Download PLM & Digital Manufacturing - Industrial Engineering 2011

Data Communication & Networking in
Manufacturing System
Nanang Ali Sutisna
Master Eng. in Computer Integrated Manufacture
Senior PLM Consultant, IBM Indonesia (retired)
Senior Manager, Product Development
Multistrada Arah Sarana
Data Communication & Networking in
Manufacturing System
Chapter 8
The Role of Networking in
Manufacturing Industry
8.1 Local Area Network Concepts
A data network is a mechanism by which many computer-based
devices (referred to as network nodes) can communicate with one
another on an "any node to any node" basis. A Local Area Network
(LAN) is so named because the nodes on that network are located
within a reasonable proximity (less than a kilometre) of one another.
A point to point link between two nodes can therefore be considered
as a network with two nodes and it shares many of the
characteristics of larger networks.
8.1 Local Area Network Concepts
The rules of protocol need to be resolved before a network can
function correctly:
 common signalling techniques
 Common character representations
 complementary communications hardware
 etc.
We need the communications interchanges between nodes to be
strictly governed by these rules of protocol so that conflicts can be
resolved.
8.1 Local Area Network Concepts
The majority of networks use serial communication between nodes. With this
in mind, there are a number of ways in which we can physically interconnect
these nodes so that any one node can communicate with any other node in a
serial form. Three of these interconnections are shown in Figure 7.1.
8.1 Local Area Network Concepts
The need for addressing means that regardless of the physical
network arrangement, data must be placed into suitable packets for
transfer. Each packet of data moving through a network needs to
contain some source and target addressing information. This
enables a receiving device know which device is transmitting to it
and where to send response messages. The concept of packet
addressing is shown schematically in Figure 7.2.
8.1 Local Area Network Concepts
With the exception of traffic control (contention) and addressing
functions, the issues related to networks are essentially the same
as those in point to point links.
We still need:
 error checking mechanisms in the form of Block Check Sums (or
more commonly, Cyclic Redundancy Checks)
 hardware to perform parallel to serial (and vice-versa) conversion
on each node
 need layers of data handling software that can provide our
applications programs with powerful communications subprograms
8.2 Network Topologies
8.2 Network Topologies
The selection of a networking topology is governed by a number of
factors including:
• the type of protocol selected to govern the network
• the availability of interfacing equipment
• the type of equipment being networked
• the environment in which the network is located.
In the manufacturing environment, the most common high-level
network topology is the bus structure and the majority of protocols
for this environment are based upon this topology. Star networks
are also in widespread use in manufacturing to link CAD systems to
a range of different CNC machines.
Star Network Topology
The star network has an intelligent central node, referred to as the "star
node".
The star node performs the following function:
• makes the decisions related to connecting any pair of nodes together. It
therefore needs to be able to resolve any contentions that may arise.
• responsible for tasks such as queuing requests for "link establishment"
between nodes.
Several advantages of Start Network:
1. The star node is transparent to communicating nodes once a connection
has been made. In other words, devices 1 and 2 can talk to each other
through "protocol A" and devices 3 and 4 can talk to each other through
"protocol B". It is then also possible that device 3 can talk to device 1
through "protocol A". This has merits in manufacturing where it is not
always practical to have all nodes using a single protocol and yet it may
still be necessary to have all nodes capable of talking to one another. A
good example of this would be where devices 1, 2 and 3 are computers
and device 4 is a robot or CNC machine (with a fixed protocol).
2. The physical medium used between any one node and the star node can
be varied to suit the operating environment. For example, if device 1 is in
the factory then it can be linked to the star node through an optic fibre
cable. If device 2 is in the office, close to the star node, then it can be
linked via a twisted-pair cable and so on.
Star Network Topology
Disadvantages to the star network topology:
1. All communication is dependent upon the star node - if it fails then
all communications ceases. In a simple star network, the star node
may be a microprocessor-controlled, serial port PABX (a multiplexer).
In this case it is feasible to maintain some redundancy since the cost
of the star node is minimal. At the other extreme however, a large
star network could have a mini-computer as the star node, with
many intelligent terminals communicating to one another through it.
In this situation it is not practical to maintain redundancy.
2. High cost of cabling. Have long cables all meandering their way
towards a central node
3. Cable maintenance. In large star networks, many cables need to
converge on the central node. This makes trouble-shooting more
difficult and time-consuming
Bus Network Topology
The bus network is perhaps the most common form of the networking
topologies - particularly in the industrial environment.
A bus network is similar to the internal bus structure used for
communications within a microprocessor system environment. The major
differences are that in bus networks, data transfer is serial (not parallel)
and secondly that there is no simple "master-slave" relationship between
devices and therefore many contention situations can arise.
Advantages:
Bus networks offer a flexibility in terms of cable utilisation, which cannot be
achieved with other network topologies. The fact that a bus network is
based upon a trunk cable, which is laid throughout an entire area, means
that video and voice channels can share the same cable, through the use of
modulation techniques. This greatly increases the cost effectiveness of the
bus network.
