Data Communication & Networking in Manufacturing System

1. 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

2. Data Communication & Networking in Manufacturing System Chapter 8The Role of Networking in Manufacturing Industry

3. 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.

4. 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.

5. 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.

6. 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.

7. 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 sub-programs

8. 8.2 Network Topologies

9. 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.

10. 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: • 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). • 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.

11. Star Network Topology Disadvantages to the star network topology: 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. High cost of cabling. Have long cables all meandering their way towards a central node Cable maintenance. In large star networks, many cables need to converge on the central node. This makes trouble-shooting more difficult and time-consuming

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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

19. 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

20. 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.

21. 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

23. 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 7-layer Communications model that is shown in Figure 7.6

24. 8.4 ISO / OSI Seven Layer Model