There are many new communication protocols that have arisen in the vacuum of the fieldbus. The goal of each system is to use the advances in microelectronics and network theory to simplify automation. As "open" systems, the protocols should be widely available to sensor, actuator, and controller manufacturers so that users could select the best value without worrying about whether the devices could be made to work together. Yet, conferences still feature round-table discussions striving to define what "open" means or nail down the differences between a sensor-level, device-level, and enterprise-level network.
To help you sort through the various possibilities, Design News asked the proponents of various protocols to give us a simple explanation of the strengths and weaknesses of each approach, and asked them to outline the applications for which their preferred system is best suited. Here, then is our bus "tour."
ASI simplifies two-wire transmission Otto W. Madelung, Secretary of the ASI Association and Michael Crew, Marketing Manager, Siemens Energy & Automation
ASI, the Actuator Sensor Interface, is a bus system from a purely technical point of view, but in practice is an interface. A consortium of instrumentation and control vendors formed the ASI Association to develop a simple solution for networking low-level devices into an enterprise-wide control and monitoring system. Their goal was achieved by combining new advancements in three basic areas of study:
Master/slave electronics of ASI enables data and energy to be transmitted along an unshielded two-wire cable. The slave has no processor and no software. The user enters an address using a hand-held device.
The master is universal and self-configuring, so many network structures are possible. The user need not enter any definitions or settings, like transmission rate, data and telegram types, and access requirements. By pushing a button, the master accepts a network configuration and incorporates this into its subsequent control procedure. It interchanges I/O patterns with the user program similar to PLC technology in use for years. It can also pass on diagnostic data.
Flat cable replaces conventional wiring, saving users time, money, and trouble, while allowing other intelligent products to be employed. The cable is geometrically coded to prevent misconnections.
The ASI cable feeds 30 Vdc power to sensors and actuators. We recommend no more than 100 mA per slave should be drawn from the net and that the net should be loaded to a maximum of 2 amps. A separate external energy supply is permissible and, in some cases, important: higher power demand, deviating voltage, alternating current, and emergency stop.
ASI is ideal for several application areas, wherever sensors and actuators need to be distributed over a wide area, or where the transmission media is exposed to high levels of electromagnetic interference, for example transfer lines or material-handling equipment.
There's no length limit to Seriplex® Edwin Newell, Product Planning Manager, Square D Co.
A large, international material-handling company needs a conveyor control system that's quick and inexpensive to install, highly flexible, and doesn't require high levels of technical expertise to start up. The control system must handle digital and simple analog signalling for communications and diagnostics on hundreds of I/O devices. Ideally, the system should have a proven track record.
The application consists of a system of motorized conveyor rollers and photoelectric sensors to move packages of varying weights and sizes. Each motorized roller powers up to eight "slave" rollers attached by a series of belts. At each motorized roller, a photoelectric sensor is stationed to detect the approach of the next package. When the sensor spots the package, it signals the motorized roller to turn on, and then off once the package has passed. Each combination of motorized roller, slave rollers, and sensor is a "zone," in an application with more than 300 zones spread over several thousand feet.
A multibit control system such as Seriplex® Control Bus is ideal for this application. The system offers reduced costs and startup time through the elimination of thousands of feet of parallel wiring. No need to pull, label, strip, connect, and test individual wires (four from the roller motor, four from the sensor) from each of the 300 zones to I/O blocks or directly to a programmable logic controller (PLC).
With Seriplex technology, a four-wire shielded cable can be daisy-chained to devices at each zone, up to 5,000 ft. The devices can be designed with an application-specific integrated circuit built in and connected directly to the network. Another option is to connect standard devices to intelligent I/O connection modules distributed near the sensors and motors.
There's no limit on the length of the drop from the network cable to the devices. The cable can accommodate any wiring configuration such as star, loop, tree, multidrop, master/slave, or peer-to-peer. The network is fast and deterministic, which means signals are sent at the same speed, every time. Seriplex has been dubbed "the electrician's network" because it's easily installed and connected after a brief introduction.
