Foundation of PLC
1. The central processing unit
Although referred to as the brain of the system, the Central Processing Unit in a normal installation is the unsung hero, buried in a control cabinet, all but forgotten.
2. Basic Functionality
In a programmable controller system, the central processing unit(CPU) provides both the heart and the brain required for successful and timely control execution. It rapidly and efficiently scans all of the system inputs, examines and solves the application logic, and updates all of the system outputs. In addition, it also gives itself a checkup each scan to ensure that its structure is still intact. In this chapter we will examine the central processing unit as it relates to the entire system. Included will be the various functional blocks in the CPU, typical scan techniques, I/O interface and memory users, power supplies, and system diagnostics.
3. Typical Function Block Interactions
In practice, the central processing unit can vary in its architecture, but consists of the basic building block structure illustrated in Fig.1.1.The processing section consists of one or more microprocessors and their associated circuitry. While it is true that some of the luxury of using microprocessors, most modern systems use either a single microprocessors such as the AMD 2903, usedin a bit slice architecture. This multiple microprocessor system to break the control system tasks into many small components which can be executed in parallel. The result of this approach is to achieve execution speeds that are orders of magnitude faster than their single-tasking counterparts. In addition to efficiently processing direct I/O control information and being programmable, the real advantage that microprocessor-based system have over their hardwired relay counterparts is the ability to acquire and manipulate numerical data easily. It is this attribute that makes programmable controllers the powerhouses that they are today in solving tough factory automation problems. The factory of tomorrow will run efficiently only if quality information about process needs and status of the process equipment are known on a realtimes basis. This can and will come about only if the unit level controllers, including programmable controllers, are empowered with the ability to collect, analyze, concentrate, and deliver data about the process. As the
market continues to exhibit this demand, manufactures are likely to outfit their controllers with more and more variable memory, and enhanced instruction sets to perform these tasks.
4. Scan Techniques
By definition and design, the programmable controller is dedicated to the continuous, repetitive task of examining the system inputs, solving the continuous, repetitive task of examining the system inputs, solving the current control logic ，and updating the system outputs. This task is referred to as scanning (sometimes called sweeping), and is accomplished in slightly different ways in each manufacturer‟s programmable controller. Since many of the variations are not material to the basic functionality of the system, we will only examine the basic varieties.
You‟ll notice that the I/O servicing is at the end of the scan cycle, and is also an integral part of the scan timing. This type of scan is referred to as synchronous scan and is used with very fast machines that can update all of the I/O without lengthening the scan time materially. A typical scan time in a modern programmable controller ranges from 10~100 ms (milliseconds). Most controllers have a mechanism, watchdog timer, to measure the scan length each cycle and normally 150~250 ms. Referring to Fig.1.1 again, the synchronous scan contains four other actives in addition to the I/O scan. Housekeeping refers to a small number of routine chores performed by the programmable controller to ensure that its internal structure is still healthy and functioning properly. Next comes the communication windows to allow structured communications to other devices in the system, or externally. Included in this group would be the programming device, special microprocessor-based communications modules to allow ultimate communication of the programmable controller system to another intelligent device. Next in line comes the executive routine, in which the actual base intelligence of the system is used to interpret the current control program. This interpretation is then used in the next step to solve the current control logic program. The last step of this basic scan process is to integrate the currently interpreted control logic program with the most current input statuses from the I/O scan, and to update the output statuses with the current results.
