Choose carefully your industrial-strength comms protocol

Graham Morphew
(07/02/2004 9:00 AM EDT)

  New microprocessors created specifically for embedded industrial control applications and equipped with multiple communication peripherals herald the growing need to connect industrial systems with the outside world. Industrial control and automation equipment users are looking for the ability to securely integrate the factory floor with back-office and front-office systems for greater productivity and lower total cost of operation. They are also demanding reduced maintenance requirements.

In response, suppliers are using processors to help improve the function and reduce the cost of their systems by shifting from proprietary communication protocols and complex wiring methods to open-standards protocols that use networking technology borrowed from other industries.

Much of this change stems from the need for greater communication between industrial devices and the end user, and between those devices and other, similar devices or control units. Many conventional industrial protocols and interfaces are proprietary, resulting in connections that require skilled maintenance and are difficult to replace and upgrade. New protocols and networking equipment are being introduced to the mix, but many of the cables and connectors are not normally ruggedized for use in harsh industrial environments, so specialized and costly equipment is required.

Several different methods are now being used for communication between industrial devices and the world around them. They include communication protocols such as TCP/IP, Universal Serial Bus (USB) and controller-area network (CAN), which are relatively new to industrial applications, as well as the more traditional serial, two-wire interfaces. All of those are becoming required components of an industrial design. Each is best-suited to a specific use.

The simplest communication interfaces for industrial equipment are serial interfaces such as RS-232, RS-422 and RS-485, used for communication among controllers, simple sensors, servos, valves and actuators. These multimaster interfaces are capable of medium-speed data transfer rates of 1 to 20 megabits/second depending on the technology. Several simple, higher-level protocols exist for these interfaces.

But because each was developed by a different industrial equipment manufacturer and subsequently became a de facto industry standard, their use is fragmented. Some of the protocols that are used today on top of RS-232, RS-422 and RS-485 are Profibus, Bitbus, Modbus and Interbus. In addition, because of their networking topology they require a great deal of effort for maintenance and network upgrades.

The CAN protocol is a serial bus system that comes to industrial design from automotive applications. Therefore, its cables and connectors are already ruggedized, and the equipment is relatively inexpensive and widely available. CAN's disadvantages are a small data payload — up to 8 bytes per transmission — and low-speed data transfers, at a maximum of 1 Mbit/second. It is a simple protocol well-suited for command-and-control-type interfaces. Its sophisticated error detection mechanisms and retransmission of faulty messages guarantee data integrity. In industrial design, it is best employed for communication among systems, sensors, devices and actuators, including printed-circuit boards.

Since CAN was designed for use in automotive power train systems, it can also be deployed in real-time control in industrial applications. A number of higher-level protocols based on CAN have been developed for industrial control and automation, including CANOpen, DeviceNet and Smart Distributed System.

CANOpen was designed for motion-oriented machine control networks such as handling systems. It is found in many applications, including medical equipment, maritime electronics, public transportation and building automation.

DeviceNet is an open, low-level network that provides connections between simple industrial devices (such as sensors and actuators) and such higher-level devices as programmable logic controllers and computers. This network employs the Common Industrial Protocol to provide control, configuration and data collection capabilities for industrial devices.

The Smart Distributed System protocol, meanwhile, is an advanced bus system for intelligent sensors and actuators.

USB originates in the PC environment, where it allows hot-swappable, plug-and-play interactions among PCs and a variety of peripherals on the same bus. This capability provides improved configuration flexibility over traditional serial and parallel ports, since many industrial devices and networks interface directly to a PC for programming, monitoring, data collection and diagnostics of the industrial bus. It may therefore be used for occasional PC connections for uploads/downloads and diagnostics. The cost for industrial USB equipment is slightly higher than for Ethernet-based industrial products.

USB data employs a multidrop, half-duplex twisted pair, similar in operation to a two-wire RS-485 system. This differential signaling offers a reasonable amount of noise immunity for harsh environments. Since power is distributed on the bus, low-power peripherals can easily operate without an external supply. USB's speed is quite good: 12 Mbits/s for the 1.1 specification and 480 Mbits/s for 2.0.

It was not designed for industrial applications, however. Distances are very limited; any single bus segment cannot exceed 5 meters, and there is no provision for signal isolation. USB is confined to a single master/many slaves architecture, its software can be complicated, and it has no real broadcast capability. Moreover, its cables and connectors are not ruggedized. It is best used for converters, such as serial-to-USB or Ethernet-to-USB. It is also suitable for systems using peripheral devices like printers, card readers and bar code scanners.

Ethernet and the de facto TCP/IP protocol running on top of it are well-understood because of their wide deployment in existing company local-area networks. Their advantages in industrial applications are many. User interfaces to back-office functions can be easily made with a Web browser, e-mail capabilities may be used to alert offsite staff of problems on the factory floor, and XML can be used to share data between the industrial network and office automation equipment. Management of industrial devices or complete systems can be done remotely over the Internet.

This widespread usage also poses security risks, but built-in security is available using Internet Protocol Secure, Secure Socket Layer and Secure Shell protocols. Ethernet provides relatively high speeds: 10 to 100 Mbits/s is the most common. Although Ethernet is perceived as a low-cost technology, that is only true for equipment designed for front-office use. Costs continue to drop, but ruggedized connectors and cables are relatively rare and still expensive.

Even low-grade, industrial-specification versions — for example, an industrial RJ-45 connector — may cost up to 30 times as much as the office-grade version of the same item. Those protocols may therefore be overkill for many industrial applications.

Many standard protocols that were previously run on two-wire serial interfaces are now being run on top of TCP/IP. Protocols such as Profinet and Modbus TCP/IP fit into this category. Profinet is a standard for manufacturing and process automation communication between industrial equipment and IT systems. The Modbus TCP/IP protocol has been developed for industrial control and automation. Other TCP/IP-based protocols are Ethernet/IP, used for industrial networking between equipment and IT systems, as well as between buildings, and BACnet/IP, used for building control and automation systems, such as heating/ventilation/air conditioning, fire detection and security.

There is considerable overlap of functionality among these protocols, but devices based on one will not communicate with devices based on another. In other words, a Modbus TCP/IP device can be placed on the same Ethernet network as a Profinet device, but the Modbus TCP/IP device cannot communicate with the Profinet device. Unlike many other physical interfaces, however, with Ethernet, a single device can easily support many different high-level protocols.

With the help of communication protocols that are mostly new to the industrial environment, industrial control and automation equipment is being integrated with back-office and front-office systems, resulting in greater efficiency and productivity, as well as lower operating costs. Work still needs to be done, however, on reducing redundancy among competing industry standards.

Graham Morphew (graham.morphew@arc.com) is a software technical marketing manager at ARC International (Stittsville, Ontario).