PLC Data Communication System

PLC Data Communication System

Data communications refers to the different ways that PLC microprocessor-based systems talk to each other and to other devices. The two general types of communications links that can be established between the PLC and other devices are point-to-point links and network links.

plc data communication system
Fig. 1

Figure 1 illustrates a point-to-point serial communications link. Serial communications is used with devices such as printers, operator workstations, motor drives, bar code readers, computers, or another PLC.

Serial communications interfaces are either built into the processor module or come as separate modules. A serial module installed in each controller is normally all that is required for two PLCs of the same manufacturer to establish a point-to-point link. As control systems become more complex, they require more effective communications schemes between the system components.

A local area network or LAN is a system that interconnects data communications components within a limited geographical area, typically no more than one or two miles. Figure 2 illustrates a LAN communication link.

Fig. 2

Network communications supports communication among multiple PLCs and other devices. PLC networks allow:

  • Sharing of information such as the current state of status bits among PLCs that may determine the action of one another.
  • Monitoring of information from a central location.
  • Programs to be uploaded or downloaded from a central location.
  • Several PLCs to operate in unison to accomplish a common goal.

Transmission media are the cable through which data and control signals flow on a network. The transmission media used in data communications systems include coaxial cable, twisted pair, or fiber optics ( Figure  3 ).

Fig. 3

Each cable has different electrical capabilities and may be more or less suitable to a specific environment or network type. Not all networks transmit information through cable.

Wireless Wi-Fi Ethernet networks, such as the DF1 Radio Modem, communicate through radio waves, which are transmitted through the air. In industrial applications, LANs have most often been used as the communication system for distributed control systems (DCS).

Recall that a DCS system uses individual controllers to control the subsystems of a machine or process. This approach contrasts with centralized control in which a single controller governs the entire operation.

A second major use of local area networks is that of supervisory control and data acquisition (SCADA). A LAN allows data collection and processing for a group of controllers to be accomplished using one host computer as the central point for collecting data.

Fig. 4

There are three general levels of functionality of industrial networks. Figure 4 shows an illustration of the three levels, which can be summarized as follows:

  • Device Level —The device level involves various sensor and actuator devices of machines and processes. These may include devices such as sensors, switches, drives, motors, and valves.
  • Control Level —The control level would be the networks industrial controllers are on. This level may include controllers such as PLCs and robot controllers. Communications on the control level includes sharing I/O and program data between controllers.
  • Information Level —The information level is a plant-wide network typically composed of the company’s business networks and computers. This level may include scheduling, sales, management, and corporate wide information.

Each device connected on a network is known as a node or station. As signals travel along a network cable, they degrade and become distorted in a process that is called attenuation. If a cable is long enough, the attenuation will finally make a signal unrecognizable. A repeater is a device that amplifies a signal to its original strength in order to enable its signals to travel further. Different network types will have different specifications for cable length and type without a repeater.

Fig. 5

Network topology is the physical layout of devices on a network formed by the network cables when nodes are attached. The star topology illustrated in Figure 5 and its operation can be summarized as follows:

  • A network controller switch or hub is connected to several PLC network nodes.
  • Currently, most Ethernet networks use switches rather than hubs. A switch performs the same basic function as a hub but effectively increases the speed, size, and data handling capacity of the network.
  • The configuration allows for bidirectional communication between switch/hub and each PLC.
  • All transmission must be between the switch/hub and the PLCs because the network controller hub controls all communication.
  • All transmissions must be sent to the switch/hub, which then sends them to the correct PLC.
  • One problem with the star topology is that if the switch/hub goes down, the entire LAN is down.
  • This type of system works best when information is transmitted primarily between the main controller and remote PLCs. However, if most communication is to occur between PLCs, the operation speed is affected.
  • Also, the star system can use substantial amounts of communication conductors to connect all remote PLCs to one central location.
Fig. 6

Bus topology, illustrated in Figure 6, is a network configuration in which all stations are connected in parallel with the communication medium and all stations receive information from every other station on the network. The operation of a bus topology network can be summarized as follows:

  • Uses a single bus trunk cable to which individual PLC nodes are attached by a cable drop that taps off the main cable.
  • Each PLC is interfaced to the bus using a network interface module that is attached using a drop cable or connector.
  • Due to the nature of the bus technology, and the way the data are transmitted on the network, each end of the bus must be terminated with a terminating resistor.
  • As the data move along the total bus, each PLC node is listening for its own node identification address and accepts only information sent to that address.
  • Because of the simple linear layout, bus networks require less cable than all other topologies.
  • No single station controls the network and stations can communicate freely to one another.
  • Bus networks are very useful in distributive control systems, because each station or node has equal independent control capability and can exchange information at any given time.
  • Another advantage of the bus network is that you can add or remove stations from the network with a minimum amount of system reconfiguration.
  • This network’s main disadvantage is that all the nodes rely on a common bus trunk line, and a break in that common line can affect many nodes.

