As the information explosion continues to grow and networks proliferate throughout industry, the demand is increasing for information to be on-line and available from any location. Managers want important data in a timely manner so that critical decisions can be made and processes can be brought in-line to maximize profits and productivity.
Often, this information must come from machines, instruments or processes in locations that are not conducive to having a PC nearby. So, it is a natural progression for data acquisition technology to migrate to a low-cost platform for moving data onto a network.
Many new products have integrated data acquisition technology with Ethernet capability. These Ethernet data acquisition systems can act as a low-cost interface from machines, sensors and instruments to corporate and enterprise-wide networks. Then, data is easily shared between managers, on-line data bases and software applications for more processing and decision-making. The Ethernet systems also maintain a truly open-systems environment for the acquisition and management of mission-critical data.
Traditional Approaches for Remote I/O
Networked I/O is certainly not a new concept. Companies have relied on the capability to perform remote I/O over networks for years. Typically, communications between the remote I/O system and the host computer have been accomplished through a serial standard such as RS-232 or RS-422/485. These have generally taken the form of some type of sensor bus or device bus, such as CAN, DeviceNet and LONworks. These standards have advantages and disadvantages compared to Ethernet.
RS-232
RS-232 is by far the simplest and least expensive. Every PC shipped today can communicate over an RS-232 line. Only a compatible cable and a terminal program are required.
RS-232 is limited to about 50 feet, but it can operate over huge distances with a little help from a modem. Only one device can be connected to a single RS-232 port. While virtually all computers have an RS-232 port, few have more than two. Most PC operating systems cannot handle more than four without special software.
RS-232 also is typically not electrically isolated from the host computer. For example, if a power line were inadvertently dropped across the RS-232 line, the host computer most likely would be destroyed. As a result, RS-232 may be appropriate for situations in which very few remote I/O systems are needed and where the likelihood of electrical transients is low.
RS-422/485
RS-422/485 are communications media developed with industrial applications in mind. They use differential signaling that allows reliable communications at speeds of up to 115-kbaud over 4,000 feet or more.
More importantly, they are both multidropped standards. As a result, many devices can share a single RS-422/485 port as long as some means exists to prevent more than one device from transmitting at any one time.
Usually, RS-422/485 ports also are electrically isolated from the host computer, limiting the likelihood of a catastrophe. And, RS-422/485 networks are robust and deterministic, important factors in industrial I/O situations.
One drawback of RS-422/485 is cost. A typical RS-422/485 interface board costs $150 to $1,300 depending on the features and the number of ports.
Another cost problem for RS-422/485 networks is wiring. This can overshadow any other cost in the system. You will have to run your own wiring; and if you move one of your remote I/O devices, you will have to move its wiring, too. If you have many I/O devices to interconnect, this could get very expensive.
A second drawback of RS-422/485 is the lack of a clear standard for a communications protocol. For a multidrop system to work, each device must have a unique address to distinguish it from all the other devices on the line.
Since all communications are over a single pair of wires, there must be some method to tell which part of a data stream is address information and which part is data. The host computer and every device on the network must agree on how this is done or nothing will work.
Many RS-422/485 protocols were developed in the 1980s for such applications. Profibus, Interbus and CAN were a few. These were designed with proprietary interests in mind, and each subsequently received backing from several vendors in an attempt to develop an open international standard with widespread support.
However, competing interests have prevented any of these buses to emerge as a clear leader. Since interoperability is such an important aspect to consider when designing a system, it would be wise to consider a truly open protocol that can deliver this capability.
Additionally, several organizations associated with ISA are developing a standard called Fieldbus, which promises to be an open, universally supported standard for controllers, sensors, actuators and transducers. However, Fieldbus has been in the works for more than eight years and no standard protocol or widespread support has emerged. Like most of the existing standards, Fieldbus components currently are considerably more expensive than Ethernet.
Ethernet
Ethernet has been around for more than 10 years and has become a commodity in modern business environments. Ethernet also has many characteristics that make it suitable for industrial networked and remote-sensor I/O applications. It combines the low cost of RS-232 with the multidrop capability of RS-422/485 and provides a clear standard for communications.
