Tugas resume paper ke-7

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