I/O bus networks can be divided into two categories: device bus networks and process bus networks.

Device bus networks interface with low-level information devices such as pushbuttons and limit switches that primarily transmit data relating to the on/off state of the device and its operational status. Device bus networks can be further classified as bit-wide or byte-wide buses.

Device bus networks that include discrete devices as well as small analog devices are called byte-wide bus networks. These networks can transfer 50 or more bytes of data at a time. Device bus networks that interface only with discrete devices are called bit-wide bus networks. Bit-wide networks transfer less than 8 bits of information to and from simple discrete devices.

Process bus networks are capable of communicating several hundred bytes of data per transmission. The majority of devices used in process bus networks are analog, whereas most devices used in device bus networks are discrete.

Process bus networks connect with high-level information devices such as smart process valves and flowmeters, which are typically used in process control applications. Process buses are slower because of their large data packet size.

Most analog control devices are used in controlling such process variables as flow and temperature, which are typically slow to respond. A protocol is a set of rules that two or more devices must follow if they are to communicate with each other.

Protocols are to computers what language is to humans. This article is in English, and to understand it, you must be able to read English. Similarly, for two devices on a network to successfully communicate, they must both understand the same protocols.

A network protocol defines how data is arranged and coded for transmission on a network. In the past, communications networks were often proprietary systems designed to a specific vendor’s standards; users were forced to buy all their control components from a single supplier.

This is because of the different communications protocols, command sequences, error-checking schemes, and communications media used by each manufacturer. Today, the trend is toward open network systems based on international standards developed through industry associations.

Fig. 7

Gateways ( Figure  7 ) make communication possible between different architectures and protocols. They repackage and convert data going from one network to another network so that the one can understand the other’s application data.

Gateways can change the format of a message so that it will conform to the application program at the receiving end of the transfer. If network access translation is their only function, the interfaces are known as bridges. If the interface also adjusts data formats or performs data transmission control, then it is called a gateway. A bus topology network requires some method of controlling a particular device’s access to the bus. An access method is the manner in which a PLC accesses the network to transmit information.

Network access control ensures that data are transmitted in an organized manner preventing the occurrence of more than one message on the network at a time. Although many access methods exist, the most common are token passing, collision detection, and polling.

In a token passing network, a node can transmit data on the network only when it has possession of a token. A token is simply a small packet that is passed from node to node as illustrated in Figure 8 .

Fig. 8

When a node finishes transmitting messages, it sends a special message to the next node in the sequence, granting it the token. The token passes sequentially from node to node, allowing each an opportunity to transmit without interference. Tokens usually have a time limit to prevent a single node from tying up the token for a long period of time.

Ethernet networks use a collision detection access control scheme. With this access method, nodes listen for activity on the network and transmit only if there are no other messages on the network. On Ethernet networks there is the possibility that nodes will transmit data at the same time.

When this happens a collision is detected. Each node that had sent out a message will wait a random amount of time and will resend its data if it does not detect any network activity.

The access method most often used in master/slave protocols is polling. The master/slave network is one in which a master controller controls all communications originating from other controllers.

Fig. 9

This configuration is illustrated in Figure 9 and consists of several slave controllers and one master controller. Its operation can be summarized as follows:

  • The master controller sends data to the slave controllers.
  • When the master needs data from a slave, it will poll (address) the slave and wait for a response.
  • No communication takes place without the master initiating it.
  • Direct communication among slave devices is not possible.
  • Information to be transferred between slaves must be sent first to the network master unit, which will, in turn, retransmit the message to the designated slave device.
  • Master/slave networks use two pairs of conductors. One pair of wires is used for the master to transmit data and the slave to receive them. On the other pair, the slaves transmit and the master receives.

A peer-to-peer network has a distributive means of control, as opposed to a master/slave network in which one node controls all communications originating from other nodes.

PLC Data Communication System

Fig. 10

The Allen-Bradley Data Highway, shown in Figure 10, is an example of a peer-to-peer network of programmable controllers and computers linked together to form a data communication system. The operation of the network can be summarized as follows:

  • Peer-to-peer networks use the token passing media access method.
  • Each device has the ability to request use of, and then take control of, the network for the purpose of transmitting information to or requesting information from other network devices.
  • Each device is identified by an address.
  • When the network is operating, the token passes from one device to the next sequentially.
  • The device that is transmitting the token also knows the address of the next station that will receive the token.
  • Each device receives the packet information and uses it, if needed.
  • Any additional information that the node has will be sent in a new packet.

There are two methods of transmitting PLC digital data: parallel and serial transmission. In parallel data transmission, all bits of the binary data are transmitted simultaneously, as illustrated in Figure  11.

