By Clark Kinnaird
AN IMPORTANT TREND in factory automation is the increase in networked connectivity between sensors, controls, actuators and other system components. Automation designers face many challenges and tradeoffs in the development of a successful network solution.
This article, the substance of which was presented as a White Paper, “Specifying Industrial Field Bus for Automation,” at the 2005 SPS Electric Automation America Conference in May, discusses the choices necessary to select a network technology appropriate for specific applications. We’ll compare the technical constraints of several common network options, primarily for discrete applications, and discuss guidelines for selecting media and protocol.
The intended audience are system designers who develop networked automation, but who might not be familiar with the details of data transmission design. The takeaway should be a general overview of some of the common network options available, and some knowledge of the kinds of questions to pose when choosing an architecture, a protocol, and an implementation.
There’s Ideal, and There’s Real
There are at least two scenarios that can arise when selecting a bus network for an application. In one, the engineer starts with a blank sheet of paper and can specify any network that best meets the needs of the application at hand. In the other scenario, the engineer is constrained to specify a network compatible with an existing installation.
In an ideal world, all networks would be compatible, and it would be easy to translate from one to another. In actuality, interconnecting different networks can be very difficult, so we’ll leave that can of worms alone. If we assume the choice of network for the second scenario essentially is pre-determined, it makes sense to focus this article on the first scenario.
As we look for ways to compare different network choices, we find a long list of possibilities. Specific parameters of comparison and design concerns include:
- Data rate and data latency
- Physical interconnect medium
- Noise immunity
- Bit error rate and bus faults
- Allowable interconnection length
- Allowable number of network nodes
- Ease of adding additional nodes
- Power consumption, cost, reliability and isolation requirements
We’ll look at several of these, discuss why each of the characteristics is of concern for automation networks, and what tradeoffs come into play for making appropriate selection. We’ll see how there is no universal answer but, in optimizing one parameter, there are tradeoffs and compromises involving other parameters.
The Likely Suspects
Available network choices include:
- 4-20 mA--This analog current loop network is slow but simple. It is limited to one transmitter per loop, but can have several receivers. The analog format limits the higher-level functionality, but this still is a widespread implementation choice for communicating simple sensor measurements to a central controller.
- HART--The Highway Addressable Remote Transducer network augments the 4-20 mA loop with a modulated signal. This allows transmission of digital information, although the data rate is relatively slow.
- RS-232--This standard interface has been around for a long time and still is used in many simple interfaces for initial setup of systems, diagnostics and other non-time-critical functions. A single-ended network, RS-232 does not have the same noise immunity as other standards that take advantage of differential signaling.
- RS-485--An outgrowth of RS-232 and RS-422, the RS-485 electrical specification is the basis for several industrial network standards, including Profibus, Interbus, Modbus and others. The strengths of RS-485 are its immunity to noise and ground offsets, bidirectional and multiple-driver capability, and party-line simplicity.
- Interbus--A ring-based network, Interbus uses RS-485 signaling with point-to-point connections, and full-duplex operation to make an adaptable bi-directional structure. Other variations of Interbus use fiber or infrared media for signaling.
- Modbus--This bus has several variations, the most common is based on RS-485 signaling. Other implementations use Ethernet or RS-232. In addition to industrial automation, it also is used in building control applications.
- Profibus-DP--Based on RS-485 signaling technology, Profibus-DP (Process Fieldbus, Decentralized Peripheral) is commonly used for factory automation, especially in Europe. The Profibus standard specifies the protocol, electrical layer, termination, signaling rates and grounding/isolation schemes. There are other variants of Profibus for fiber media, intrinsically safe applications, and motor control applications.
- DeviceNet--Based on the Controller Area Network (CAN) signaling specification, the DeviceNet standard specifies the electrical layer (voltages, current loading, termination and isolation/grounding) and protocol requirements for a device-level network.
- ControlNet--With a high signal-to-noise ratio and coaxial media, ControlNet is a robust, relatively high-speed industrial network. Its strengths are deterministic timing, robust electrical characteristics, and simplicity of expansion.
- Industrial Ethernet--Several variations of Industrial Ethernet now are available. These include Profinet, EtherCAT, EtherNet/IP (Industrial Protocol), Ethernet Powerlink and others. Each is based on the IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Standard for Local Area Networks. Each variation has differences and is not easily interconnected with the others, mainly because of the manner by which the requirement for known data latency (deterministic timing) is handled. Versions with 10 Mbps and 100 Mbps are commonly used for industrial automation applications.