Download Data transmission on a Local Area Network in industrial conditions

Data transmission on a Local Area Network
in industrial conditions.
Janusz Kolbusz Tomasz Pardela
University of Information Technology and Management in Rzeszow
Industrial networks are dependent on time, thus they need real time capability. There are many
commercial protocols dedicated to be used in the industry. Most of the specialized solutions have one
downfall though, they are expensive to create, launch and operate.
The alternative is the usage of the existing LAN with TCP/IP communication protocol. Up until
now Ethernet was used mainly in the office systems but is becoming more common in the industrial
communication systems as well. The usage of time cycles in the basic protocol of LAN hasn't been
researched well enough for it to be integrated into industrial networks. More studies need to be done
before LAN network can be used for data transfers in industrial systems. The results of this research
will help us determine how useful LAN can be in controlling technological process in specific cases.
Depending on a structure of the bus, either physical collisions in the medium or logical problems
in the switch may occur. The probability of such collisions runs higher the busier the information flow
is in the network. When it comes to process control, data availability should be immediate. Time cycles
are not such an important matter when it comes to visual process. For data registry, it is important when
a specific event happens, not when it is delivered to the registering station. Quality of Service
mechanism can be used to determine and sort all the data that was sent. QoS helps to prioritize
information, which allows it to be collected in the proper ordinance.
The following research was registered in LAN network during the busiest data traffic.
Studied environment consisted of structural wiring system made with fiberglass. All of the other
cable connections were made with twisted-pair wire, - category 5e. Managing switches and a router
which provided Internet connection were used as active devices. The network is controlled by “Active
Directory” base which is installed on the server with DHCP and DNS services enabled. There are 30
computers working in the network. The server is also used as a server for visualization of program
Citect. CitectSCADA provides an access to information explaining the works of automatic systems as
well as giving the ability to control each and every executive element.
Fig.1 The schematics of the network
Research environment
The studies of the information flow in the industrial networks are based on Supervisory Control
And Data Acquisition system by Citect. This system uses basic environment of the Ethernet network.
CitectSCADA program works in a client-server kind of structure. This type of structure is implemented
on basic level tasks made by the system. Each task appears as an independent client and/or basic server.
Necessary data is used or indicated as accessible by it's client-server functionality.
CitectSCADA manages 5 tasks, which are:

communication between I/O devices
 alarm scan
 report generation
 trend registration
 user interface operation
Every type of the I/O device uses a unique communication protocol which allows data exchange
with overriding systems such as CitectSCADA. The speed of this communication is determined by the
system's data interconnections and is limited by it's I/O devices and the construction of the protocol.
Such limitations are caused by the fact that I/O devices don't react instantaneously when prompted for
specific data and many of the protocols are simply ineffective.
Communication in the CitectSCADA program starts with a request from the client, then the I/O
device will only release this specific information . An I/O server optimizes client's requests, e.g.
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combines two requests into one. When a response for a single question comes back, the size of data
block is restricted. When all of the client's questions are grouped together, there are less requests
coming to an I/O device , shortening the time of response.
This client-server structure helps to optimize communication's productivity by using buffering
mechanism on the I/O server. Records read by an I/O server are saved in it's memory for the time
requested by user (approximately 300 ms). During this time the I/O server releases data to clients,
however, incoming requests don't require reading other records of the I/O devices. CitectSCADA
program used in research is a network version. It has a “runtime” license and 250 process variables.
The license consists of 3 parallel connections to clients' stations (stationary personal computers with
Windows XP system).
Fig.2 Basic version of the Citect structure
The server is a central element gathering data directly from PLC controllers. Visual client
(operating station) reads data filed on the server. Shown below is the research made on the information
flow between Visual server and PLC controllers, placed in different localizations. TCP/IP was the
protocol used for communications and snifter Ethereal was the program used for the study.
Devices converting interface RS485/232 to Ethernet network
In communication between a controller and visual server there is an element that converts
interface RS485/232 to Ethernet network. The element used to convert standard RS485/232 to Ethernet
network in this research is the network interface card which is built into the operating console. The
operating console communicates with the controller PLC FX2N through a serial connection, then data
is sent through the Ethernet network. Operating console is equipped with interface RS-232, which
through an additional card 485ADP built onto the controller FX2N, communicates with monitored
records in the controller. Physical connection from the Ethernet's side are created through an interface
of the network card built into the operating console. Protocol used for communication is Mitsubishi
MELSEC-FX2N series PLCs.
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Fig.3 Communication with the PLC controller
Another method applied to connect the controller PLC to the Ethernet network is converter Tibbo
DS100. The device is a server equipped with serial interface SDS (Serial Device Server) which is able
to connect any device equipped with serial interface to Ethernet. This type of converter allows us to
install an asynchronous serial interface RS232/RS485/RS422 in any place in the network. It also
supports communication between any devices connected with this interface and any other host located
in the network. There is also software available which allows us to create a virtual serial port in the PC
computer and access to serial interface of the converter connected to any place in the network just like
to the local port COM of this computer. Two converters connected to the network create translucent
data transfers between serial ports of devices attached to those converters.
Fig.4 RS485/422 to Ethernet
Routing process in Tibbo converter can be observed in Device Server Manager application.
