How to reduce PLC input points

PLC in practical applications often run into two problems: one is the PLC I/O points is not enough and needs to be expanded, however increased I/O points will increase costs; second, the PLC has selected an Extensible I/O points are limited and cannot be increased. Therefore, on the premise of meet the system requirements, the rational use of I/O points to minimize the I/O required points is of great significance. Here‘s some common measures to reduce I/O points.

1. grouping input


General systems have a variety of ways, but the system also select only run one way, that is, work procedures cannot be executed. Therefore, according to their corresponding input signal can be divided into several groups work in different ways, PLC runtime uses only one set of signals, so the group can share PLC inputs, thus making the reduced entry point.


As shown in Figure 6-12, the system of automatic and manual work in two ways, which S1~S8 to work automatically to the input signal, Q1~Q8 the input signal used to manual work. Two sets of input signal X0~X7 share PLC input points, such as S1 and Q1 are common entry points to X0. Working mode selector switch to switch SA automatic and manual signal input circuit and PLC by X10 recognition is automatic or manual, to perform automated or manual procedures.

Figure 6-12 grouping input


Diode is to prevent parasitic circuit in the figure, set up wrong input signal. For example, when SA to pull the automatic position, if closing S1, S2 disconnected, Q1, Q2, closed, should be X0 enter and X1 input, but if no diode isolation, the current flow from the X0 after Q1 Q2 → → S1 COM form a parasitic circuit, so that X1 wrongly connected. Therefore, must be diode series cut off parasitic circuit, avoiding mistakes of the input signal.


2. the matrix input


As shown in Figure 6-13 is a 3×3 matrix input circuit, PLC three outputs Y0, Y1, Y2, and three input points X0, X1 and X2 to achieve the 9 input switch input device. Illustration, Y0, Y1, Y2 on the public side of COM and enter the public side of the relay COM together. Dang Y0, and Y1, and Y2 turns guide pass, is entered end X0, and X1, and X2 also turns get different of three group entered equipment of State, that Y0 connected Shi read into Q1, and Q2, and Q3 of pass broken state, Y1 connected Shi read into Q4, and Q5, and Q6 of pass broken state, Y2 connected Shi read into Q7, and Q8, and Q9 of pass broken state.


If Y0 is switched on, if Q1 is closed, the current flow from the X0 side, D1 Y0-end → → Q1, and Y0contact, flow from the output common COM, and finally back to enter the COM port, so as to make the relay X0 is connected. Y0 should be used in the ladder diagram programs normally open contacts and X0 normally open series, represents the Q1 input signal.


Diode is off in the role of parasitic circuit.


6-13 matrix input


Matrix input methods in addition to the hardware connection of the figure 6-12, you must also write the corresponding PLC program. Because the matrix input signal is known as a time to be read into the PLC, so read the input signal as a series of intermittent pulsed signals, should pay attention to this issue. In addition, should ensure that the input signal is greater than the width of Y0, Y1, Y2, took turns conducting the time, otherwise the input signal may be lost.


3. the combination of input


Can’t connect the input signal can be used to enter coding. As shown in Figure 6-14A, the three input signal Q1, Q2, Q3, as long as take two input points, and as shown in Figure 6-14B decoding process and into Q1, Q2, Q3, three signal correspond to M0, M1, M2. Using this method special attention should be paid to ensure that the input switch signal does not switch on.


Figure 6-14 combined input


A) hardware connection diagram b) ladder program


4. input devices, multifunction


In a traditional relay circuits, a master appliance (switch, button, etc) produced only a function of the signal. PLC systems, powerful logic with PLC functionality to implement an input device under different conditions, have different signals. Here is a simple example to illustrate.


As shown in Figure 6-15 the ladder using only X0 a button input to control outputs Y0 from the broken.


Figure 6-15 using a button control on starting, maintaining, stop circuit


Diagram, Y0 is disconnected, press the button (the X0 pass), M0, Y0 and lock press the button again, the M0, because at this point Y0, M1, the normally closed contacts disconnect Y0. The touch of a button, connect X0, Y0; press the button again, X0 and through the next, Y0 loss. Change the traditional relay control box with two buttons (Start button and the stop button), thereby reducing the PLC input points.


Similarly, we can use this approach to achieve an input with three or more features.


5. combined type


Some feature the same switch input devices input. If it is a normally closed contact, input are concatenated; if it is a normally open contact, parallel input. Therefore, one of several input devices can share PLC input points.


6. some input devices not PLC


Some simple functions, the input signal of the system involves a very narrow, such as some of the manual button, the motor overload protection thermal relay contact, sometimes there is no need as input to the PLC, put them in an external circuit can meet the requirements, as shown in Figure 6-16.

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