【正文】 s product only. Open protocols are based on industry standards such as TCP/IP or ISO/OSI models and are openly published. Modulation: Network modulation refers to the way messages are encoded for transmission over a cable. The two most mon types are broadband and baseband. Network transmission interfaces The vast majority of PLC munications is done via RS232C and twisted pair cables. Most PLCs have an RS232 port and are capable of handling munications with host puters, printers, terminals, and other devices. Maximum transmission speed is Kbps. The distance and data transmission rates a。s access method prevents the occurrence of more than one message on the work at a time. There are two mon access methods. Collision detection is where the nodes listen to the work and transmit only if there are no other messages on the work. If two nodes transmit simultaneously, the collision is detected and both nodes retransmit until their messages get through properly. Token passing allows each node to transmit only if it39。s the PLC programmer39。 powerful programming and documentation software is available for program development. Programs then can be written on the puter in relay ladder logic and downloaded into the PLC. In this way, you can create, modify, debug, and monitor PLC programs via a puter terminal. In addition to host puters, PLCs often must interface with other devices, such as operator interface terminals for large security and building management systems. Although many intelligent devices can municate directly with PLCs via conventional RS232C ports and serial ASCII code, some don39。s no master PLC. However, it39。s memory. This entire cycle occurs hundreds of times per second. Peertopeer works Peertopeer works, as shown in Fig. 4B, enhance reliability by decentralizing the control functions without sacrificing coordinated control. In this type of work, numerous PLCs are connected to one another in a daisychain fashion, and a mon memory table is duplicated in the memory of each. In this way, when any PLC writes data to this memory area, the information is automatically transferred to all other PLCs in the work. They then can use this information in their own operating programs. With peertopeer works, each PLC in the work is responsible for its own control site and only needs to be programmed for its own area of responsibility. This aspect of the work significantly reduces programming and debugging plexity。s required is an ASCII interface for the connection(s), along with considerable work with software. Remote I/0 systems A remote I/O configuration, as shown in Fig. 4A, has the actual inputs and outputs at some distance from the controller and CPU. This type of system, which can be described as a masterandslave configuration, allows many distant digital and analog points to be controlled by a single PLC. Typically, remote I/Os are connected to the CPU via twisted pair or fiber optic cables. Remote I/O configurations can be extremely costeffective control solutions where only a few I/O points are needed in widely separated areas. In this situation, it39。s see how they are used in describing the available PLC work options. PLC work options PLC works provide you with a variety of working options to meet specific control and munications requirements. Typical options include remote I/O, peertopeer, and host puter munications, as well as LANs. These works can provide reliable and costeffective munications between as few as two or as many as several hundred PLCs, puters, and other intelligent devices. Many PLC vendors offer proprietary working systems that are unique and will not municate with another make of PLC. This is because of the different munications protocols, mand sequences, errorchecking schemes, and munications media used by each manufacturer. However, it is possible to make different PLCs talk to one another。s used to transfer data to, or receive data from, a PLC in a PLC/puter work. 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 work. Each node incorporates a device that can municate with all other devices on the work. Protocol: The definition of how data is arranged and coded for transmission on a work. 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 munications that describes specific wiring connections, voltage levels, and other operating parameters for electronic data munications. 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 munications access point on a device that is set up for serial munications. 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 work 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