The measures in Table 1, other than CRC for data integrity, are indicated in the appropriate column check for a range of other types of communication errors that can arise during transmission of a message between any two points. Each of these measures, as implied by the short explanation in brackets, provides the following benefit and increase in confidence of the reliability of the transmitted information:
- Consecutive Number – Confirming that the message transmitted is received and reassembled in the proper sequence is important, especially for messages that have more than one route option to get from point A to B.
- Time Out – Many buses have some form of acknowledgement mechanism, however, the majority of the Industrial Ethernet protocols use UDP, which does not support message acknowledgement. Therefore, an independent dedicated tool must be used.
- Codename – This is a way to be sure that messages are transferred between the two end devices/nodes for which they are intended and no others.
Using a safety layer as just described provides the advantage of easy and fast implementation and also allows safety margins to be ideally dimensioned and machine clock rates to be increased to meet the overall system safety/SIL requirements.
The functionality of the safety protocol is not concerned which transport protocol is used, because all safety-related mechanisms are integrated exclusively on the application layer of the protocol, and the safety bus functionality is thereby independent of the underlying transport layer.
The safety bus network does not benefit from any error detection mechanisms of underlying transmission channels, and thus supports the securing of whole communication paths, even backplanes, inside controllers or remote I/O.
Using the black channel approach ensures that the safety quality is independent of the communication channel.
Is the black channel concept really "black magic"? No. At most it is "sleight of hand," since just like the black box, it moves responsibility for making the "trick" work from the medium or messenger to the parts of the system actually doing the work and having the intelligence to tell the difference.
Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional and an authority on Foundation Fieldbus and industrial communications technologies. His website is www.industrialautomationnetworks.com.