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DP-Ethernet: the Profibus DP protocol implemented on

Ethernet

S. Vitturi

National Research Council, ISIB-CNR, Corso Stati Uniti 4, I-35127 Padova, Italy Received 7 January 2002; revised 27 September 2002; accepted 10 October 2002

Abstract

The use of Ethernet networks at the low level of factory automation systems, which is even more frequent, requires the adoption of real time protocols to implement the typical functions of this level.This paper investigates the possibility of using of a very popular fieldbus protocol,

Profibus DP, as a real time protocol for Ethernet. The proposal, named DP-Ethernet, makes use of the IEEE 802.2 Logical Link Control to implement the Profibus DP functions on Ethernet. The paper shows package, are given. q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ethernet; Profibus DP; Real time; Protocol

1£®Introduction

Communication networks currently systems. Unfortunately, the lack of a well clear standardisation process together with the continuously changing communication requirements factory automation systems. This scenario, at the , because, thanks to the impressive growth of the Internet, some very popular protocols such as for example FTP, HTTP, based on the TCPIP suite are even more adopted also in industrial applications. These protocols are usually implemented on Ethernet networks whose physical extension is limited to that of the plant under control: the resulting networks are normally referred to as Intranets.

Conversely, the situation is particularly critical at the lowest level of factory automation systems, called ¡®device level¡¯, where fieldbuses are typically used. In this area there a proliferation of proprietary products, which are incompatible among each other. At present, many of these products included in some international standards such as for example IEC 61158, EN 50170 and EN50254. However, although some considerable efforts to reach a form of (e.g. the NOHA ESPRIT project), they are still incompatible. An interesting tendency originated recently is represented by the possibility of using Ethernet networks also at the device level of factory automation systems. The advantages of such a solution are indubitable, among them: a very easy integration with the this case, the market, and there is the concrete risk of assisting to a new ¡®fieldbus war¡¯.

For this purpose it is worth mentioning that the IAONA organization encourages the growth of open networking in factory automation systems and the adoption of Ethernet at all levels. IAONA, in particular practice, the paper proposes of implementing an Ethernet network with the Profibus DP protocol placed on top of its data link layer. As it will be shown, this solution, which for commodity will be later on referred to as DP-Ethernet, may be realized in a simple way. Moreover, as in this case the interface to the user applications does not change, DP-Ethernet could replace Profibus DP in any already established application. In detail, the structure of the paper is

the following: Section2 reports the main features of Profibus DP.

Particular attention will be given to the functions available to the user applications and to their mapping onto data link layer services. Section 3 illustrates the features of Ethernet and of the Logical Link Control (LLC) protocol, which are extensively used by DP-Ethernet. Section 4 describes 5 takes into consideration some typical configurations of DP-Ethernet and evaluates their performances using a suitable software simulation package.

2. Profibus DP

Profibus DP is a protocol designed to perform cyclic process controllers and field devices, such as sensors and actuators, in a master-slave configuration.

The first version of the Profibus DP standard was issued in 1994. Subsequently it was included in the European Standard EN50170. In 1997 an extension, named DPV1, added acyclic functionalities to this fieldbus and, finally, at the beginning of 2000, a completely revised version of Profibus DP became part of the IEC 61158 international fieldbus standard. The communication profile of Profibus DP is shown in Fig. 1. As can be seen, some of the ISOOSI layers are empty, and the Profibus DP protocol is placed on top of the data link layer, named Fieldbus Data Link, FDL. The access to the physical medium realized by FDL is based on a technique very similar to that specified by the IEEE 802.4 standard, token bus. The token is circulated among active stations which form a logical ring, but also passive stations can be connected to the network: as they do not receive the token, they can only answer to specific requests coming from an active station. An important parameter, the target token rotation time, TTR, is set in all the stations during the network configuration phase and represents the maximum time requested by a complete token circulation in the logical ring.

When receiving the token, a station computes the maximum time it can be used (token TTR and the actual duration of the last token rotation (real token rotation time, TRR). The FDL protocol specifies two possible priorities for the Protocol Data Units (PDUs), transmitted on the network: be present on the network: Class 1 masters, which are typically control devices, such as for example PLCs, CNCs, PCs. Class 2 masters, used for network configuration and administration tasks. Slaves, which are input¨Coutput devices employed to realise the interface with the plant. Because the use of class 2 masters the following the term masters will

be used to indicate uniquely class 1 master devices. Although the Profibus DP standard allows for the realisation of network configurations with more than one master device (multimaster), most of the already existing applications are based on monomaster network configurations. As can be seen in Fig. 1, Profibus DP master and slave stations. The User Interface of a master the following. After power on, a slave waits for its initialisation from a master: this is realised by means of two functions, named DDLM_set_prm (set parameters) and DDLM_chk_cfg (check configuration). At the end of a successful initialisation, the slave enters the data exchange phase, during which it cyclically exchanges input¨Coutput data with the master by means of the DDLM_data_exchange function. In this phase a slave, when polled, may signal the presence of a diagnostic message.

As a consequence, the master, at the end of the current polling cycle, is forced to read the slave diagnostic with the DDLM_slave_diag function. Moreover, during the data exchange phase, a master can send some Fig. 1. Profibus DP communication profile. global control commands to the slaves in order to synchronise the acquisition of the inputs andor the sending of the outputs: to this purpose the DDLM_global_control function is used. This function can address either a single slave or a group of slaves. The DDLM can send up to 246 data octets to a destination station.

This latter is requested to acknowledge the correct reception of the data. In the response frame the destination station can include, if previously prepared, a maximum of 246 data octets to send back to the source. SDN is a connectionless unconfirmed service used to send up to 246 data octets to either a single station or a group of stations. In this case, the destinations do not send back the acknowledgments of the correct reception, but the source station generates a local confirm meaning that the data correctly submitted to FDL. The DDLM uses the SDN service to implement the DDLM_global_control function and the SRD service to implement all the other functions. SDN is necessary because it is the only FDL service which can address a group of stations, as necessitated by the global control. In order to specify the different User Interface functions, the DDLM makes use of the Service Access Points, SAPs, which are present in the PDUs of FDL. Thus, for example, a DDLM_slave_diag request is implemented by a SRD request where the source SAP #62 and the destination SAP #60 are specified. Whereas, the DDLM_set_prm function is implemented specifying the source SAP #62 and the destination SAP #61. As an example, Fig. 2 shows the sequence of the primitives necessary to realize the DDLM_chk_cfg function (destination SAP source SAP 62).