Fig. 11

Parallel transmission of data can be summarized as follows:

  • Eight transmission lines are required to transmit the 8-bit binary number.
  • Each bit requires its own separate data path and all bits of a word are transmitted at the same time.
  • Parallel data transmission is less common but faster than serial transmission.
  • A common example of parallel data transmission is the connection between a computer and a printer.
Fig. 12

In serial transmission one bit of the binary data is transferred at a time, as illustrated in Figure 12. Serial transmission of data can be summarized as follows:

  • In serial transmission, bits are sent sequentially on the same channel (wire) which reduces costs for wire but also slows the speed of transmission.
  • Serial data can be transmitted effectively over much greater distances than can parallel data.
  • Each data word in the serial transmission must be denoted with a known start bit sequence followed by the data bits that contain the intelligence of the data transmission and a stop bit.
  • An extra bit, termed a parity bit, may be used to provide some error-detecting ability.

A duplex communication system is a system composed of two connected devices that can communicate with one another in both directions at the same time. A half-duplex system provides for communication in both directions, but only one direction at a time (not simultaneously). Half-duplex transmission is use for master/slave communications.

Full-duplex transmission allows the transmission of data in both directions simultaneously and can be used for peer-to-peer communications. The different networking schemes replace traditional point-to-point hardwiring.

Network control of systems minimizes the amount of wiring that needs to be done. With traditional wiring multiple wires from each device, fed through control cabinets, often result in large wire bundles running through the system. Due to the sheer volume of wires, installation time is considerable and troubleshooting is complex. If a network is used all devices can be directly connected to a single transmission media cable.

High-speed industrial networking technologies offer a variety of methods for connecting devices. PLC network configurations may be either open or proprietary (vendor-unique). Following is an overview of some of the industrial communication technologies that play a critical role in today’s control systems.

Fig. 13

Data Highway:  The Allen-Bradley Data Highway networks, Data Highway Plus (DH+) and DH-485, are proprietary communications networks. They use peer-to-peer communication implementing token passing. The medium is shielded twisted pair cable. Figure 13 shows the DH+ network connection for a SLC 5/04 controller. The three-pin Phoenix connector is used to form the network transmission media.

Serial Communication:  Serial data communication is implemented using standards such as RS-232, RS-422, and RS-485. The RS in the standard’s name means recommended standard that specifies the electrical, mechanical, and functional characteristics for serial communications.

Serial communication interfaces are either built into the processor module or come as a separate communications interface module, as illustrated in Figure 14.

Fig. 14

The simplest type of connection is the RS-232 serial port. The RS interfaces are used to connect to devices such as vision systems, barcode readers, and operator terminals that must transfer quantities of data at a reasonably high rate between the remote device and the PLC.

The RS-232 type of serial transmission is designed to communicate between one computer and one controller and is usually limited to lengths up to 50 feet. RS-422 and RS-485 serial transmission types are designed to communicate between one computer and multiple controllers, have a high level of noise immunity, and are usually limited to lengths of 650 feet (for RS-485) or 1650 feet (for RS-422).

DeviceNet: DeviceNet is an open device-level network. It is relatively low speed but efficient at handling the short messages to and from I/O modules. As PLCs have become more powerful, they are being required to control an increasing number of I/O field devices. Therefore, at times it may not be practical to separately wire each sensor and actuator directly into I/O modules. Figure 15 shows a comparison between conventional and DeviceNet I/O systems.

Fig. 15

Conventional systems have racks of inputs and outputs with each I/O device wired back to the controller. The DeviceNet protocol dramatically reduces costs by integrating all I/O devices on a 4-wire trunk network with data and power conductors in the same cable. This direct connectivity reduces costly and time-consuming wiring. The basic function of a DeviceNet I/O bus network is to communicate information with, as well as supply power to, the field devices that are connected to the bus.

Fig. 16

The PLC drives the field devices directly with the use of a network scanner instead of I/O modules, as illustrated in Figure 16. The scanner module communicates with DeviceNet devices over the network to:

  • Read inputs from a device.
  • Write outputs to a device.
  • Download configuration data.
  • Monitor a device’s operational status.

The scanner module communicates with the controller to exchange information which includes:

  • Device I/O data
  • Status information
  • Configuration data

DeviceNet also has the unique feature of having power on the network. This allows devices with limited power requirements to be powered directly from the network, further reducing connection points and physical size.

DeviceNet uses the Common Industrial Protocol, called CIP, which is strictly object oriented. Each object has attributes (data), services (commands), and behavior (reaction to events). Two different types of objects are defined in the CIP specification: communication objects and application-specific objects.

A DeviceNet network can support up to 64 nodes and the network end-to-end distance is variable, based on network speed. Figure 17 shows an example of a typical layout of the trunk wiring for a DeviceNet network. Communications data is carried over two wires with a second pair of wires carrying power.

