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TCP/IP vs field buses for control applications. The eternal discussion

posted Sep 8, 2008, 11:17 PM by Daniel Berenguer

Why not use TCP/IP for control applications instead of those complicated field buses?. Indeed, this discussion is appearing every day in most environments with control needs. But which are the typical positions around this subject?

Software engineers typically defend TCP/IP as communication channel for control applications. Their main argument is that this communication channel can be found in most computers and appliances nowadays. Moreover, Ethernet and Wi-Fi hardware interfaces present low prices compared to some years ago.

TCP/IP is one of the few channels that allow combining big packets of data with short control-oriented messages. This feature makes this technology specially suitable for home applications where multimedia and control can share a common communication system. Besides, Ethernet-certified cables and RJ45 connectors are very cheap compared to some control-oriented cabling systems. Most buildings already contain a LAN infrastructure with the necessary hardware and connecting IP control systems to those infrastructures is as easy as installing a new computer in the LAN.

On the other hand, control engineers don't usually like the idea of relaying on the TCP/IP technology for controlling critical applications. Furthermore, Ethernet follows a physical star topology and this complicates the installation process in networks with big amounts of endpoints. On the other hand, Wi-Fi is often avoided in industrial environments due to the electromagnetic noise potentially produced by this technology. Control-specific technologies usually follow a bus topology, reducing cable runs and providing a separate communication channel for the control messaging. Control interfaces as CAN, RS485, Lonworks, LIN and others can be used with low-power microcontrollers as the communication protocol is often easier to maintain than the TCP/IP stack. These simple controllers participate in the bus as listeners, transmitters or both but will never have to worry about a possible overhead of information coming from a computer or a media server.

Nevertheless, integrators know that TCP/IP is an excellent complement to the industrial control solutions. Ethernet is still the natural way of connecting a PC to a network and the inclusion of the web technology in remote monitoring applications force us to mix somehow the best of both worlds. Multimedia, temperature sensing, web access, binary control, SCADAs, ... when all these applications have to be integrated into a single solution, then the use of a hybrid network is often the best solution.

The following schema, extracted from the opnode project, is an example of integration of TCP/IP and several control-oriented technologies:

As you can see, TCP/IP or "the green network" is used in the above example to transport multimedia data and also as integration point between different control technologies. The link between every control technology and the IP world is relayed on a high performance gateway that translates and filters the commands coming from both channels, avoiding then the overhead of data on the control side and reducing the amount of short commands on the IP LAN.

This architecture is being widely used in industrial and building applications. No matter which control bus is installed, it will be connected to the LAN infrastructure in some point.

After this explanation, a new term comes into focus: the cost per endpoint. This parameter gives an idea about the cost (price and power consumption) of controlling a single endpoint using a given technology or mix of technologies. IP controllers are more expensive in price and consumption. Thus, the only way of reducing the cost per endpoint is to add more control points to the device. This is the reason because most IP controllers are designed to control important amounts of endpoints whilst other less expensive control-oriented technologies provide devices controlling just the temperature of one room.