Ring Network Topology
In a ring network, neighbouring nodes are interconnected with point
to point serial links until a complete ring is formed. Data in ring
networks is passed unidirectionally from node to node. Each device
receives a message and then retransmits it. This is shown in Figure
7.4.
Ring Network Topology
A device in a network ring originates a message that is passed
around the loop from node to node. Nodes in between the source
and the destination do not alter the message.
However, when the destination node receives the message, it
modifies the control portion of the message packet and places it
back onto the loop.
The originator of the message packet determines whether or not
the message has reached its target correctly by the modifications
on the returning packet.
Ring networks are relatively commonplace in the office
environment, where the area they cover is relatively small.
The response time of networks based upon the ring topology can be
very good with an appropriate protocol.
Ring Network Topology
A potential problem for the ring network topology arises because
devices are all interlinked with point to point links. Hence one is
tempted to ask what happens when a device fails - does the network
stop?
As it turns out, there are by-pass mechanisms built into ring
networks so that devices that are down (or just switched off) provide
a short-circuit path and do not result in network failure.
However, the ring network completely fails if any one of the point to
point links is severed.
In terms of cabling in ring networks it is evident that if one node is
far removed from all other nodes then the cost of transmission
medium will be higher than that in the bus network.
For this reason, ring networks are most commonly found in the office
environment for short-distance communications.
8.3 Contention Schemes
The technique of modulation allows us to utilise a transmission
medium with a high degree of efficiency. Modulation is all about
changing the physical representation of information, by creating
communications channels, so that many different information
systems can share the same transmission medium. This gives us
the opportunity of using the same transmission medium for
video, audio and digital data transmission - however, within any
one channel conflicts can still arise.
In a network, or more specifically in a bus network, we have a
situation where there is the potential for many nodes to attempt
to place data onto the transmission medium at the same time.
This uncontrolled transmission could make decoding impossible.
The physical conflict is called a contention situation.
8.3 Contention Schemes
Figure 7.5 shows a bus network in which devices are all tied
together through a two wire conducting cable (signal + common
line). Since all devices in this network are "intelligent", they are
capable of placing data (represented by voltage levels) onto the
bus at any time.
8.3 Contention Schemes
Contentions can be resolved in any number of different ways, but
there are two generic techniques of contention resolution that are
in widespread use:
 CSMA/CD
 Token Passing
8.3 Contention Schemes
CSMA/CD
CSMA/CD is an abbreviation for "Carrier Sense, Multiple Access with
Collision Detection". The CSMA/CD system sounds complex but is
straightforward to implement in practice. It is used within a number of
different bus networks.
Each device in a CSMA/CD system is allowed to attempt to transmit on the
network bus at any time. In other words, multiple access. However, prior to
attempting a transmission, each device must monitor the bus for the
presence of a carrier signal, emanating from another node. This is called
"carrier sensing".
If a carrier is already present on the bus (another node is already
transmitting), then the device must wait until that transmission has ceased
before attempting to place a message packet (frame) on the bus. Even
when a device has the right to transmit, it must still monitor the bus to
ensure that the signal that is being sent is the same as that on the bus.
The CSMA/CD system is not ideally suited to the industrial environment. The
irony of CSMA/CD is that the time delay for messages is longest when the
network is busiest and the network is generally busiest when abnormal or
emergency conditions arise
8.3 Contention Schemes
Token Passing Schemes
In principle, the so-called "token passing" scheme sounds much simpler than
the CSMA/CD system. In practice it is more difficult to implement. The
scheme is based upon a binary bit pattern that is referred to as a "token".
Before any node is permitted to place message frames onto a network, it just
be in possession of the token. Once a node has the token, it is permitted to
transmit a message frame and must then pass the token on to another node.
The movement of the token from node to node forms a "logical ring" between
devices.
The token is itself a message frame (packet) with a special control section
that defines its characteristics. A node wishing to use the token modifies these
characteristics so that it can become a message frame. The node can then
place data into the message frame. The token passing scheme is
deterministic, because it is possible to precisely define the maximum delay
that will arise in transmitting a data frame. It is for this reason that the
scheme is often promoted as a basis for industrial networks.
8.3 Contention Schemes
Token Passing disadvantages:
There are a number of problems with the token passing scheme.
Since it is possible for a device to fail while it is in possession of
the token, steps must be taken to ensure that there is a
mechanism for regenerating a lost token.
The token passing scheme also introduces delays into the
network even under light traffic conditions. In other words, a
token is still passed from device to device, whether or not that
device is to broadcast on the network
8.4 ISO / OSI Seven Layer Model
The problem with trying to rationalise networking standards is that
it is a very complex task to decide upon the scope of a standard for
any individual networking requirement.
The International Standards Organisation (ISO) tackled this
problem by developing a framework for what is referred to as
"Open Systems Interconnection" or "OSI". The objective of this
framework was to place all the requirements, for making a number
of computers communicate with one another, into seven functional
groups called "layers". The end result of this work was the OSI 7layer Communications model that is shown in Figure 7.6
8.4 ISO / OSI Seven Layer Model