Steve Mackay

IEEE 488 standard

This chapter provides a general overview of the IEEE 488 standard. The term IEEE 488 is usually associated with three standards—IEEE 488.1, IEEE 488.2, and the SCPI. The IEEE Standard 488.2 was introduced to define data formats, status reporting, controller functionality, error handling, and common commands. IEEE 488.2 concentrates on software protocol issues and maintains full compatibility with devices that comply with the hardware-oriented IEEE 488.1 standard. Standard commands for programmable instruments (SCPI) uses IEEE 488.2 as a basis and defines a common command set that can be used for programming instruments with any hardware link. The term general purpose interface bus (GPIB) is often used interchangeably with IEEE 488 standard. The GPIB is an interface design that allows the simultaneous connection of up to 15 devices or instruments on a common parallel data communications bus. This allows instruments to be controlled or data to be transferred to a controller, printer, or plotter. GPIP defines methods for the orderly transfer of data, addressing of individual units, standard bus management commands, and the physical details of the interface.

ProfiBus PA/DP/FMS overview

This chapter provides an insight into the PROcess FIeld BUS (Profibus) networking standard and its several versions. Profibus is a widely accepted international networking standard, commonly found in process control and in large assembly and material handling machines. It supports single-cable wiring of multi-input sensor blocks, pneumatic valves, complex intelligent devices, smaller sub-networks (such as AS-i), and operator interfaces. It is an open, vendor-independent standard and adheres to the OSI model and ensures that devices from a variety of different vendors can communicate together easily and effectively. It has been standardized under the German National standard as DIN 19 245 Parts 1 and 2 and, in addition, has also been ratified under the European national standard EN 50170 Volume 2. Profibus DP (distributed peripheral) allows the use of multiple master devices, in which case each slave device is assigned to one master. This means that multiple masters can read inputs from the device but only one master can write outputs to that device. Profibus FMS (fieldbus message specification) is a peer-to-peer messaging format, which allows masters to communicate with one another. The Profibus PA protocol is the same as the latest Profibus DP with V1 diagnostic extensions, except that voltage and current levels are reduced to meet the requirements of intrinsic safety (class I division II) for the process industry.

Foundation Fieldbus overview

This chapter provides an insight into the Foundation Fieldbus (FF). The FF aims to preserve the desirable features of the present 4-20 mA standard (such as a standardized interface to the communications link, bus power derived from the link and intrinsic safety options) while taking advantage of the new digital technologies. FF takes full advantage of the emerging “smart” field devices and modem digital communications technology, allowing end-user benefits such as reduced wiring, communications of multiple process variables from a single instrument, advanced diagnostics, interoperability between devices of different manufacturers, enhanced field level control, reduced start-up time, and simpler integration. FF consists of four layers. Three of them correspond to OSI layers 1, 2, and 7. The fourth is the so-called “user layer” that sits on top of layer 7 and is often said to represent OSI “layer 8”, although the OSI model does not include such a layer. The user layer provides a standardized interface between the application software and the actual field device. High-speed Ethernet (HSE) is the FFs backbone network running at 100 Mbits/second. HSE field devices are connected to the backbone via HSE-linking devices.

EIA-485 overview

This chapter focuses on Electronic Industries Alliance (EIA)-485 standard and its features. The EIA-485A standard is one of the most versatile of the EIA interface standards. It is an extension of EIA-422 and allows the same distance and data speed but increases the number of transmitters and receivers permitted on the line. EIA-485 permits a “multidrop” network connection on two wires and allows reliable serial data communication for distances of up to 1200 m, data rates of up to 10 Mbps, up to 32 line drivers on the same line, and up to 32 line receivers on the same line. According to the EIA-485 standard, there can be 32 “standard” transceivers on the network. Some manufacturers supply devices that are equivalent to ½ or ¼ standard device, in which case this number can be increased to 64 or 128. If more transceivers are required, repeaters have to be used to extend the network. The EIA-485 interface standard is very useful for systems where several instruments or controllers may be connected on the same line.

System design methodology

This chapter focuses on the methodology related to systems engineering. For any methodology, the environment in which the system will operate has to be specified first, because it imposes specific constraints on the system design. The environment involves everything surrounding, affecting, or supporting the system and its sub-systems, but does not include the system itself. If the two communicating devices are close together, there is very little electrical interference (noise) present and there is a good grounding system with both end systems at the same reference potential, then the RS-232 is an option. If the two end stations are up to 1200 m (4000 ft) apart and/or there is a fair amount of noise present, the the RS-422 or 485 can be a solution. Networked systems in a plant can be placed conceptually in one of the three levels. The lowest (bottom layer) is often referred to as the device layer. Systems at this level are deployed on the plant floor and link sensors and actuators, both digital and analog, with controllers. The middle layer or control layer typically interconnects PLCs, SCADA systems, and HMIs. The upper layer is referred to as the enterprise or information layer. Typical networks at this level include Ethernet and Token ring, connecting various computer systems.