Routing in between Ethernet port and serial port RS485/422/232 in the DS100 device is created
through two independent in-memory buffers. Each of these buffers is used independently for both flow
directions. In-memory buffers are necessary, because standard Ethernet and standard RS485/422/232
work differently and their speeds vary. Port Ethernet receives and sends data in groups and serial port
receives and sends data streams, where each byte is independent.
Ethernet to Serial data routing Device Server (DS) processes packages from Ethernet one byte
after another and then sends them to the serial port. The Device Server does not filter received data
from Ethernet port.
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Serial to Ethernet data routing In this case separating data into groups is necessary. After data is
separated and grouped into different packages, it is being sent to Ethernet via a port. The picture below
presents configuration of the routing process. Data encapsulation is made with protocol TCP in the
local port 1001, remote port is numbered 1477. The connection is active, buffer's capacity-510 byte.
Fig.5 “Routing – Status” for current conection.
Visual Server parameters research
The information flow in between the visual server and PLC controllers placed in two different
locations was monitored. The amount of Bytes/Tick was measured between Citecta server
(192.168.2.10) and the network card (192.168.2.5 port:6004) built into the operating console. The
operating console was communicating with the controller PLC FX2N through a serial connection and
then data was transferred via Ethernet network. Measurement of the traffic was taken for 900 sec.
Ethereal formula (ip.addr==192.168.2.5 and ip.addr==192.168.2.10)
First monitored controller
The amount of transferred Bytes/Cycles was measured between Citecta server (192.168.2.10
port:1268) and a network card (192.168.2.5 port 6004) built into the operating console.
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Fig.6. The traffic between server and PLC controller #1
Second monitored controller
The amount of transferred bytes/cycles was measured between server Citecta (192.168.2.10
port:3220) and FX1N controller with a serial port RS422 connected with a converter RS232/422/485 to
Ethernet network, ip assigned statically to the converter is: (192.168.2.6 port 1001). Measurement of
the traffic was taken for 900 sec. Ethereal formula (ip. addr==192.168.2.6 and ip. Addr==192.168.2.10)
Fig.7 The traffic between server and PLC controller #2
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The graphs shown above present the traffic, recorded between field devices and a visual server, it
is the first level of management in a hierarchical management structure. Below is the graph that
presents all of the information flow during the research.
Fig.8 All of the traffic recorded on the server
The size of each package in the traffic between the server and field devices is no bigger then 510
Bytes/Tick. In all of the traffic, the packages were consisting of 400000 Bytes/Tick.
Control and monitoring station parameters research
Monitored traffic consists of TCP packages at the client station (operating console) of the visual
system Citect. Citect's client application port TCP is tcp:1191. The graph below presents second level
of management in a hierarchical management structure. The measurement was taken for 900 sec.
Etheral formula tcp.srcport==1191.
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Fig.9 The traffic between server and client station Citect
Fig.10 All of the traffic on the client
On the graph below is a presentation of the traffic on the network card client station during our
research which is significantly smaller then the traffic on the visual server. The graph below shows the
value of Bytes/Tick for the x axis to compare with the traffic of packages presented on the other graphs
presented above.
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Fig.11 All of the traffic on the network card client station.
Point-to-point communication parameters research
The graph below shows the traffic between two devices (connection point-to-point). Transmitting
medium is a concentric cable with stoppers on the ends. Network cards 10 Mbit with slots BNC. The
measurement of the amount of Bytes/cycles was taken between a server with the network card
(192.168.1.8 port: 1025) and a network card in the PLC controller (1920168.1.5 port 3008). Alternately
two packages are sent from the PLC controller to the server then two packages are sent from the server
to the controller. In this type of network, collision is not going to happen so the traffic is determined.
Data sent by transmitting medium is used to record technological process.
Fig.12 The traffic between two devices(connection point-to-point)
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Fig.13 All of the traffic on the server network card
The graphs presented above show identical traffic, which states lack of differences between a network
traffic with specific application port on the server and the whole traffic on the same interface.
Summary
The main quality that should characterize industrial network is real time capability in data
transfers. Such a quality is represented by those networks, where the time of operating a package in a
network nodes is finite and stated either in accurate numbers or an interval, which also means that the
time when data is available is finite and stated in accurate numbers or an interval. The program or
mechanism that guarantees the implementation of time determined data transfers is necessary to state
the work of the protocol in real time.
Real time capability can also be improved by:
 designating Ethernet network just for controlling matters
 limitation of delays and increase of network capacity by increase of speed of transmissions
 improvement of real time capability by introducing switches, devices that will provide dedicated
connection point-to-point
 usage of faster protocol UDP/IP
 package prioritizing (norm IEEE 802.1p) by filling in typical Ethernet bracket by priority field
 algorithmic compensation of delays
 decrease in amount of packages
The research shown above proves that usage of protocol TCP/IP in computer systems used in
industry is possible. The studies were carried during the busiest traffic in the network, therefore no
collisions were detected regardless of how explosive the character of the traffic was. Economical aspect
of these studies is undeniable. Technical realization of this project is possible with usage of easily
accessible tools both hard- and software. The future of automatic systems is dependent on full
integration of institutional network and data communications. That integration should be based on
standards and protocols of Ethernet and Internet networks. Another very important element connected
with issues researched above is the usage of radio networks. These can be used in places characterized
as high dynamic areas in mechanical matters.
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