protocol this is a relevant station parameter, since it specifies the amount of
time a station has to transfer its real-time
traffic. In Profibus,
if a master receives a late token (TRR was greater
than TTR)
only one high priority message may be transmitted.
As a consequence, in Profibus, low priority traffic
may drastically affect
the high priority traffic capabilities. In
fact, if the low priority traffic is not constrained when a master receives an
early token (TRR smaller than TTR), the master may use all the available
time (TTH = TTR – TRR) to
process low priority traffic, delaying the token rotation. In this case, the
subsequent masters may be limited to only one high priority message
transmission when holding the token. Ilustrasi
situasi tersebut terdapat pada gambar berikut.
The major contribution of this paper is to
prove that is possible to guarantee real-time communication behaviour using Profibus fieldbus networks, thus allowing
its use to support distributed computer-controlled systems.
In order to achieve real-time
communication behaviour using Profibus networks,
we propose two distinct
approaches: an Unconstrained Low Priority
Traffic profile and a Constrained Low Priority Traffic profile. For both profiles we derive
deadline constraints, which, if verified, guarantee real-time messages
deadlines.
STRENGTH: Jika sebuah
master telat menerima sebuah token (TRR lebih besar dari TTR)
hanya satu high priority message yang dapat dikirim.
WEAKNESS: Jika master menerima token lebih awal (TRR lebih kecil TTR), master dapat menggunakan semua waktu yang tersedia (TTH = TTR –
TRR) untuk memproses low priority traffic. masters
tetap dibatasi hanya mengirim satu high
priority message ketika holding
the token
