Sequential logic process

Sequential logic is coupled to device states. For example, device state 0 could be a valve closed, agitator off, and so on, while device state 1 could be the valve open or the agitator on. Basically, the sequential logic determines when the process should proceed from the current set of operating conditions to the next. Sequential logic must encompass both normal and abnormal process operations, such as equipment failures. 

An automated batch facility utilizes sequential logic and discrete devices to change the process state subject to the interlocks. Discrete devices such as two-position valves can be driven to one of two possible states (open/closed). Such devices can be equipped with limit switches that indicate the state of the device. The discrete-device driver is the software routine that generates the output to a discrete device such as a valve and that also monitors process measurements to ascertain that the discrete device actually attains the desired state. 

This section has illustrated the important part that the Process statement plays in the design of sequential logic. In Chapter 4, its function in combinational design was also shown. It is good design practice to separate combinational and synchronous sections by using different processes. Box 5.1 offers some guidelines on how to use the process statement and illustrates its syntax. 

The SYNC WAIT architecture demonstrated how a Walt statement can be used to infer a sequential section. The whole process must be synchronous and therefore only s)mchronous initialization is possible with a Wait statement. The drcuit produced by this form of sequential logic template is... 

A process containing either a Walt or an If statement must be used to infer sequential logic. 

A function is very similar to a Process statement in its operation except that it cannot contain signal assignment or Wait statements. This means that a function caimot be used to create sequential logic. As the RIPPLE functions illustrate, variables that are local to the function can be declared in the declarative part and its execution is sequential. All objects declared locally are discarded when the final result is returned. The s)mtax of a function and other important details are given in Box 6.4. 

What happens, though, as the control logic becomes more complicated For example, if the output X is dependent on inputs A, B and C, or even the inputs A, B and C and ff e present outputs W and Z, how is ffiis sort of operation best represented Storing the present state of the outputs obviously requires some form of sequential logic, and this must be implemented using a Process statement. It is already known that tfie Case statement generally creates a more efficient circuit than its If-Else equivalent. It is also a neat structure in which to construct the jfinite state machine that will usually be used to implement such control operations. 

Planning is defined as the process of analysing each job so as to determine the nature of the job and the results desired specify the logical sequence of the job and apply humanpower and estimates for each sequential step Hst predeterminable material, tools, and special equipment and estimate the total cost to meet the required results. 

Reverse Logical Opposite Reverse flow. Sequential process steps performed in reverse order. 

One possible method of automatic optimization was mentioned by Kahn (Kl) quite some time ago. This used the Monte Carlo technique to make a random design of cases over a broad area known to contain the optimum. After a limited number of cases had been calculated, the best case was selected and another random design of cases was made. For the second design, however, the dispersion of cases was reduced so that the investigation became more localized. After a sufficient number of repetitions of this process the optimum would be determined to a sufficient degree of accuracy. The method is perhaps the antithesis of a logical or orderly calculation procedure, but it can be programmed for automatic sequential calculation. 

It is desirable to list the sequence of space and point steps which together constitute global adsorption. This is not a new concept and such descriptions have frequently been presented (I, 14), particularly for fluid reactions on porous catalyst particles. The first space process, axial dispersion, is not a part of the. sequence, but it does affect the observed kinetics, and is logically considered as a space process. Its significance depends upon the reciprocal of the axial Peclet number, EJ(2R)v. The sequential steps are ... 

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