The same economic forces that drove the PC into the industrial marketplace are now working to drive Ethernet there. While these networks have traditionally been used in office environments, industrial users cannot afford to ignore the cost and performance benefits of Ethernet in their factories. Ethernet has become the medium of choice to communicate management data throughout the enterprise.
True Open System
Ethernet is an open standard worldwide. Originally developed by Xerox, it was standardized in the mid-1980s by IEEE. Since then, it has proliferated so that most new buildings are prewired for Ethernet.
While there are many protocols available for Ethernet, they can typically coexist on the same network. The protocol of choice, however, is TCP/IP.
TCP/IP is a suite of protocols that has virtually universal support. It comes as part of all Windows 95 and NT operating systems, it is used almost universally in UNIX environments, and it is the protocol used on the Internet. By using TCP/IP, you can be confident that you will have little or no trouble with interoperability.
The Bandwidth You Need
Available in 10-Mb/s or 100-Mb/s versions, Ethernet provides wide bandwidth for remote I/O systems. This is nearly three orders of magnitude higher than a typical RS-422/485 network running at 19.2 kbaud. Wide bandwidth translates into fast response time to real-world events. It also allows many devices to communicate on the same network.
If you must ensure that your sensor network is insulated from your corporate information system, intelligent devices called bridges and routers are readily available. Using one of these devices creates a partition that only passes traffic between the segments that is intended for a device on the other side of the partition. An example of this is shown in Figure 1.
Cost-Effective Network
The price of an Ethernet interface has dropped because of the economy of scale of manufacturing these devices in such enormous volumes. Ethernet hubs are available for as low as $60 and interface boards are less than $40.
Expanding your network as your company or your application grows is simple and inexpensive with Ethernet. A simple $60 hub will add eight more drops to your network that can be wired up to 100 meters from the hub. And if you need to move a device connected to Ethernet, moving the Ethernet connection is as simple as unplugging the connection and plugging in to the nearest hub in its new location.
Network maintenance is also less expensive than maintenance of an RS-422/485 network. Many larger facilities have in-house personnel who can maintain an Ethernet installation. For smaller facilities, the task can be contracted to readily available experts.
Separate Network for Sensor Applications?
Though many facilities are prewired for Ethernet, some systems still employ conventional networks for sensor applications. In these systems, a computer interfaces the sensor network to the corporate Ethernet information system (Figure 2).
Using a TCP/IP-ready platform such as an Ethernet data acquisition system helps connect sensors and transducers to the information system without a costly, full-blown PC (Figure 3). This way, data is readily shared in real time with the resources, applications and data bases available on the corporate information system.
This solution also easily accommodates remote applications where sensors or transducers are located a long distance from any computers. A 10Base-T cable can be wired to the remote location, providing inexpensive access.
I/O Applications
Ethernet is a peer-to-peer network. Any device on the network can transmit or broadcast data at any time as long as no other device is broadcasting. Since data can be transferred asynchronously, a remote I/O device can immediately notify a host if an event occurs. With RS-422/485 systems, the I/O devices must be polled by the controller, introducing a greater latency in the response to events.
The nondeterministic nature of Ethernet may limit its usefulness in certain applications. Because of the random nature of messaging on Ethernet, the amount of time it takes to deliver a particular packet of information cannot be guaranteed. Typically, delivery time is dictated by the amount of traffic on the network, where more traffic will cause longer delivery times. Remember that longer delivery times are still in the millisecond range.
Due to this characteristic, Ethernet may not be suitable for process-control applications where timing is critical. For these types of applications, standards such as Profibus that guarantee delivery time may be more appropriate. However, Ethernet is perfectly suitable for around 90% of all sensor applications.
Monitor Events from Anywhere
While the allowable distance from an Ethernet hub to any device is a maximum of 100 meters, it is possible to extend the range of the network indefinitely by using such devices as repeaters, routers and bridges. What’s more, readily available hardware and software allow you to connect your Ethernet network to the Internet.
