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Ethernet and USB in automation systems
 Ethernet is here with us for quite a long time. In this year, it will be already twenty years since the inception of 10Base-T standard with transmission speed of 10 Mbit/s, which brought communication through UTP - Unshielded Twisted Pair and star bus topology. Since 1990, the Ethernet way is open to all industrial applications. Although the Ethernet is for a long time the most successful and most spread type in computer network connection, in systems of industrial automation was being asserted long and complicated.

The Ethernet was in minds of technicians for a long time connected with shared transmission medium in the form of thick and thin coaxial cable or hub with wide collision domains and above all with nondeterministic principle of collision solution by CSMA/CD (Carrier Sense with Multiple Access and Collision Detection). All stations in collision domain can receive together, but only one can transmit. If there is a collision, caused by transmitting of more than one station, all stations turn off for random period of time, before any of them tries it again. At low network load, this very effective method caused exponential increase of collisions when increasing the load.

These problems disappeared by the start of using switches. Nowadays, the collision domain usually contains only two participants - end device and switch. Cables contain up to four twisted pairs and transmission runs concurrently with receive in full duplex mode. Transmission medium is not shared and it does not make any collisions. Transmission speed can steadily reach the most possible level. Such network can be decent basis of communication even for very complicated and extensive automation systems.

As well as the Ethernet represents the best variant for wide networks, in case of connecting peripheral units to computers, the best choice is USB connection. This standard of universal serial bus originated in 1995 and its quicker variant USB 2.0 with 480 Mbit/s bit rate exists more than ten years too. USB represents obvious qualitative progress against all formerly used serial and parallel interfaces. Its reliability, endurance, transmission speed and response time predetermines it for majority connected peripheral units in task of data collection and industrial automation.

Ethernet nor USB are not new and untried technologies. These standards did not have an easy way to the branch of industrial automation. Their dominance is obvious and their qualities indisputable. Building an Ethernet network is actually extremely easy, cheap and quick and all system integrators handle this task without difficulties. And building a network of peripheries communicating over USB is even easier.

Serious requirement on communication in automation systems is working in real time. Working in real time does not require only short responses and high speed transmissions, but above all, deterministicity. It means, the system must guarantee the response in requisite time - and time response requirements differs case-by-case. The guaranteed time response is related to stability of time response as well. The requirements on tolerance of time response significantly differ in single applications.

According to time requirements and the requirements on stability of time response of communication network, it is possible to distinguish approximately three types of industrial Ethernet network:

  1. Real time is solved through software under standard layers IP, UDP a TCP. This organization works in all standard networks and they reach time responses counted in milliseconds with the right segmentation of the network. This standard architecture is the cheapest, easiest to realize and satisfy in the majority of industrial networks. As an example of such solutions is e.g. Modbus/TCP or DataLab.

  2. Real time is solved through software substituting standard layers IP, UDP a TCP. These networks work with standard network adapters, communication software works in real time directly under Ethernet packets and in reserved segments of network reaches responses and precision suitable even for i pro demanding applications e.g. on area of drives and robotics.

  3. The highest requirements on real time are solved by combination of special hardware and software. It is impossible to use standard network adapters here and these networks usually work with its own identification of single nodes without necessity of assigning IP addresses. Communication packets are not usually read, interpreted and consecutively further posted, but permeate through by single node with minimum delay counted in nanoseconds to another device. Data are given on reserved space in data telegram, which circle on network. Real time communication packets have the priority in network before packets of standard network layers IP, UDP and TCP.

Three principles of solving the communication in real time in Ethernet networks

Three principles of solving the communication in real time in Ethernet networks

In different network solutions of industrial Ethernet is for keeping the demands on punctuality of message delivery able to use:

  • star structure with switches and not with hub

  • communication models publisher / subscriber and producer / consumer instead of client / server model

  • priority Ethernet packets

  • high-speed variants of Ethernet

  • network segmentation

  • synchronization on principle of distributed real time hours

Ethernet network is important medium for connecting and integration of many automation applications into one scope with the option of central management and supervision. Single applications within the enterprise can work independently without the necessity to communicate with other systems in real time and yet can be part of one centrally managed complex. Stand-alone applications do not have to have their own displays or any other human - machine interface, they can be equipped by uninterruptible power supply and can permanently run irrespective of the state of company computer network.

Applications, which have no displays of their own nor even own computer, can be run in the environment of the Ethernet network. A number of stand-alone applications can run on single dedicated server. The management and maintenance of system becomes even more simple and it is also possible to spare considerable finance on buying a upgrading computers and control units. The reliability of network infrastructure here is substantive, but nowadays it is not usually a problem.

An example of architecture of automation applications built on cooperation of Ethernet and USB

An example of architecture of automation applications built on cooperation of Ethernet and USB

Single applications share common server, do not have own computers nor displays

Single applications share common server, do not have own computers nor displays

Connection over Ethernet:

  • + practically unlimited distance of connection (dedicated segment with twisted pair to circa 100 m)

  • + time response counted in milliseconds

  • - power supply is not always part of communication standard ( standard Power over Ethernet defines options of power supply of Ethernet devices through data conductors from CAT5 cabling, voltage of 44 – 57V  is usually used with max. current of 550 mA )

  • - it is necessary to assign fixed IP address

  • - it is necessary to configure the device through inbuilt HTTP server before the first use

Connection over USB:

  • + time response counted in milliseconds

  • + high transmission speed 480MB / sec, sufficient transmissive capacity even for high definition digital cameras

  • + power supply is fixed part of bus

  • + plug-and-play without the necessity of configuration

  • - the distance of connection to 5m (with active extension to circa 30m)

DataLab industrial input / output unit with Ethernet connection

DataLab industrial input / output unit with Ethernet connection

DataCam industrial digital camera with USB connection

DataCam industrial digital camera with USB connection

DataLab IO/ETH units with Ethernet interface are fully compatible with all I/O modules of DataLab IO system. Only USB interface is replaced by 10/100 Mbit Full Duplex Ethernet interface and the unit communicates through TCP/IP protocol. TCP/IP protocol can be easily routed through various network, the physical distance of the unit from a computer is practically not limited. From the point of view of application program equipment, the USB and Ethernet units do not differ. It suffices in parametrical file of identical driver to choose the way of communication and set the proper addresses. Then the applications communicate identically with single units. The communication with Ethernet units is encrypted and that prevents from unauthorized interventions in the technology.

The components for industrial automation with the option of Ethernet and USB connection make integration of applications to connected centrally managed environment easier and make the building of modern integrated systems significantly cheaper.


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