CAN Newsletter magazine
Why should I use a safety-related CAN protocol? Is it necessary? And if so, which protocol is the most feasible? We take a look at where the demand for safety protocols comes from and at existing technical solutions.
If you want to place a product like a machine or a vehicle on the market, you have to consider the legal requirements. Those are specified in national laws like the Product Liability Act or European directives like the Machinery Directive or the Regulation for Vehicle Type-Approval. All of them refer to technology, which is described by the harmonized product or sector standards. For any kind of machine with safety-related parts of a control system, the most commonly used standard is ISO 13849. Concerning data communication, it refers to IEC 61508-2, providing a choice of two data communication architectures. With the White-Channel, the entire transmission path has to be developed compliant to the standard, whereby with the Black-Channel, only the end-points are considered safety-relevant and the transmission is protected via a safety protocol. In both cases, for non-rail applications, IEC 61784-3 “Functional safety fieldbuses” is referred to, whose principals have been implemented for example in the CANopen Safety standard EN 50325-5.
As CAN is still a widespread bus-system in the industry, the article takes a look at the technical solutions for this standard. With the rising need for safety-related CAN communication, several companies have come up with ideas on how it can be realized. As an example, Pilz developed Safetybus p, which is an event-driven CAN protocol and primary used in fabric automation. By adding additional measures to the OSI layer 2 and 7, it is made suitable for safety applications up to SIL 3 according to IEC 61508. Transmission errors and device errors are detected by a combination of sequential numbers, timeout detection, echo check, IDs for transmitter and receiver, as well as data protection with CRC.
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