【正文】 e remote I/O to change the state of outputs as dictated by the control program in the PLC39。s memory. This entire cycle occurs hundreds of times per second.Peertopeer networksPeertopeer networks, as shown in Fig. 4B, enhance reliability by decentralizing the control functions without sacrificing coordinated control. In this type of network, 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 network. They then can use this information in their own operating programs.With peertopeer networks, each PLC in the network is responsible for its own control site and only needs to be programmed for its own area of responsibility. This aspect of the network significantly reduces programming and debugging plexity。 because all munications occur transparently to the user, munications programming is reduced to simple readandwrite statements.In a peertopeer system, there39。s no master PLC. However, it39。s possible to designate one of the PLCs as a master for use as a type of group controller. This PLC then can be used to accept input information from an operator input terminal, for example, sending all the necessary parameters to other PLCs and coordinating the sequencing of various events.Host puter linksPLCs also can be connected with puters or other intelligent devices. In fact, most PLCs, from the small to the very large, can be directly connected to a puter or part of a multi drop host puter network via RS232C or RS422 ports. This bination of puter and controller maximizes the capabilities of the PLC, for control and data acquisition, as well as the puter, for data processing, documentation, and operator interface.In a PLC/puter network, as shown in Fig. 4C, all munications are initiated by the host puter, which is connected to all the PLCs in a daisychain fashion. This puter individually addresses each of its networked PLCs and asks for specific information. The addressed PLC then sends this information to the puter for storage and further analysis. This cycle occurs hundreds of times per second.Host puters also can aid in programming PLCs。 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。t have the software ability to interface with individual PLC models. Instead, they typically send and receive data in fixed formats. It39。s the PLC programmer39。s responsibility to provide the necessary software interface.The easiest way to provide such an interface to fixedformat intelligent devices is to use an ASCII/BASIC module on the PLC. This module is essentially a small puter that plugs into the bus of the PLC. Equipped with RS232 ports and programmed in BASIC, the module easily can handle ASCII munications with peripheral devices, data acquisition functions, programming sequences, number crunching, report and display generation, and other requirements.Access, protocol, and modulation functions of LANsBy using standard interfaces and protocols, LANs allow a mix of devices (PLCs, PCs, mainframe puters, operator interface terminals, etc.) from many different vendors to municate with others on the network.Access: A LAN39。s access method prevents the occurrence of more than one message on the network at a time. There are two mon access methods.Collision detection is where the nodes listen to the network and transmit only if there are no other messages on the network. 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 in possession of a special electronic message called a token. The token is passed from node to node, allowing each an opportunity to transmit without interference. Tokens usually have a time limit to prevent a single node from tying up the token for a long period of time.Protocol: Network protocols define the way messages are arranged and coded for transmission on the LAN. The following are two mon types.Proprietary protocols are unique message arrangements and coding developed by a specific vendor for use with that vendor39。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 interfacesThe 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 are standards for the various interfaces. Their actual performance is a function of the driving devices and varies significantly between manufacturers. As such, you should consult the manufacturer39。s specifications for actual distance and data transmission rate capabilities.The only real limitation on RS232C is the 50ft remended distance between devices. While RS232C installations often can achieve cabling distances greater than this, the unbalanced design of the interface results in a greater susceptibility to surrounding electrical noise and reduced data integrity. This is particularly true where electroma