Once connected, the system handles the binary control signals from the sensors and the conveyor roller motors. Analog signalling provides additional diagnostic information over the same network. For example a roller might be stalled, which would be indicated by a current reading above a preset limit. Or, the photosensor using built-in lens diagnostics communicates over the network insufficient signal intensity, which could indicate a dirty lens.
Seriplex technology is proven, with an installed base of more than 200,000 I/O points in more than 500 locations.
INTERBUS-S: the gates are open Tom Rosenberg, Product Manager, Interbus Systems Group, Phoenix Contact, Inc.
INTERBUS-S is an open-standard (DIN 19258), ring-based, distributed device network for manufacturing industries. An efficient protocol for today's high-speed requirements, INTERBUS-S is used widely in automotive assembly, material handling, and printing applications.
An INTERBUS-S system consists of a controller board installed into a computer or PLC that communicates to a variety of sensor/actuator-type devices via various I/O modules. The network can be distributed up to 42,000 feet with remote bus cable lengths up to 1,300 feet. Highly flexible, an INTERBUS-S system can connect 256 I/O drops for a total of 4,096 digital input and 4,096 digital output points (or combinations of digital and analog signal types). It operates at 500K baud, providing full duplex, total frame transmission with extremely high data throughput (4,096 digital I/Os in 14 msec). The network allows full migration over the complete range of devices--with no gateways. Comprehensive diagnostics allow the user to pinpoint the cause and location of errors, including power loss, cable break, CRC errors, and module defects. Finally, system integrity is assured with standard profiles for robotics, drives, process controllers, encoders, pneumatic valves, and operator interfaces. Over 400 device manufacturers worldwide support this standard.
Advantages of the INTERBUS-S system compared to conventional, closed-protocol systems include:
Vendor selection--proprietary networks limit users to one vendor. The open standard allows and encourages multiple suppliers.
Product selection--Proprietary systems may limit product offerings. One proprietary vendor, for example, offers no pneumatic products. In an open-architecture system, a wide number of vendors assures a broad product selection.
Smart Distributed System: CAN do. Brad Kautzer, SDS Business Development Specialist, MICRO SWITCH Div., Honeywell, Inc.
The Smart Distributed System (SDS) is an open CAN-based I/O network standard optimized for high-speed communication between field devices in manufacturing automation applications. Originally developed by Honeywell and installed in industrial applications since mid-1992, SDS is more powerful than bit-level on/off sensor networks, yet faster and less complex than the high-end process fieldbuses. The scalable protocol supports multiple message structures to ensure efficient device-data communications. Message types include binary and multiple binary I/O, analog, character (ASCII), and real numbers.
When Orange Engineering & Machine (OEM) Company, Anaheim, CA, first saw SDS, they immediately realized benefits for their machines. OEM designs, manufactures, and supports large lamination presses and material-handling equipment for the electronics industry. Each custom-designed, labor-intensive system is built, tested, torn down, shipped, and reassembled at the customer site. The ability of the SDS bus to simplify the process and reduce labor and technical costs has made it an important tool.
The Smart Distributed System changes conventional I/O from change-of-state devices to intelligent control devices through an open, robust network. Simplified wiring offers a key benefit. Large presses and long conveyors are integrated over the expanse of the fabrication system as well as numerous sensors, pneumatic actuators, and hydraulic actuators.
OEM's first use of the technology was for a large, complex project going to a US-based electronics manufacturer in Singapore. Approximately 30 different pieces of machinery had to be assembled and wired. Ten different items had to be controlled. The SDS-controlled application consisted of over 200 sensors and actuators.
With so much I/O, SDS's plug-and-play, single four-wire cabling reduced field wiring installation costs compared with traditional "spaghetti wire" methods. Set-up time, which usually took as long as three weeks, was reduced to four days. Debugging time was cut from two weeks to less than a week. In Singapore, local labor performed the installation, cutting wiring costs in half. Overall, installation time was reduced over 50%, field installation time by 75%, and field debugging was largely eliminated.
