【正文】 ure A1 figure (1). The equivalent ladder logic looks very similar, figure A1figure(2). This circuit is based on the start switch being connected to input number 400, the stop switch to 401 and the motor output to 436. The ladder is a low level modelling tool, the PLC program is a translation of the ladder logic into mneumonics and number that vary from PLC to PLC. Figure A1 For a Mitsubishi PLC the corresponding set of instruction is given. The PLC first scans its internal memory to establish the state of the system. It will then run the program to establish all logical connections between input and output before actioning the outputs. In a plex control situation with a very large number of logical inputs and logical binations it is difficult to predict the results of illegal inputs such as damaged input switches or sensors. This lack of control causes normally stable systems to behave unpredictably causing damage to plant and violating safety. PLCs in flexible manufacturing PLCs are used extensively in flexible manufacturing systems. The cross effect of one output on another is difficult and often impossible to detect, consequently programmers do not take into account all possible permutations. The following example prises three machining centres. Machine 1 is a pillar drill operating on a single axis with sensors positioned at the top and bottom of the machine slide. Machine 2 is a planer with vertical movement and a rotating machine head with three selectable heads. Machine 3 is a horizontal miller with movement on two axis and sensors positioned at the end of each stroke. The three conveyor belts convey the ponents from the starting bay to each of the machining centres. Each of the ponents is individually coded, the code being entered into the PLC when it enters the starting bay. The ponent is then tracked through the system. As the ponent passes each of the machining centres a sensor informs the PLC of its arrival, the machining centre can now perform the appropriate operation for the ponent. In a system such as this it is relatively easy to loose control which results in jumped sequences, deadlock and lost ponents. The project39。 內(nèi)部記憶體，包括定時器，計數(shù)器，登記表和謹(jǐn)慎的內(nèi)部繼電器。機(jī) 1是唯一工作在軸的位置與傳感器安置在機(jī)器幻燈片 圖 B2 的頂部和底部。圖 B5表明，梯形邏輯確實是一個符號的 PLC程序，是假設(shè)內(nèi)部存儲器