TABLE OF CONTENTS The increasing use of automation in industry has led to the need for communications and control on a plant-wide basis with programmable controllers, computers, robots, and CNC machines interconnected. The term local area network (LAN) is used to describe a communications network designed to link computers and their peripherals within the same building or site.
Networks can take three basic forms. With the star form (Figure 4.26(a)) the terminals are each directly linked to a central computer, termed the host, or master with the terminals being termed slaves. The host contains the memory, processing and switching equipment to enable the terminals to communicate. Access to the terminals is by the host asking each terminal in turn whether it wants to talk or listen. With the bus or single highway type of network (Figure 4.26(b)), each of the terminals is linked into a single cable and so each transmitter/receiver has a direct path to each other transmitter/receiver in the network. Methods, i.e. protocols, have to be adopted to ensure that no more than one terminal talks at once, otherwise confusion can occur. A terminal has to be able to detect whether another terminal is talking before it starts to talk. With the ring network (Figure 4.26(c)), a continuous cable, in the form of a ring, links all the terminals. Again methods have to be employed to enable communications from different terminals without messages becoming mixed up. The single highway and the ring methods are...
