1 Introduction
PLC has the advantages of small size, high cost performance, low power consumption, strong anti-interference ability, convenient programming and so on, and is widely used in industrial control field. However, in the actual application process, the output point of the controlled object is less frequent than the input point, and there are more points to be detected in the control task, or more operation buttons, so that the number of output points of the plc can be easily selected during the selection. The satisfaction of the requirements, but for the input point may be difficult to meet, for the above situation can usually take the following measures: (1) select the number of output points to meet the requirements of the plc, by configuring a dedicated input module to increase the number of input points, make the input The number of points meets the requirements of the control system. This method increases the cost of the control system and reduces the cost performance of the system. (2) Selecting a number of plcs with a large number of input points, so that the number of input points is satisfied by the control system, and the number of output points is increased, so that the output points are redundant and idle, resulting in waste of resources. (3) It is still the plc that selects the number of output points to meet the requirements, but adds some peripheral circuits when expanding. This part of the circuit is mainly composed of decoders, which can greatly reduce the initial investment of the system. (4) Using plc software programming implementation, the advantage is that under the premise that the number of plc output points meets the system requirements, select the number of input points with less plc, without adding additional hardware, using the inherent resources of the plc, through coding The method implements an extension of the number of input points. This paper introduces two input point expansion methods based on software programming methods by combining plc input and output points.
2. Input point expansion method based on software programming method
An important feature of plc is that the input and output points of each group are highly independent, which is mainly reflected in the common end of the input and output points. On the one hand, individual input and output points can have their own common end. On the other hand, multiple input and output points can share a common end, so that the combination of input and output points is relatively easy. Through these combinations, we can increase the number of input points by borrowing the principle of matrix keyboard scanning and the principle of input node combination matrix.
2.1 Using the matrix keyboard scanning principle to expand the number of plc input points
Take m input points of plc as the line retrace line input end of the input node matrix, take n output points of plc as the column select line output end of the input node matrix, and simultaneously use the common end com of the input end and the common end of the output end Com is connected, the state of the n column selection lines is controlled by an internal program, thereby realizing the input node matrix column scanning; by detecting the state of the m input points, the line scanning of the input node matrix is ​​completed; thus, the input node matrix can be uniquely determined. The closed state of a joint. With the node matrix, it is convenient to expand from m input points and n output points into m × n input points.
Figure 1 is a schematic diagram of extending 4 × 2 input points using the matrix keyboard scanning principle. When the input and output of plc move, it must form a closed loop. The following describes the working process of the system with input nodes s0 and s1:
(1) When the plc output points y0 and y1 are turned off, the loop of the input point i0 is not connected. At this time, even if the input nodes s1 and s2 are closed, the plc cannot detect the closing of the node.
(2) When the plc output point y0 is closed and y1 is off, if the input node s0 is closed, the plc input point i0 can be valid; meanwhile, since y1 is disconnected, the s1 closure is invalid.
(3) When the output point y1 of plc is closed and y0 is off, if the input node s1 is closed, the plc input point i0 can be valid; meanwhile, since y0 is disconnected, s2 is closed.
Through the above analysis, the state of the time-sharing control output points y0 and y1 can be known, and the closed state of the input nodes s1 and s2 can be uniquely determined, and the same can be extended to the input nodes s2, s3, s4, s5, s6, s7. . When using this method, the scan time of the keyboard must be determined, and the length of the scan depends on the output point form of the plc. For the plc of the transistor, thyristor and solid state relay output, the scan time can be made shorter if the control requirements are met; for the plc of the relay output, the scan time should be extended appropriately considering the life of the contact.
2.2 Using the input point combination matrix method to expand the number of plc input points
The premise of extending the number of plc input points using the matrix keyboard scanning principle is that the plc must have the remaining output points. If not, this scheme is inevitably not feasible. At this time, the input point must be used. The input point expansion method based on the input point combination matrix is ​​introduced below.
Taking m input points of plc to form m input node groups, taking n input points of plc constitutes n input node state detection ends, that is, each input node group contains n nodes, so that m×n can be realized Enter the extension of the point. When a contact is closed, the corresponding input node group and the input node detection end have signals sent to the plc, and the input node state can be uniquely determined by the input node.
Figure 2 is a schematic diagram of expanding 3 x 4 input points using an input node combination matrix. Figure 2 contains three input node groups and four input state detection terminals, that is, each group contains four input nodes. The role of the diode in Figure 2 is to prevent mutual interference when the nodes are closed. The following describes the working process of the system with the input node s0.
(1) When the input node s0 is disconnected, there is no input to the corresponding input node group input terminal x0 and the input state detection terminal x6, indicating that s0 is off.
(2) When the input node s0 is closed, the corresponding input node group input terminal x0 and the input state detection terminal x6 both have signals entering plc, indicating that s0 is closed.
Through the above analysis, it can be concluded that the uniqueness of the combination of the input point x0 and the input point x6 can uniquely determine the state of the input node s0, thereby achieving the purpose of expanding the number of input points, which can be derived from the truth table of the attached table. inferred. In the attached table 1, "1" indicates that the internal contact of the plc input point is closed, and "0" indicates disconnection.
This method can easily expand the number of plc input points. Compared with the former method, the applicability to plc is strong. The choice of scan time depends on the scan time of the application.
3, the conclusion
Using plc's own input and output points to expand the actual number of input points of plc without adding additional hardware, increasing the cost performance of the system. For the above two methods of expanding the number of plc input points, the method is verified in the laboratory, the operation is reliable, easy to understand, and the application value is considerable.
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