Communication protocols

There are quite a lot of common communications protocols, which ensure link between PLC, PC, executing mechanisms, the Internet etc.

Among most well known and most utilized communication protocols we can name — Profibus, DeviceNet, Ethernet, Modbus, AS-I, Lionwork, CAN, HART. In a lot of cases, it is not at all unambiguously clear, on basis of what protocol (or protocol combination) to realize certain automation project. Optimum protocol choices are determined by the equipment location topology of the given process control task, total number of devices, high-speed as well as field of application. Popularity of several protocols using in USA and Europe is very different. For example AS-I, Profibus and CAN are particularly more popular in Europe, however, DeviceNet is used in USA relatively much more wider than in Europe.

For technical and application characteristics description of the different protocols you may use the following table:

NetworkNetwork LevelPhysical MediaTransmission SpeedNumber of NodesMax. Distance
ARCNet Enterprise Twisted pair, Coax, Fiber Up to 10M 255 10,000 m
ASI Device Flat Cable or Twisted Pair 167K 31 100 m
CANopen Device Twisted pair Up to 1 M 127 1000 m
ControlNET Control Coax, Fiber 5M 99 30,000 m
DeviceNet Device Twisted pair Up to 500K 64 500 m
EtherNet Enterprise 10BaseT, 100BaseT, 100Base FiberLink 10M, 100M 1024 4000 m
Foundation Fieldbus Control Twisted pair 31.25K — 100M 240 1900 m
Interbus Device Twisted pair, Fiber 500K 256 400 m
LonWorks Device Twisted pair, Fieber, power-line 1.25M 32000 2000 m
Modbus RS-485 Twisted pair 38.4K 250 500 m
Profibus — DP/PA Device/ Control Twisted pair, Fiber 12M 127 24,000 m

Further we will show an example from our practice, how the system in which were used AS-I, Profibus and Ethernet communication protocols was selected:

Examined project is related to flour transportation, storage, homogenization and other operations in the flour silo. Object elements dimensionally are located quite steady.

In this system number of digital inputs/outputs were around 1500. Performance of the project realization was selected to be based on Siemens Simatic S7-400 (with processor CPU 414).

We analyze three process control system versions:

  1. Direct connection of the executing mechanisms' control signals, sensors and end-switches to the centralized inputs/outputs of the centralized controller.
  2. Use of distributed input/output stations that are connected with the central PLC with mediation of PROFIBUS communication line.
  3. Application of AS interface lines, in order to use distributed input/output modules for operation of electro magnetic valves and installation of sensor information. AS interface modules connect with main PLC by using PROFIBUS line.

All three versions were analyzed within' context of the given task, by taking into consideration technical solution and complex expenses.

First version is improper, because, due to the large number of inputs/outputs, object controller circuit will have pipelines with capacious installation. It will inconvenience and make the installation performance, system diagnostic and maintenance more expensive, as well as will reduce fire safety due to the large number of cables.

Second version is based on the distributed input/output devices that connect with the main controller via PROFIBUS line. Application of this version is profitable if the object is separated into the groups. In addition, these groups should be relatively consistent and distance between them should be quite long. In particular case distances were quite near and object elements were evenly dissipated in the room. Therefore this version, in terms of outer installation will have very few preferences in comparison with the first version. Besides, in the configuration of the task this version will be the most expensive.

The most advantageous, from all aspects, in this case is the third version.

Given version allows to significantly decrease capacity of the installation pipes, ensures convenient system extension, well observable condition for diagnostics and maintenance. Besides this version is also quite economical. Costs of it are for 20% lower than for the second version and almost the same as for the first version.

Of course it was also necessary to use other communication protocols.

Wherewith, in this case main controller connects with system elements in four ways:

  1. Main controller inputs/outputs are directly connected with MCC drive for the provision of control commutation signal for its startup/shutdown and overload control. Main controller and MCC direct circuit is based on ensuring operational safety demands.
  2. Frequency converters are controlled, by using PROFIBUS communication line. This PROFIBUS line is also used for link with DP/AS interface converters.
  3. All electromagnetic valves, level sensors, pressure sensors, conveyer movement sensors and valve end-switches are connected to input/output modules, that ensure information exchange with the main controller via AS interface. Given input/output modules are connected to the appropriate production line of AS interface. For provision of such interface links in the configuration of this task there were necessary 6 DP/AS interface converters that are connected to the central processor via PROFIBUS communication line. It was necessary to use 6 of these converters, because 31 input/output modules could be connected to one DP/AS interface converter. Total number of DP/AS interface converters is 7, as one AS interface line was supposed to be a reserve line. One of such modules has 4 digital inputs and outputs. Modules of analogue values have two inputs or outputs. DP/AS — it was possible to interconnect number of interface converters with the production lines.
  4. Link with the computer was ensured with the help of Ethernet line.

If for the follow-up of certain parameter it would be necessary to provide faster operation, then an opportunity that given sensor should be connected straight to the centralized inputs and outputs should be considered. Such version could apply to the recording of the conveyer movement sensor impulses.


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