Application example of a modular machine design

Plant and machinery are becoming increasingly modular. This means that they are being segregated into mechatronic units with separate functions. In a concept such as this, the electrical engineering follows the mechanical structure of the machine, bringing wide-ranging benefi ts. Once the machine modules have been developed they can be reused in various machines, which ultimately reduces the development effort. Modules can also be manufactured separately and joined together only during final assembly. What's more, modules can be developed in isolation from each other, so tasks can be run in parallel, saving time during development.

This type of engineering follows the building-block principle and enables customized solutions to be implemented at lower cost. Current fieldbus systems prevent this modular approach, as they are mainly based on a centralized master/slave approach. In safety technology in particular, one central instance is usually available: the master.
The publisher/subscriber communication principle applied universally on SafetyNET p does not use a central instance, thereby enabling a modular machine design.

Safe communication in distributed control systems

The publisher/subscriber communication principle is used universally on SafetyNET p. To enable the publisher/subscriber approach to also be used for safe communication, some new security mechanisms have been developed for SafetyNET p. For example, telegram delays can be managed by a runtime measurement initiated by the receiver. The advantage over previous standard solutions is that the transmitter of the message does not need to know the receiver. So the publisher/subscriber approach can also be applied in safety technology, which enables distributed, safe control systems.

Safe telegram structure

Cyclical data in SafetyNET p is communicated as safe PDOs (SPDOs) and has the following format:

• PID ( Packet Identifier):
• Used with the SID for unique data packet identification
• Length: Complete length of packet in Bytes
• Process data: Safe process data
• SID (Safe ID): 16 Bit unique network-wide ID, through which both the sender and the SPDO are uniquely identifiable
• Counter No.: 8 Bit cyclical counter for life sign monitoring on subscribers
• CRC: 32 Bit check sum covering the whole safe data packet

Safe communication via SafetyNET p

SafetyNET p can also communicate safety related data through an integrated safe communication layer.
The security mechanisms are designed up to SIL 3 in accordance with IEC 61508. The safety-related data is sent encapsulated within SafetyNET p telegrams. As a result, all other network components such as switches or cable may be standard Ethernet components, which have no impact on safety. Even non-safety related network subscribers such as PCs or standard control systems, for example, have no impact on safety-related communication. As a result it is possible to mix the operation of safety and non-safety-related devices within a network. On SafetyNET p, safety-related objects are stored in a safe object directory, similar to the CANopen object directory.

Safe communication in the OSI reference model
On SafetyNET p, the safe application layer is implemented in Layer 7, the application layer of the OSI reference model. Cyclical, safety-related objects are communicated via safe process data objects (SPDO). SPDOs are mapped on the cyclical data channel, the CDCN, and sent in defi ned intervals. When necessary, acyclical, non-time-critical safety-related data is sent as SSDOs ( safe service data objects) via the MSCN ( Message Channel).