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In a PLC/computer network, as shown in Fig. 4C, all communications are initiated by the host computer, which is connected to all the PLCs in a daisy-chain fashion. This computer inpidually addresses each of its networked PLCs and asks for specific information. The addressed PLC then sends this information to the computer for storage and further analysis. This cycle occurs hundreds of times per second.
Host computers also can aid in programming PLCs; powerful programming and documentation software is available for program development. Programs then can be written on the computer in relay ladder logic and downloaded into the PLC. In this way, you can create, modify, debug, and monitor PLC programs via a computer terminal.
In addition to host computers, PLCs often must interface with other devices, such as operator interface terminals for large security and building management systems. Although many intelligent devices can communicate directly with PLCs via conventional RS232C ports and serial ASCII code, some don't have the software ability to interface with inpidual PLC models. Instead, they typically send and receive data in fixed formats. It's the PLC programmer's responsibility to provide the necessary software interface.
The easiest way to provide such an interface to fixed-format intelligent devices is to use an ASCII/BASIC module on the PLC. This module is essentially a small computer that plugs into the bus of the PLC. Equipped with RS232 ports and programmed in BASIC, the module easily can handle ASCII communications with peripheral devices, data acquisition functions, programming sequences, "number crunching," report and display generation, and other requirements.
Access, protocol, and modulation functions of LANs
By using standard interfaces and protocols, LANs allow a mix of devices (PLCs, PCs, mainframe computers, operator interface terminals, etc.) from many different vendors to communicate with others on the network.
Access: A LAN's access method prevents the occurrence of more than one message on the network at a time. There are two common 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 it'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 common types.
Proprietary protocols are unique message arrangements and coding developed by a specific vendor for use with that vendor'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 common types are broadband and baseband.
Network transmission interfaces
The vast majority of PLC communications is done via RS232C and twisted pair cables. Most PLCs have an RS232 port and are capable of handling communications with host computers, printers, terminals, and other devices. Maximum transmission speed is 19.2 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 manufacturer's specifications for actual distance and data transmission rate capabilities.
The only real limitation on RS232C is the 50-ft recommended 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 electromagnetic interference (EMI) and radio-frequency interference (RFI) are known to exist.
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