PLC电梯控制英文文献五千字翻译(适用于本科毕业设计)培训资料.ppt

1;2.5 System Block Diagram A Programmable Controller is a specialized computer.Since it is a computer, it has all the basic component parts that any other computer has; a Central Processing Unit,Memory, Input Interfacing and Output Interfacing.A typical programmable controller block diagram is shown in Figure 2.5.;3;is input/output updating, it is referred to as I/O Update.The updating of discrete input and output information is accomplished with the use of input and output image registers set aside in the PLC memory.Each discrete input point has associated with it one bit of an input image register.Likewise, each discrete output point has one bit of an output image register associated with it.When I/O updating occurs, each input point that is ON at that time will cause a 1 to be set at the bit address associated with that particular input.If the input is off, a 0 will be set into the bit address.Memory in todays PLCs is generally configured in 16 bit words.This means that one;Figure 2.6;the light (coil) has been either turned on or off it will remain in that state until the next time the PLC solves that particular rung.After solving a rung, the PLC moves on to solve the next rung in the same manner and so forth until the entire ladder has been executed and solved.One rule that is different from general electrical operation is the direction of current flow in the rung.In a ladder logic, rung current can only flow from left to right and up and down; never from right to left. As an example, in the ladder shown in Figure 2.7, coil CR1 will energize if any of the following conditions exist:; At I/O update, the states of all coils which are designated as outputs are transferred from the output image register to the output unit and the states of all inputs are transferred to the input image register.Note that any input changes that occur during the solution of the ladder are ignored because they are only recorded at I/O update time.The state of eac