Networks and Communications
Networking From the Device to the Internet
The drive to integrate the manufacturing enterprise from the simplest device to the highest-level information system represents a task which few networks are able to manage seamlessly. The problem, however, is that a laundry list of network options currently exist - enough to confuse even the most informed manager - and separating those that employ dated technologies is a labor-intensive, time-consuming task. Make no mistake, though, there is no one "cure all" network. Distinct differences in the way tasks are performed and data is handled for various applications require more than one type of network in most facilities. However, linking these networks to provide seamless communication from the device to the Internet provides an even greater productivity obstacle. As a result, this application story will highlight the benefits of open, object-orientated communication on the factory floor, explain the need for more than one network, define seamless integration in terms of the producer/consumer model and describe how it is achieved today (and will be tomorrow).
Benefits of Object-Orientated Communication
During the reign of proprietary communication networks, many controls component suppliers were faced with the daunting task of building dozens of network interfaces for a single product. This costly practice increased the price of products and demanded a significant amount of human resources. Meanwhile, users of proprietary systems were forced to standardize on a network and choose from the products that were available for it. The resulting high interest in the emergence of open networks isn't too much of a surprise. The transition to open networks has allowed product developers to concentrate on just a few networks, while allowing users to mix-and-match best-in-class products from a variety of suppliers.
The transition to open networks has also evoked a migration to newer, more efficient types of object-oriented communication. We see object-oriented systems used in almost every new technology. The driving force was the computer industry where common platforms, open operating systems, and common programming languages allowed developers to leverage existing knowledge into newer projects. Today, all new software developments are object-orientated. Migrating this technology from the PC industry into industrial-communication networks was one of the challenging projects for Rockwell Automation in the early 90s.
Object-orientated networks offer a lot of advantages in a world of open networks. ControlNet, DeviceNet, EtherNet/IP and Foundation Fieldbus are all object-orientated networks. And as a result, developers can use standard objects provided from these networks, or can create their own vendor-specific objects, which fit their specific needs. For instance, a valve company can write their own product-specific code, add some data, wrap it in an object and it will work anywhere in the network, and on any of the networks. Objects are the only natural, modular, replaceable, re-usable and self contained building blocks available to developers and end users to take advantages of modern networking. This object orientation contributes not only to better interoperability, but by defining device profiles (minimum objects required for a given device type) interchangeability among vendors is possible.
The main benefit, however, of transferring information via an open, object-oriented network is an increase in the amount and the rate of data flowing back to the control system. The improved communication flow affects maintainability and quality of the manufacturing operation. For example, communicating device diagnostics can save operation and maintenance costs, especially with hazardous or difficult-to-reach processes, and can help improve performance by detecting deteriorating conditions before they become problems.
Diagnostic messages such as "limit switch LS101 is broken" originating from devices like sensors, actuators, I/O and operator interface systems can drastically reduce the time it takes to locate devices needing maintenance. Two-way communication also aids in the flexible reconfiguration of device parameters, providing expanded control over the process as a result of access to information. Devices can also be easily added or removed from the network.
The Need for More than One Network
Because of the different tasks in most control system, there are typically three basic network levels: Information, Control, and Device. At the Information level, large amounts of data are sent non-deterministically. Typically, Ethernet is used at this level for functions such as system-wide data collection and reports. At the Control level, PLCs and PCs typically control I/O racks and I/O devices such as variable speed drives and dedicated HMI. For this layer ControlNet is widely used in Industrial applications where time critical interlocking between PLCs and guarantied I/O updates rates is an extremely important functionality. At the Device level are two types of networks. The first type primarily handles communication to and from discrete devices. DeviceNet is a popular example in this area. The other type primarily handles communication to and from process devices. And Foundation Fieldbus is a popular example in this area. As time goes on, new commercial technologies will continue to emerge and may eventually find a home in manufacturing applications. The bottom line is that there won't be - at least in the near future - a single network that can serve all purposes.
Seamless Integration
Product suppliers should have a vested interest in making it as easy as possible for end users to communicate from device to device, and from network to network. Users with multiple networks in their facilities shouldn't have to spend countless hours doing special programming and configuring in order to achieve seamless integration. To understand how best to proceed, it is important to know how messages are transferred using object-orientated communication. Messages that need to be sent from device to device or network to network are reformatted into smaller, more manageable pieces called data packets. Each packet consists of a header (data transmission alert signal, synchronization information, source address and destination addresses), a data section and a trailer (validation information, cyclical redundancy check, etc.).
Methods used to achieve seamless communication include a common language, routable protocols, linking devices, hardware and software routers, and gateways (in part). The secret on how to achieve this seamless communication with newest technologies lies in object-orientated communication methods. For easy communication between two networks, the data packets must be in the same language and have a standard set of rules for communication.
Producer/Consumer Functionality Provides a Common Language Over Any Wire
In the past, network communication models were based on what was referred to as Source/Destination or point-to-point models. Using this model, a data packet that needed to be delivered to more than one destination had to be sent one at a time. Many end users determined that this procedure wasted bandwidth and made synchronization between devices every difficult (as data arrives at each device at a different time). An object-orientated communications model providing Producer/Consumer functionality was introduced to make data handling more efficient. On Producer/Consumer-based networks, messages are identified by content rather than as source to destination. If a node needs data, it will accept that identifier (a unique number) and consume it. With Producer/Consumer networks, multiple nodes can consume the same data at the same time from a single producer, resulting in more efficient use of the bandwidth. In other words, the node with the data just needs to send it once, via multi-cast, and other devices on the network that need the data can consume it.