TCP/IP overview

Transmission Control Protocol/Internet Protocol (TCP/IP) is the de facto global standard for the Internet (network) and host-to-host (transport) layer implementation of internetwork applications because of the popularity of the Internet. TCP/IP is not limited to the TCP and IP protocols, but consists of a multitude of interrelated protocols that occupy the upper three layers of the Advanced Research Projects Agency (ARPA) model. TCP/IP does not include the bottom network interface layer, but depends on it for access to the medium. TCP fragments large portions of data into smaller segments if necessary, reconstructs the data stream from packets received, issues acknowledgments of data received, provides socket services for multiple connections to ports on remote hosts, performs packet verification and error control, and performs flow control. In TCP/IP, the Internet layer is primarily responsible for the routing of packets from one host to another. Each packet contains the address information needed for its routing through the internetwork to the destination host. The dominant protocol at this level is the IP. The host-to-host layer is primarily responsible for data integrity between the sender host and the receiver host, regardless of the path or distance used to convey the message. The process/application layer provides the user or application programs with interfaces to the TCP/IP stack such as file transfer protocol (FTP), trivial file transfer protocol (TFTP), and simple mail transfer protocol (SMTP).

Industrial Ethernet overview

This chapter provides an insight into the industrial Ethernet systems. The concept of Ethernet network was first developed by Xerox Corporation, using “ether” as the transmission medium and created a network between the sites. Early Ethernet (of the 10 Mbps variety) uses the carrier sense multiple access/collision detect (CSMA/CD) access method. This results in a system that can operate with little delay, if lightly loaded, but access to the medium can become very slow if the network is heavily loaded. Ethernet network interface cards are relatively cheap and produced in vast quantities. Modem Ethernet systems are a far cry from the original design. From 100BASE-T onwards, they are capable of full-duplex (sending and receiving at the same time via switches, without collisions) and the Ethernet frame has been modified to make provision for prioritization and virtual LANs. There are several key technology areas involved in the design of Ethernet-based industrial automation architecture. These include available switching technologies, quality of service (QOS) issues, the integration of existing (legacy) field buses, sensor bus integration, high availability and resiliency, security issues, long-distance communication, and network management. For high availability systems, a single network interface represents a single point of failure (SPOF) that can bring the system down.

AS-interface (AS-i) overview

This chapter provides an insight into the actuator sensor interface (AS-i). The AS-i is an open system network developed by eleven manufacturers who created the AS-i association to develop the AS-i specifications. Some of the more famous members of the association include Pepperl-Fuchs, Allen-Bradley, Banner Engineering, Datalogic Products, Siemens, Telemecanique, Turck, Omron, Eaton, and Festo. AS-i is a bit-oriented communication link, designed to connect binary sensors and actuators. Most of these devices do not require multiple bytes to adequately convey the necessary information about the device status, so the AS-i communication interface is designed for bit-oriented messages in order to increase message efficiency for these types of devices. AS-i uses a two-wire untwisted, unshielded cable that serves as both communication link and power supply for up to thirty-one slaves. A single master module controls communication over the AS-i network, which can be connected in various configurations such as bus, ring, or tree. The data-link layer of the AS-i network consists of a master call-up and slave response. The master call-up is exactly fourteen bits in length while the slave response is 7 bits.

Modbus Plus protocol overview

This chapter provides an insight into the Modbus Plus protocol and its main features. Modbus Plus is a local area network (LAN) system for industrial control applications. Networked devices can exchange messages for the control and monitoring of processes at remote locations in the industrial plant. The Modbus Plus network was one of the first token-passing protocol networks that pioneered the development of other more advanced deterministic protocols of today. Each device on a network segment must be assigned a unique network address within the range 1 through 64 inclusive. Multiple network segments may be bridged together to form large systems. Messages are passed from one device to another by the appropriate use of a route within the message. The route will include the network address of the target device, and any inter-network addresses required. Each network supports up to 64 addressable node devices. Up to 32 nodes can connect directly to the network cable over a length of 1500 ft. Network nodes also function as peer members of a logical ring, gaining access to the network upon receipt of a token frame. Moreover, protocols for token passing and messaging are transparent to the user application.

Fiber optics overview

This chapter focuses on fiber optic communications and its main features. Fiber optic communication uses light signals guided through a fiber core. Fiber optic cables act as waveguides for light, with all the energy guided through the central core of the cable. The light is guided because of the presence of a lower refractive index cladding surrounding the central core. None of the energy in the signal is able to escape into the cladding and no energy is able to enter the core from any external sources. Therefore, the transmissions are not subjected to electromagnetic interference. Fiber optic cables deliver more reliable transmissions over greater distances, although at a somewhat greater cost. Cables of this type differ in their physical dimensions and composition and in the wavelength(s) of light with which the cable transmits. The chapter discusses ways to fix problems with splicing, laser and LED transmitters, driver incompatibility, incorrect bending radius in installation, and interface to cable connectors.