Since TCP/IP is the protocol used on the Internet, applications that use TCP/IP can access remote I/O data from anywhere in the world. For example, you could monitor the status of operations at your facility in Hong Kong from your desk in Philadelphia. In testing applications, this is very useful for tests that run for long periods of time.
With Ethernet, your remote I/O data can be available enterprise-wide in real time using the same network you use for everything else. This means that a production manager can interrogate the state of an assembly line or batch process at any time and know what is occurring right now. Additionally, all of this data can be viewed at the corporate management level if desired.
A Variety of Applications
The industry has picked up on the emerging trend that incorporates data acquisition systems with an Ethernet interface. Companies such as Real Time Integration, BayTech, Fluke, Measurement Technology and Intelligent Instrumentation have recently introduced various types of data acquisition systems with built-in Ethernet communications capability.
Many of these systems have recently been installed in a range of industries to perform a variety of applications. In each case, cost and ease of installation are primary benefits when considering previously available alternate solutions.
Seed Storage Research
At the U.S. Department of Agriculture Seed Storage Facility in Ft. Collins, CO, an EDAS™ system from Intelligent Instrumentation helps determine optimum conditions for long-term seed storage. The facility stores crop particles, such as seeds and pollen, for agricultural research and use. The goal is to provide conditions that allow the longest storage time while maximizing the survival rate of the seeds.
The facility uses many large test chambers and freezers for the different kinds of seeds. Environmental variables such as temperature and humidity are periodically monitored and logged. The data acquisition system also detects equipment status to report out-of-tolerance conditions.
This data acquisition system was selected because the facility was already wired with a 10Base-T Ethernet network. Before the data acquisition system was installed, researchers periodically walked through the facility and recorded the data on a sheet of paper. Data was manually logged into a PC for analysis.
Besides automating the process and freeing up researchers to spend time on other important projects, one of the major benefits of the data acquisition system is fault detection. If a test chamber experiences an equipment malfunction, an alarm provides immediate notification. This prevents test conditions from changing to the point where weeks of testing are wasted.
Monitor and Control of Medical Chambers
An OEM of large environmental chambers embeds EDAS systems into some of its chambers. These chambers are wired with many sensors to detect temperature, pressure, humidity and other environmental conditions in various locations throughout the chamber. Each chamber could contain several hundred sensors.
Currently, these sensors are monitored by a PC-based data acquisition system. A PC equipped with Intelligent Instrumentation data acquisition boards is installed on the outside of the chamber. To connect to the PC, the sensors are wired directly to a termination system mounted inside the chamber, next to the entrance. A variety of expensive filtering and signal conditioning is required to compensate for the resulting loss in signal quality due to the long sensor wires.
The manufacturer intends to replace the miles of sensor wires and expensive filtering equipment that occupy each chamber with an EDAS system. Several EDAS units will be placed in strategic locations throughout the chamber to minimize the amount of signal lead wire required to connect to all sensors. Then, low-cost, high-performance 10Base-T cable will bus the sensor data to the PC monitoring system outside the chamber.
The PC no longer will require internal data acquisition cards. Instead, only a modified version of the existing monitoring program, a network interface card (NIC) and a TCP/IP protocol stack will be needed. Each NIC is approximately $70 and the TCP/IP stack is already included with the operating system.
Summary
In the short time Ethernet data acquisition systems have been available, they have proven themselves capable of an infinite number of applications, from pipeline monitoring to wine-making in Italy. By selecting Ethernet as their communications medium, users are choosing a widely implemented open industry standard that has a proven track record and continues to penetrate more markets. Most new facilities are prewired with 10Base-T wiring for Ethernet networks, and many facilities are already using Ethernet for a variety of functions.
About the Author
Robert Winkler has been the product marketing engineer for data acquisition and control systems at Intelligent Instrumentation since 1994. Previously, he served as a communications officer in the U.S. Army Information Systems Engineering Command, specializing in local area networks. Winkler holds a B.S.E.E. degree from Lehigh University. Intelligent Instrumentation, 6550 S. Bay Colony Dr., MS 130, Tucson, AZ 85706, (520) 573-0887.
Copyright 1997 Nelson Publishing Inc.
January 1997
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