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2.2.3 Communications

Communications methods can be applied to transfer larger amounts of information to and from the controller. With this method the digital and analog I/O described above along with text and numerical data can be transferred. There are many different methods of communication based input and output protocols. Many of the communication techniques described below have been adapted to allow remotely mounted devices and I/O blocks to be distributed to remote locations on a machine or within a system and controlled from a central point. Often the remotely mounted I/O points can be semi-autonomous in controlling their local stations with only periodic communication to the central controller. Serial

Serial communications are strings of digital ones and zeroes sent over a single wire. They can alternate between sending and receiving data or have a dedicated line for each signal. The protocols for the data sent across the lines can vary widely but a few of the common types of serial communications are RS232, RS422 and RS485. The RS in these designations is an acronym for “recommended standard” and does not describe the actual communication protocol being used.

RS232 communications typically use separate send and receive lines. These are labeled as TX for transmit and RX for receive. They can also use other lines such as CTS and CTR for clear to send and clear to receive as a traffic control or hardware handshaking method. There are a number of parameters such as baud rate (the speed of the communication or bit rate), number of bits per character (7 or 8), whether or not a “stop” bit is used and whether the CTS and CTR lines are used (hardware handshaking). RS 232 signals are a common protocol used between computer terminals and various manufacturer’s control platforms. A 9 pin serial port is often included on a computer or control system which makes it a convenient tool for downloading programming into control devices. The TX and RX lines can either connect to the same pin numbers on each end or connect RX-TX and TX-RX which is known as a “null-modem” configuration.

RS422 and RS485 communications use a pair of twisted wires to carry the transmit and receive signals bi-directionally. For this reason they are often referred to a “twisted pair” communication. RS422 and RS485 can be used over a much longer distance and at a much higher data rate than RS232. RS422 is a multi-drop configuration whereas RS485 is a multi-point or “daisy-chain” configuration. These are often referred to as balanced or differential signaling. Parallel

Parallel communications allow multiple bits to be transmitted simultaneously over parallel lines. This can increase the throughput of data over RS232 signals but increases the cost of the cabling between two points. A common use of parallel cabling is between a computer’s parallel port and a printer. Another common use of parallel communications is between CPU chips and the various registers used for processing data on a controller board. This configuration is easily visible when looking at the many parallel traces on a circuit board or the multicolored ribbon cables that often connect boards to each other. The backplanes of many control systems that connect controllers to their I/O cards are often parallel busses. Ethernet

Ethernet is a framework for computer networking technology that describes the wiring and signaling characteristics used in Local Area Networks or LANs. The medium used for cabling Ethernet communications can be in the form of twisted pair wiring, coaxial cabling or fiber-optic lines between points. As with the other communication methods described in this section, Ethernet only describes the physical characteristics of the system in terms of wiring and not the communication protocol used across the wires or fibers. Because of the widespread use of Ethernet in computing, nearly every computer is now equipped with an Ethernet port. Switches and hubs are used to connect computers and control devices in wide ranging configurations. There are two different pin configurations for standard Ethernet cables, one with direct terminal to terminal configurations used with switches and hubs and another known as a “crossover” cable for direct port to port connection.

Ethernet communications are very fast in comparison to serial and parallel communications and can transfer large amounts of data quickly. Addresses must be configured for each device on a network in the format of “” Addresses can be configured directly by typing it into a field or configured automatically by a server using DHCP, or Dynamic Host Configuration Protocol. Typical LAN (Local Area Network) systems use DHCP to prevent duplication of addresses. A subnet mask is also used to prevent crosstalk between different connected networks.

TCP/IP is the set of communication protocols used for the internet and other similar networks. This is now commonly referred to as the Internet Protocol Suite. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. Today’s IP networking represents a synthesis of several developments that began to evolve in the 1960s and 1970s, namely the Internet and LANs (Local Area Networks), which emerged in the mid- to late-1980s, together with the advent of the World Wide Web in the early 1990s.
The Internet Protocol Suite, like many protocol suites, may be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted. USB

USB, or Universal Serial Bus is a configuration widely used in computer peripheral devices but is beginning to be adopted into automation systems. It was originally designed as a replacement for some of the RS232 and other serial connections on the backs of PCs. Along with communications between peripheral devices it can also provide a limited amount of current to power devices. USB signals are transmitted on twisted pair data cable. Unlike some of the physical-only specifications described previously, the USB standard also includes frame and communications protocols for more commonality between devices from different manufacturers. Special Automation Protocols

Many automation component vendors have developed their own protocols for communications using the various physical forms described above. Communication of data between controllers and operator interface touchscreens are often developed by the manufacturer and as such are not used between different manufacturers. To facilitate communications between different manufacturer’s devices, drivers are made available to allow devices to be easily interfaced.

Because of the interconnection problems between devices from different manufacturers, protocols have been adopted for communication and I/O control as standards.

DeviceNet is a communication protocol used in the automation industry to interconnect control devices for data exchange. It was originally developed by the Allen-Bradley company which is now owned by Rockwell Automation. It was adopted as an “open” concept and as such has been shared with third –party vendors. It is common to distribute power for remote devices through the DeviceNet media. It is layered on top of the CAN (Controller Area Network) protocol which was originally developed by Bosch. It is commonly used for remote I/O and control of servo and other motor control systems.

Profibus is a bit-serial field bus protocol developed by a group of companies in Germany. It is a global market leader among protocols because it can be used in both production automation and process automation. Profibus PA is a low current variation used to monitor measuring equipment in process automation (PA). Profibus DP (Decentralized Peripherals) is used to operate sensors and actuators via a centralized controller in a production environment. Siemens is a major player in the Profibus market.

Fieldbus, ControlNet, ASIbus and Ethernet/IP are other widely used communication and remote I/O protocols.

Miscellaneous I/O


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