Protocols: Sorting through the options to choose one

Though the terms are often used interchangeably, there is a difference between communications and networking.

Networks are the infrastructure used to get the data from one node to another, while communications includes the language (protocol) so that once the message gets to the intended recipient they can “translate” all of the ones and zeros to something understandable and usable by the two devices.

Of course, as stated in the past, the network itself must be properly designed and working in order for the ones and zeros to get between its various nodes. The protocol stack determines how, when and to whom the messages are sent.

Just as we have different languages and even dialects within languages, there are also many industrial protocols available to meet the different needs of different industries — each of which have unique requirements. There are presently 19 different protocols in the IEC fieldbus standard, each meeting the needs of one or more industries.

Profibus recognizes the needs of different industries within its family of protocols; not only at the field or serial level of Profibus DP and PA, but also with its different Ethernet variants, including IRT, with a custom stack for nanosecond resolution of, for example, multi-axis machines. The field level or serial versions of Profibus all communicate with their host using the DP protocol, as Profibus PA does not connect directly, but rather through a PA/DP gateway.

Profibus PA, being designed for the process industries, follows very similar rules as Foundation Fieldbus, operating at 31.25 kbit/s and a maximum cable length of 1,400 metres with power and data over the single pair.

There are, of course, other protocols for other industries such as BACnet and Lonworks in building automation and CAN in automobiles — each designed to meet the needs of the industry for which they will be used. However, if no other option is available, a single fieldbus protocol could be made to work for every industry and application.

The majority of the buses have an Ethernet version that fortunately, because they follow the OSI seven-layer model, use the same underlying language despite the medium by which the packet or data is carried. The fieldbus packet is much like a sea container in that the data/message can be transmitted several different ways without a concern or alteration to the contents of the container/packet. This shows, once again, that just like we can use Ethernet or the telephone to carry a wide range of data types, messages or languages, the same is true for industrial protocols.

If there were only one protocol, it would be like building a house with only a hammer, which as we know means everything is a nail. However, not only is there a variety of nails to suit the task at hand, there is also a range of hammers as well.

Yes, the network is important; however, what really drives the decision in determining the correct protocol for your project is the application. Each protocol has been designed to work best in a few applications/industries, which means that you as an engineer will have to decide which tool is best suited to your project.

This column originally appeared in the June 2012 issue of Manufacturing AUTOMATION.