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March 1, 2002

6 Min Read
Which way to factory automation? Take the bus

If you have not yet gotten into it, chances are you will be looking at factory automation (FA) quite soon. We're not talking about installing robotics. We're talking about data and the benefits of networking every molding machine, granulator, dryer, and accessory device in a molding or moldmaking facility. With just a few mouse-clicks or keystrokes you can look at what's happening in any part of your operation. Up-to-date inventory; real-time machine process parameters; output by machine, department, or the whole shop; scrap rates; and more are literally at your fingertips. 

An FA system is at heart a data network. Therefore, armed with the information it provides, you can use that same network to tweak and adjust anything attached to it. And thanks to the Internet, you can do this from almost anywhere in the world. 

Because FA is data networking technology, there is a learning curve to be scaled and new terms and acronyms to sort out. You most likely have heard some of these already since machinery and peripheral suppliers are building FA capability and conformance into their products. Does it seem like many of them end in "bus"? Many do, and at first that may seem confusing. The most important thing, however, is that your computers and machinery can speak to each other, which requires that they be able to ride the same bus. 

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Which Bus? 
Profibus, Modbus, Interbus, Fieldbus . . . there is a large variety of networking protocols and standards on the market—some better known than others. Go to ourworld.cs.com/rahulsebos for an idea of how many protocols have been developed. Also, see Table 1, below. 

Since the numbers can be a bit daunting, let's try to simplify this a bit. In a data network, the data transport mechanism between nodes (equipment, control, computer) is called a bus, so named because data packets, like bus passengers, can enter and exit at any station on the line. 

The FA networks, also called industrial automation (IA) networks, mostly—but not always—fit into one of three categories: general purpose information network; fieldbus, which is sometimes referred to as a control bus; and device bus, also called bit-level bus. The image on this page provides examples of each type. 

The device bus is the simplest of the three levels, offering basic connectivity between robots, molding machines, and material handling devices via an I/O device that is usually in the control unit. 

A fieldbus goes beyond basic connectivity to permit the exchange of periodic
data from and to the I/O devices. A fieldbus also supports point-to-point messaging and network management tasks that assist in coordinating the I/O devices through configuration and diagnostics. Many fieldbuses employ a technique called virtualizing to create a network identity for each I/O unit, thus allowing use of equipment from different manufacturers. 

These days it is a fieldbus name or standard that is most commonly included in the specs of plastics molding equipment makers. Some of the most frequently cited include Fieldbus Foundation, CANopen, DeviceNet, Interbus, Modbus, and Profibus. As an example, Maguire just announced an adapter module for its weigh scale blenders that supports DeviceNet, Modbus, and Profibus (see "Adapter Module Helps Blenders Communicate," January 2002 IMM, p. 112). 

Although there are a variety of fieldbus standards available, a molding professional is ultimately at the mercy of machinery and software suppliers that decide which protocols their equipment will support. Most suppliers have a lot of information on fieldbuses, and we have included websites in the list of fieldbus types in Table 1. If you are looking at factory automation, it is worth the time and effort to try to understand network structures and specifications—and to see where they are going. 

Table 1. Common industrial fieldbus protocols

Fieldbus
CANopen

Developer
CAN In Automation
www.can-cia.de

Standard(s)
CiA DS 301

Openness
Multiple chip and product vendors

ControlNet

Allen-Bradley
www.controlnet.org

ControlNet
International

Chips from Allen-Bradley, 17 product vendors

DeviceNet*

Allen-Bradley
www.odva.org

ISO 11898, 11519

Multiple chip and product vendors

Foundation 

Fieldbus Foundation

ISA SP50/IEC 61158

Chips, software, products from multiple vendors

Interbus**

Phoenix Contact
Interbus Club
www.interbusclub.com

DIN 19258
EN 50.254

Products from more than400 manufacturers

Modbus**

EN 1434-3 (layer 7)
IEC 870-5 (layer 2)

Open specification

Profibus**

EN 50170/DIN 19245 part 3 (DP)/4 (PA),
IEC 1158-2 (PA)

ASICs from Siemens and Profichip, more than 300 product vendors

*Ethernet/IP info is at DeviceNet website.  **Providers with Ethernet solutions.


Rewriting the Rules 
The three-level structure shown above—information, fieldbus, and device—is not rigid. A number of buses have taken on tasks of several levels, but not all. For example, AS-interface (AS-i) reaffirms its dedication to the device level. They say simplicity and low cost are their advantages when all that is needed is to connect devices for signal passing. 

DeviceNet and CANbus began life as device buses, the latter in automobiles, and each has been enhanced to offer some of the performance of a fieldbus. Some fieldbus suppliers have integrated simple device interconnection. In general, network suppliers are enlarging the capabilities of their offerings to cross the levels, and many of them say their ultimate objective is to support the network of an entire enterprise. 

One of the biggest obstacles to a full-enterprise network has been the differing architectures among the three bus types. The largest gap has been between the general data networks and the field and device buses. 

The most ubiquitous data corporate networking standard is Ethernet, unless you count the Internet, which in this case you should. It was the very structure of Ethernet that created difficulty with the other levels. Ethernet is subject to data collisions and delays that mean little in a business environment, but could play havoc with sequenced control signals. The technical details of how this has been solved could fill this magazine. What is important is that many leading fieldbus protocol developers have begun to offer full data network solutions based on Ethernet or Internet protocols, or a combination of the two. 

Fieldbus Foundation's high-speed Ethernet is an open, fault-tolerant 100 megabit/sec backbone for connecting device buses, subsystems like control nets, and enterprise data servers. Interbus with Ethernet connects from sensor to Internet using the TCP/IP protocol you probably use for e-mail. Modbus now has Modus TCP that combines the Internet's TCP/IP protocol with Ethernet. Profibus has standardized Profinet, an Ethernet-based open concept integrated with Profibus control networks. Ethernet/IP (industrial protocol) shares an application layer protocol with DeviceNet, and the two can combine to provide networking from sensor to enterprise level. 

Alphabet Soup 
No matter how you decide to implement FA or IA, be it a simple device-level system or a network spanning your entire business, you will need to take a bus, and perhaps several. 

A quick glance at the bus standards may seem like someone spilled a pot of alphabet soup. The standards and protocols in most cases are created and managed by organizations, associations, institutions, or quasi-governmental entities of various size and import. After they've been developed, protocols are typically submitted for approval by international or regional standards-making bodies like ISO, IEC, EN, or IEEE. 

Some standards are more common in Europe than North America, and vice-versa. They are virtually all trying to globalize, and as with computer operating systems and video recording formats, the standards are contending for supremacy, each thinking itself destiny's child. Besides educating yourself about networking technology, it makes sense to keep your chosen equipment suppliers close during any automation project. Their hardware has to ride whatever bus you choose. 

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