Smart Distributed System network supports the continued migration of control towards increased device-level functionality. Devices available today include the ability to configure device attributes in software and provide fault-prediction diagnostics. SDS offers interfaces to virtually all PLCs. Ultimately, advancements in distributed control and distributed intelligence may drive the need of PC-based control to alleviate the limitations of proprietary PLC architectures.
SERCOS in motion Robert P. Brennan, VP, Industry Applications Group, Indramat
The Serial Realtime Communications System (SERCOS) interface is the only international standard (IEC 1491) for motion control. It is a highly deterministic protocol which can support tightly synchronized multi-axis motion. Although first applied to machine tools, SERCOS has proven successful in many motion-control applications, such as electronic line shafting in the printing and converting industries.
Printing/converting machines traditionally have used a high-horsepower, variable-speed motor as a prime mover, coupled to a mechanical drive train running the length of the machine. With the electronic line shafting approach, these machines are sectionalized into separate stand-alone sections, each powered by its own high-performance ac servo drive. The drives at each section and the master control are connected in a ring by a single daisy-chained, fiber-optic cable, and each section is independently controlled for velocity, position, or torque.
Similar to fly-by-wire for airplanes, where electronics is replacing hydraulic and mechanical control systems, electronic line shafting is fly-by-wire for printing and converting machines. There are several important applications considerations. The electronic solution must be as rigid as the mechanical solution it replaces, yet it must be flexible to be reconfigured on demand. And the system must operate at high speeds with high resolution.
These unique protocol requirements are met by SERCOS. It operates with 32 bits of resolution, an accuracy not available with D/A converters in an analog system. SERCOS allows the servo systems to provide the required stiffness for system rigidity, as well as extremely fine resolution for operation at low speeds and at the high speeds required of printing/converting machines. SERCOS is the realtime, synchronous, deterministic protocol required for a stiff, highly controllable system. Just as in multi-axis interpolation for metal cutting, printing/converting requires all axes to be at a known position at a known time. SERCOS updates all drives on a ring at a fixed clock speed, selectable at 2,4,6, etc. msec. All drives on that ring synchronize to the SERCOS master telegram and perform sub-interpolation, returning position information with an axis-to-axis accuracy of ±7 microsec.
SERCOS-based electronic line shafting applications offer improved product quality, replacement of hundreds of individual wires and termination points by a single fiber-optic cable, and the inherent noise-immunity of fiber optics. Additionally, they offer high levels of electronic diagnostics, reduction of mechanical maintenance, and high machine flexibility for production of more products on a single machine and for future machine expansions.
The SERCOS protocol supports up to 254 intelligent digital drives and I/O systems per fiber-optic ring, with multiple rings available. It allows up to 30 meters between devices, which would be over 7,500 meters per ring of 254 devices. There is no direct overall length limitation. A major advantage of SERCOS is that it allows the use of intelligent subsystems, in which intelligence and local decision-making are pushed down to the drives, with position loop closure at the drive level.
CompactPCI speeds factory computing Joe Pavlat, Vice President, New Product Development, Pro-Log Corporation, and President PCI Industrial Computer Manufacturer's Group
PCI--Peripheral Component Interconnect--is today's bus standard for high performance computers. Supported by all major microprocessor manufacturers, including Intel, Motorola, and DEC, PCI is a high-speed, processor-independent 32/63 bit bus capable of transfer rates of over 260 megabytes/second. PCI has enough bandwidth for high performance networking, fast disk access, and real-time, high-resolution video. PCI chips are currently being manufactured by the tens of millions for the desktop world.
What is CompactPCI?
Industrial computers have the same performance requirements as their desktop brethren, but must be more rugged, more modular, have better access, and have lower Mean-Time-To-Repair (MTTR). The newest standard for high-performance industrial computers is CompactPCI. CompactPCI combines off-the-shelf PCI silicon and software with the rugged Eurocard packaging popularized by VME bus.
Defined for both the 3U (100mm by 160 mm) and 6U (160mm by 233mm) card sizes, CompactPCI has the following features: standard Eurocard dimensions and packaging; high density 2mm pin-and-socket connections (IEC-1076 standard); vertical card orientation for good cooling Positive card retention; front panel I/O; excellent shock and vibration characteristics; uses standard PCI silicon and software.
