Back to Basics: PROFINET
If you ever installed a printer at your office or a router at home, you probably used an Ethernet cable. Most people think “Ethernet” just means the cable type and leave it at that. But in fact, the cable only represents the physical part of Ethernet.
This cable has a series of communication protocols. You’ve heard of two of them, the Internet Protocol (IP) and the Transmission Control Protocol (TCP). For offices and homes, these two will suffice. With them, you have all you need for an office or home – file sharing, printer and email access, and of course, netsurfing.
However, an industrial plant has different needs. On the factory floor, controllers must access data in drive systems, input/output (I/O) devices, and workstations. And here you need super-fast responses, whereas in the office a user can wait. Finally, you have to protect against abrasion, moisture, temperature changes, even damaging chemicals.
So computer-type people cooked up a new Ethernet category, the Industrial Ethernet. Then more computer types based the PROFINET protocol on that. So let’s dig deeper and see how PROFINET works.
Peeling the onion
So PROFINET connects process devices – sensors, actuators, and the like – to control systems. PROFIBUS and PROFINET International (PI) created this protocol with a little help from a few major vendors. If you are interested about what is the difference between PROFIBUS and PROFINET International (PI), you definitely check this article.
It complies with the Ethernet standard IEEE 802 in IEC 61158 and
IEC 61784. This means you can have a printer and a flow meter on the same network! How cool is that? However, PROFINET runs much faster than your office Ethernet, with cycle rates in sub-milliseconds.
To explain how this works, we have to turn to the ISO/OSI reference model. The Open Systems Interconnection (OSI) model has seven layers to connect two points in a network. Take a look:
The Ethernet standard simplifies this model to four layers:
- Ethernet = physical and data link layers
- IP = network layer
- TCP or User Datagram Protocol (UDP) = transport layer
- Other protocols = application layer
PROFINET uses these four layers, but sometimes not all of them. For example, Real Time PROFINET skips the network and transport layers, making the response much faster than the standard TCP/IP model.
Three channels swap data with a controller in PROFINET:
- TCP/IP, setting parameters, configurations, and cyclic read/write operations
- Real Time (RT), bypassing TCP/IP layers to speed automation to between 1 and 10 milliseconds
- Isochronous Real Time (IRT), switching schedules and sorting signals for precise syncing
Into and out of the IO system
PROFINET IO uses the provider-consumer model of data exchange rather than the master-slave model adopted by PROFIBUS. These devices fit in three categories:
- Controller: Usually a programmable logic controller (PLC) or a distributed control system (DCS), where the automation program runs. The controller sends output data to some IO devices and receives input data from others. Similar to a class 1 master in PROFIBUS.
- Devices: Sensors or actuators connected to the controller through Ethernet cables. These devices receive output data from the controller and send input data to the controller. Similar to slaves in PROFIBUS.
- Supervisor: PCs, human-machine interfaces (HMI), or other monitoring, commissioning, or diagnostic devices. Similar to a class 2 master in PROFIBUS.
To set a PROFINET IO system, you need at least one controller and one device. But you can set up systems in a number of ways – multiple controllers for a single device, a single controller for multiple devices, or multiple controllers for multiple devices. You’ll usually use supervisors temporarily, for troubleshooting and commissioning.
A PROFINET system also has network components, like switches and wireless access points. Some components can work as IO devices for enhanced diagnostics.
Configuring PROFINET devices
To configure a PROFINET IO device you need two things – a tool to act as a supervisor and a General Station Description (GSD) file. This GSD file, provided by the device vendor, resembles files used in PROFIBUS. However, it’s XML-based, called GSDML.
After the configuration, the GSDML file downloads to the controller, connecting it to the devices. Inputs and outputs then flow between the devices and the controller as cyclic data. More information like diagnostic data moves as acyclic data.