Unit operations, control sequence logic

The central processing unit (CPU) controls the overall operation of the computer. It is made up of electronic registers and logic circuits that execute the simple logical and arithmetic operations of which the computer is capable. When these operations are executed in appropriate sequences, the computer can accomplish complex mathematical or data-processing functions. Moreover, if one provides the appropriate electronic interface, these simple operations can be used to control experimental systems, acquire data, or print results on a teletype printer, line printer, oscilloscope, or other peripheral device. 

The heart of a eomputer is the CPfl, which in the case of a microcomputer is a microprocessor chip. A microprocessor is made up of a control unit and an control unit determines the sequence of operations by means of instructions frvmi a program stored in tho computer memory. The control unit receives informaiion from tho input device, fetches instructions and data from the memory, and transmits instructions lo the arithmetic logic unit, output. and memory. 

Other examples for CDFG are shown in Figure 2, where actual CVs are shown above extended CVs. When they have the same value, actual CVs are omitted for the figure. The ECV of node a is [1,0,1] and its ACV is [1,1,1] because conditional operation node p [1,1,1] is resolved and node q [0,1,1] is not resolved. Note that the ACV of node p is [0,0,0]. When operation can be done by simple logic gates (don t use function unit), its ACV becomes [o]. This is because such operation need not share FUs and its delay is so short that the operation can be executed with another operaion in a c-step (logic operation does not need a c-step). This characteristic is valuable in the control sequence synthesis section as indicated in a later section. 

Packaged units have traditionally had their own progranunable logic controllers (PLCs) and local control panels. This is especially true in the case of units that require extensive sequence controls for stepwise processes such as filtration and ion exchange. Many operators find it more convenient to have control supervised from a central station. From this point of view, these systems are better controlled by a distributed control system (DCS) and monitored from the control room. Some who bought systems equipped with PLCs some years ago have dispensed with the PLCs and moved control to the DCS. Many favor systems which combine local control by PLC with status and alarm signals sent to the DCS. 

However the SIS logic should not be used for direct control of start up progress, rather the main BPCS should be programmed to manage all start sequences such that blocking by the SIS should not have to occur. Remember that it is a basic principle of SIS design that it should not have to play any role in routine control operations. For this reason an SIS trip should not be used as a routine means of shutting down a plant unit. 

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