How Does It Work?
CompactPCI is electrically identical to desktop PCI. This allows the bus to take advantage of low-cost silicon that is manufactured in very large quantities. CompactPCI uses a shielded, high-density pin-and-socket connector that has low capacitance, low inductance, and controlled impedance. This is important for high speed PCI signals to operate reliably in a noisy industrial environment.
Large numbers of ground and power pins and the connector shield reduce emissions and noise susceptibility. The low connector capacitance allows CompactPCI to have up to eight slots without bridge chips. Standard PCI bridge chips (available from DEC and IBM) allow the number of slots to be extended almost without limit.
3U CompactPCI boards use a single 235 pin connector for all PCI signals (a smaller connector can be used for boards not capable of 64 bit transfers). 6U boards have two connectors. The lower connector is the same as on 3U cards. The upper connector can be used for bus expansion, expansion to other buses like ISA, STD, or VME, or can be used for user I/O. The latter usage is popular in the telecom industry.
Who Uses CompactPCI?
CompactPCI is becoming popular for applications requiring high performance and ruggedness, including those in the telecommunications, industrial control, and process industries.
Applications for CompactPCI include:
Man Machine Interface (MMI).
Compact PCI was developed by the PCI. Industrial Computer ManufacturersGroup (PICMG), a consortium of industrial computer manufacturers. With over 55 members, PICMG maintains and publishes standards for industrial PCI hardware and software.
DeviceNet reduces life-cycle costs Bill Moss, Director, Open DeviceNet Vendors Association
Large machinery and machine tools with embedded controls--particularly highly distributed, high-speed conveyor applications with multiple variable-speed drives--are ideal applications for DeviceNet. OEMs who manufacture such equipment realize significant savings in wiring, installation labor, and post-installation troubleshooting. The end user benefits from DeviceNet's diagnostic capabilities.
With a maximum bus length of 1,600 ft, the linear topology (Trunk-line/Dropline) DeviceNet bus accommodates a maximum 64 nodes per bus and 32 I/O per node. Bus addressing includes master-slave, peer-to-peer, multicast (one-to-many), and change-of-state (exception-based reporting). At a maximum speed of 500K baud, its scan time is 7.2 msec for 63 devices in master-slave format, faster in other address modes. Its data package size is zero to eight bytes, with messages in multiple of eight bytes.
Integrating different brands of equipment on the plant floor is one of the problems end users want to focus on when looking at device-level networks. The DeviceNet interface allows products from different vendors to communicate because of their common communications technology. Many large manufacturing companies have multiple facilities. DeviceNet's common interfacing technology will allow each facility to choose the vendor they are most comfortable with, and still maintain a level of consistency from plant to plant. DeviceNet products are virtually interchangeable because of the network's Device Profiles. These profiles define the parameter,diagnostics, and required features of DeviceNet products.
Key to simplifying device integration is the reduced wiring and wiring maintenance involved in a plant with a DeviceNet network. Most OEMs select a device network for the wiring savings, but the savings are not just the cost of the wire and initial stringing of the wire. It is also the cost of electricians who perform the physical terminations. OEMS find savings during the design stage of a plant because DeviceNet eliminates the need to draw wires going everywhere. The address to be configured into each device is all that is needed on the design drawings.
While wiring alone can justify DeviceNet in many cases, end users find that the network provides an additional benefit as well. The advanced diagnostic capability, for instance, allows for the immediate awareness of unstable conditions. When a device is broken or misaligned, the problem may remain undiscovered until conveyors jam, products are mispackaged, or other more serious problems occur. DeviceNet systems perform self-diagnostics internally and are able to report the working status of each device over the network--even when a machine is in standby mode. When a fault occurs, diagnostics identify the location, eliminating the time-consuming process of tracing I/O back to the control panel.
DeviceNet adds intelligence to areas in the machine or plant that never had it before. No control system has the ability to report when a light bulb filament burns out unless you go through the hassle of putting in a control code to test it. With DeviceNet, the light socket is intelligent: It can notify the operator when it is being given power but not consuming any current.