Abstract
Digital communications are widely used in many types of control systems, enabling information interchange between items of equipment and with the outside world. This has enabled control system designers to achieve better control over processes, improve interactions between equipment, and to speed fault diagnosis. Such features can have positive effects on safety, such as reducing the demand rate on a protection system. However, digital communications are also increasingly being sold by system manufacturers as a fundamental component of safety-related systems. Some significant safety benefits can be realised, but practical difficulties can result in direct challenges to safety function integrity levels, and more subtle effects through operational and maintenance issues that could lead to safety system failures. This paper explores the complications of designing safety-related systems incorporating digital communications, of assessing the adequacy of such systems, and of maintaining their integrity in a wide range of environments. Topics discussed include the effects of complex interactions within systems, the consequences of reduced isolation between control and safety systems, threats to system integrity, competence, and the use of proven-in-use components. The requirements of the functional safety standards BS EN 61508 and BS EN (IEC) 61511 are described in relation to the use of digital communications, and some basic design principles are suggested.