Fig. 17

The field devices that are connected to the network contain intelligence in the form of microprocessors or other circuits. These devices can communicate not only the on/off status of field devices but also diagnostic information about their operating state.

For example, you can detect via the network that a photoelectric sensor is losing margin because of a dirty lens, and you can correct the situation before the sensor fails to detect an object. A limit switch can report the number of motions it has performed, which may be an indication that it has reached the end of its operating life and thus requires replacement.

ControlNet: ControlNet is positioned one level above DeviceNet. It uses the Common Industrial Protocol (CIP) to combine the functionality of an I/O network and a peer-to-peer network providing high-speed performance for both functions. This open high-speed network is highly deterministic and repeatable. Determinism is the ability to reliably predict when data will be delivered, and repeatability ensures that transmit times are constant and unaffected by devices connecting to, or leaving, the network. Electronic device data sheets (EDS-Files) are required for each ControlNet device. During the setup phase the ControlNet scanner must configure each device according to the EDS-Files.

Fig. 18

The ControlNet layout shown in Figure 18 has a redundant media option in which two separate cables are installed to guard against failures such as cut cables, loose connectors, or noise.

EtherNet/IP:  EtherNet/IP (Ethernet Industrial Protocol) is an open communications protocol based on the Common Industrial Protocol (CIP) layer used in both DeviceNet and ControlNet. It allows users to link information seamlessly between devices running the EtherNet/IP protocol without custom hardware, as illustrated in Figure 19.

Fig. 19

The following are some of the important features of EtherNet/IP:

  • Sharing a common application layer between ControlNet, DeviceNet, and Ethernet/IP will make plug-and-play interoperability possible among complex devices from multiple vendors. Plug and play refers to the ability of a computer system to automatically configure devices . This allows you to plug in a device and play (operate) it without worrying about setting DIP switches , jumpers , and other configuration elements.
  • EtherNet/IP provides standardized full-duplex operation which gives a single node, in a peer-to-peer connection, full attention and therefore maximum possible bandwidth. Bandwidth refers to the data rate supported by a network, commonly expressed in terms of bits per second. The greater the bandwidth the greater the overall performance.
  • EtherNet/IP allows interoperability of industrial automation devices and control equipment on the same network used for business applications and browsing the Internet.

Modbus: Modbus is a serial communication protocol originally developed by Modicon for use with its PLCs. Basically, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves.

Modbus is an open protocol, meaning that it’s free for manufacturers to build into their equipment without having to pay royalties. It has become a standard communications protocol in industry, and is one of the most commonly available means of connecting industrial electronic devices.

Fieldbus: Fieldbus is an open, serial, two-way communications system that interconnects measurement and control equipment such as sensors, actuators, and controllers. At the base level in the hierarchy of plant networks, it serves as a network for field devices used in process control applications. There are several possible topologies for fieldbus networks.

Fig. 20

Figure 20 illustrates the daisy-chain topology. With this topology, the fieldbus cable is routed from device to device. Installations using this topology require connectors or wiring practices such that disconnection of a single device is possible without disrupting the continuity of the whole segment.

PROFIBUS-DP: PROFIBUS-DP (where DP stands for Decentralized Periphery) is an open, international fieldbus communication standard that supports both analog and discrete signals. It is functionally comparable to DeviceNet. The physical media are defined via the RS-485 or fiber optic transmission technologies.

PROFIBUS-DP communicates at speeds up to 12 Mbps over distances up to 1200 meters. Figure 21 illustrates a Siemens S7-200 Micro PLC system connection to a PROFIBUS-DP network.

Fig. 21

Supervisory Control and Data Acquisition (SCADA)

In some applications, in addition to its normal control functions, the PLC is responsible for collecting data, performing the necessary processing, and structuring the data for generating reports. As an example, you could have a PLC count parts and automatically send the data to a spreadsheet on your desktop computer. Data collection is simplified by using a SCADA (supervisory control and data acquisition) system, shown in Figure 22.

Fig. 22

Exchanging data from the plant floor to a supervisory computer allows data logging, data display, trending, downloading of recipes, setting of selected parameters, and availability of general production data. The additional supervisory control output capabilities allow you to tweak your processes accurately for maximum efficiency.

In general, unlike distributive control systems, a SCADA system usually refers to a system that coordinates but does not control processes in real time. In a typical SCADA system, independent PLCs perform I/O control functions on field devices while being supervised by a SCADA/HMI software package running on a host computer, as illustrated in Figure 23.

Fig. 23

Process control operators monitor PLC operation on the host computer and send control commands to the PLCs if required. The great advantage of a SCADA system is that data are stored automatically in a form that can be retrieved for later analysis without error or additional work.

Measurements are made under processor control and then displayed onscreen and stored to a hardcopy. Accurate measurements are easy to obtain, and there are no mechanical limitations to measurement speed.

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