NetLinx™ is the term used to describe the Rockwell Automation open network architecture that allows you to Control, Configure and Collect data seamlessly throughout your plant. It's based on networks that use the Producer/Consumer network model and a single, routable protocol that can be used over any wire. While there are dozens of networks on the market today, it would be safe to assume that in the future some will be eliminated, and new ones will be added. With the uncertainty about which wire will be favored at any given time, it only makes good business sense that users adopt networks that have a flexible migration path. NetLinx provides this migration path with its common routable protocol that is media independent and s that can be designed into the specification of any network.
This routable protocol is technically called the Control and Information Protocol (CIP). CIP is an object-orientated protocol that has been optimized for solving industrial automation needs and problems (configuration, control and collection of data). Today, there are three networks that use CIP and they include DeviceNet, ControlNet and Ethernet/IP. A fourth network Foundation Fieldbus can also integrate very well in the NetLinx architecture because it is a Producer Consumer based network.
Routable Protocols
This is the capability of routing data through and to multiple networks. These multiple networks might have different physical media and data link layers (cable, media access), and different transport mechanisms, but they use the same objects library and device profiles in the application layer.
The tremendous advantage for the user is that there is absolutely no programming necessary in the controller to communicate from Ethernet to ControlNet and/or to DeviceNet (see Figure 2). The common routable protocol - CIP takes care of all routing and interfacing between the products and Networks that make up the NetLinx architecture.
Linking Devices
If networks are based on the same model and have a routable protocol, it is very easy to connect the networks with linking devices. For instance, the ControlNet-to-Fieldbus H1 Linking Device allows a seamless integration between the device and control levels. This linking device and corresponding configuration software make it possible to access process variables from Foundation Fieldbus and transfer them throughout the manufacturing system via ControlNet, a high-speed, deterministic, intrinsically safe network. The linking device is the effort of a ControlNet International Special Interest Group that wanted to provide users with the capability to simultaneously and effortlessly leverage both the high speed (5 Mbit) and dual-media redundancy of ControlNet, and the process device interfacing capability of Foundation Fieldbus H1.
Rockwell Automation also offers a ControlNet to DeviceNet Linking device.
Hardware and Software Routers
A router is a software and hardware connection between two or more networks that permits traffic to be routed from one network to another. Routers, also know as bridges, can connect networks that use different network adapters or transmission media as long as both sides of the connection use the same protocols. The Rockwell Automation ControlLogix can support multiple network interface modules and can therefore be configured as a bridge.
Gateways
If the networks are not based on the same communication model and language, it is still possible to achieve some integration, but it is necessary to use gateways. By definition, a gateway is a hardware/software package that allows incompatible protocols to communicate. Gateways are normally used to connect controllers to a host machine, such as a mainframe. In runtime, gateways require additional time to translate the data from one protocol to the next and they add overhead to data packets - both of which impede the efficient transfer of data. They also require extensive programming in the controller to transfer data in and out of the gateway. In addition, gateways are typically quite expensive. So it is to the end user's advantage to use linking devices because of the gateway's cost and lower performance. However, ControlLogix can also be configured as a gateway device.
The Benefits of a Common Language
The NetLinx architecture is the basic ingredient needed to easily achieve seamless integration. CIP can be applied over any wire and this increases the number of wire choices for the end user. This includes networks that are used in manufacturing applications today, such as DeviceNet, ControlNet, Ethernet and Foundation Fieldbus, as well as the networks that users might want tomorrow, and such as Fast Ethernet, ATM, USB and FireWire.
A network based on the Producer/Consumer network model, which uses CIP also, provides the latest services that users rely on to increase the efficiency of their process. This includes, among others, multicast, change-of-state, cyclic data production, explicit and I/O messaging on the same wire. Explicit messages are used for uploading and downloading programs, modifying device configurations, and data logging, trending and diagnostic functions, also on the same wire. Having this capability decreases installation time because there is less programming for store/forwarding in a PLC. I/O messages on the other hand are implicit in nature. The data field contains no protocol information, only real-time I/O control data. Prior to CIP, manufacturers have had to use separate networks to manage the very different requirements of these two messaging types.
The NetLinx architecture also improves embedded web-server capabilities. Users can load up their Internet browser and enter the IP address of the device they are interested in monitoring. Rather than just having the ability to access the diagnostics of the master controller, the user can view any device on the local network.
Today's Networking Solutions
In the past, the decision on which network to install has not been an easy task. Now, with the NetLinx architecture providing a clearer migration path, the decision has just gotten easier. Users can now use a single routable protocol and have the choice of the media type they require for their given application to help assure that they can achieve seamless integration throughout their manufacturing enterprise now and well into the future.
7 Layer OSI Model
Control and Information Protocol (CIP)
|
||
| TCP/UDP/IP | ||
| CAN1) | CTDMA2) | Ethernet3) |
| DeviceNet | ControlNet | EtherNet/IP |
- CAN Control Area Network: originally developed by automotive industry for vehicle networking, later adopted from Industrial Automation
- Concurrent Time Domain Multiple Access: Data Link Layer that gives ControlNet its unique deterministic scheduled I/O capability
- Ethernet uses CSMA/CD: Carrier Sense Multiple Access with Collision Detection
Seamless Communications
Open Network Architecture — Seamless Communications from the DeviceNet to the Internet
