EWICS: plcs-subgroup

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Programmable Logic Controllers Subgroup
BRIEFING PAPER

Introduction

EWICS TC7 (European Workshop on Industrial Computer Systems, Technical Committee 7, Safety, Reliability and Security) is an international workshop of experts in the field of dependability of industrial computer systems with respect to safety, reliability and security.

The PLC Subgroup was set up at the beginning of 1992. This work has been undertaken in co-operation with other EWICS Subgroups, especially 'Systems Security' and 'Safety Aspects of Distributed System'. Different life-cycle scenarios have been discussed in the PLC Subgroup and international / national standards taken into account. The intended audience has been identified as users, procurers, developers and assessors / certifiers.

Problem Statement

PLCs are at present being increasingly used in a wide range of industrial sequence control, interlocking and safety applications. The flexibility of the PLC, coupled with the ease in which its transfer functions can be defined, accounts for its increasing popularity.These aspects, and the relatively low cost of a PLC, have resulted in traditional hard wired controllers becoming less attractive resulting in their withdrawal from the market-place. This creates further pressure to spread the use of PLCs.

The advantages gained from a PLC are accompanied by inevitable disadvantages. PLCs are easy to access and to reprogram. Therefore security and integrity are problem areas and awareness of the implications of change or access must be increased. The user of a PLC may have no knowledge or guarantee of the reliability and quality of the embedded software. The manufacturer may change the design of the hardware and the software without the user becoming aware. This means every application must be considered a new application.The transfer function is defined in the application software. This may be developed by persons with little or no knowledge of software development, verification, validation and safety principles as they apply to software.

These safety related aspects of the design, manufacture, use and maintenance of PLCs urgently need investigating guidelines provided for all the parties involved in specification, procurement, supply, use and maintenance.

Progress to Date

The guideline concentrates on the whole lifecycle of a PLC application, i.e. starting from the definition of safety requirements, giving support for the selection of a PLC system and proceeding to the implementation of the PLC system into the target application. The aspects of validation and certification as well as commissioning and use play a mayor role in the guideline. Safety implications dealt with in IEC 61508 are considered in the guideline.

This information has been disseminated through the Subgroup for commenting and discussion during meetings. Then the work concentrated on editing and reviewing the collected material and in some cases filling the outstanding gaps with the new relevant information. Presently the Guidelines for the Use of Programmable Logic Controllers is ready for dissemination.

Contacts and membership

The PLC Subgroup would welcome new members to participate in the further development of the guidelines and its dissemination. For information about the PLC Subgroup please contact:

Gerd Rabe
TÜV Nord e.V.
Grosse Bahnstrasse 31
D - 22525 Hamburg, Germany
tel: +49 40 8557 2101 fax: +49 40 8557 2429
grabe 'at' tuev-nord.com

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