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To understand the PLC's communications versatility, let's first define the terms used in describing the various systems.
ASCII: This stands for "American Standard Code for Information Interchange." As shown in Fig. 1, when the letter "A" is transmitted, for instance, it's automatically coded as "65" by the sending equipment. The receiving equipment translates the "65" back to the letter "A." Thus, different devices can communicate with each other as long as both use ASCII code.
ASCII module: This intelligent PLC module is used for connecting PLCs to other devices also capable of communicating using ASCII code as a vehicle.
Bus topology: This is a linear local area network (LAN) arrangement, as shown in Fig. 2A, in which inpidual nodes are tapped into a main communications cable at a single point and broadcast messages. These messages travel in both directions on the bus from the point of connection until they are dissipated by terminators at each end of the bus.
CPU: This stands for "central processing unit," which actually is that part of a computer, PLC, or other intelligent device where arithmetic and logical operations are performed and instructions are decoded and executed.
Daisy chain: This is a description of the connection of inpidual devices in a PLC network, where, as shown in Fig. 3, each device is connected to the next and communications signals pass from one unit to the next in a sequential fashion.
Distributed control: This is an automation concept in which portions of an automated system are controlled by separate controllers, which are located in close proximity to their area of direct control (control is decentralized and spread out over the system).
Host computer: This is a computer that's used to transfer data to, or receive data from, a PLC in a PLC/computer network.
Intelligent device: This term describes any device equipped with its own CPU.
I/O: This stands for "inputs and outputs," which are modules that handle data to the PLC (inputs) or signals from the PLC (outputs) to an external device.
Kbps: This stands for "thousand bits per second," which is a rate of measure for electronic data transfer.
Mbps: This stands for "million bits per second."
Node: This term is applied to any one of the positions or stations in a network. Each node incorporates a device that can communicate with all other devices on the network.
Protocol: The definition of how data is arranged and coded for transmission on a network.
Ring topology. This is a LAN arrangement, as shown in Fig. 2C, in which each node is connected to two other nodes, resulting in a continuous, closed, circular path or loop for messages to circulate, usually in one direction. Some ring topologies have a special "loop back" feature that allows them to continue functioning even if the main cable is severed.
RS232. This is an IEEE standard for serial communications that describes specific wiring connections, voltage levels, and other operating parameters for electronic data communications. There also are several other RS standards defined.
Serial: This is an electronic data transfer scheme in which information is transmitted one bit at a time.
Serial port: This the communications access point on a device that is set up for serial communications.
Star topology. This is a LAN arrangement in which, as shown in Fig. 2B, nodes are connected to one another through a central hub, which can be active or passive. An active hub performs network duties such as message routing and maintenance. A passive central hub simply passes the message along to all the nodes connected to it.
Topology: This relates to a specific arrangement of nodes in a LAN in relation to one another.
Transparent: This term describes automatic events or processes built into a system that require no special programming or prompting from an operator.
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