How to save PLC input and output points - Database & Sql Blog Articles

Programmable Package SG-8018CA(SG7050C) 0.67M~170M

It is generally believed that the number of plc input points is determined by the number of input signals of the system. However, in practical applications, the following measures can be used to save the number of PLC input points. The following is introduced by Mitsubishi plc.

(1) Combination input, for input signals that are not turned on at the same time, can be input by combination coding. As shown, the three input signals SB0~SB2 occupy only two input points.

(2) Group input, as shown in the figure below, the system has two modes of operation: “manual” and “automatic”. Use X0 to identify whether to use the "automatic" or "manual" operation signal. When "manual", the input signal is SB0~SB3. If you follow the normal design idea, you need 8 input points for X0~X7, if you press the method below. In fact, only 4 input points are required for X1~X4. The diodes in the figure are used to cut off parasitic circuits. If there is no diode in the figure, the system is in the automatic state, SB0, SB1, S0 are closed and S1 is disconnected. At this time, the current flows out from the com terminal, and a parasitic bribe is formed into the X0 terminal via SB0, SB1, and S0, so that the input bit X2 is incorrectly changed. On. After the diodes are connected in series with each switch, the parasitic loop is cut off to avoid errors. However, the input signal strength should be considered.

(3) Matrix input

The figure below shows the 4*4 matrix input circuit, which uses the four input points X0~X3 of the PLC to realize the functions of 16 input points, especially suitable for occasions where there are many plc output points and the input points are not enough. When Y0 is turned on, X0~X3 accepts the input signal sent by Q1~Q4; when Y1 turns on, X0~X3 accepts the input signal sent by Q5~Q8; when Y2 turns on, X0~ X3 accepts the input signal from Q9~Q12; when Y3 turns on, X0~X3 accepts the input signal from Q13~Q16. Connect the normally open point of Y0 and X0~X3 in series with the input signal Q1~Q4, and connect the normally open point of Y1 with X0~X3 as Q5~Q8, and so on.

When using it, it should be noted that in addition to the wiring according to the above figure, the corresponding software is needed to cooperate to realize the turn-on conduction of Y0~Y3. At the same time, the width of the input signal should be greater than the turn of Y0~Y3. The time may otherwise lose the input signal. The disadvantage is that the sampling frequency of the input signal is reduced to one-third of the original, and the output points Y0~Y3 can no longer be used.

(4) Input device multi-function

In a conventional relay control system, a master command (button, switch, etc.) produces only one function signal. In the system of plc system control, an input device can generate different signals under different conditions, and can be used to generate a start signal by using a button, and can also be used to generate a stop signal. As shown in the figure, only one button is used to control the on and off of Y0 through X0, that is, Y0 is turned on when X0 is turned on for the first time, and Y0 is turned off when X0 is turned on again.

(5) The combination of the access contacts, the combination of some of the same function switching input devices (normally closed contact series input, normally open contact parallel input). Some methods of protecting computer alarm circuits often use this method.

If some external input signals are always present in the ladder diagram in a whole form of "or NAND" combination, some of their corresponding contacts can be programmed as a whole input after being connected in series outside the programmable controller. The controller only accounts for one input point of the programmable controller.

For example, a load can be started and stopped in multiple places, multiple start signals can be connected in parallel, multiple stop signals are connected in series, and respectively sent to two input points of plc, as shown in the figure, occupying the same input with each start and stop signal. Compared with the point method, the ladder circuit is also simplified.

PLC output control method one, the principle is the same as the matrix input, and the output point is made into 4*4 or 5*5, which is the output point of 16 or 25 points.

The names Y0~Y7 are a0 a1 a2 a3 a4 a5 a6 a7

We arrange 4*4=16 output points

A0a4 a0a5 a0a6 a0a7 a1a4 a1a5 a1a6 a1a7, a2a4 a2a5 a2a6 a2a7 a3a4 a3a5 a3a6 a3a7

In the wiring, we wear them in the order above.

In the plc program, when a0a4 is on, the first group of switches is powered.

When a0a5a is on at the same time, the second group is powered

Note that when there are multiple points output at the same time, we have to exclude a point that is powered at the same time. Any combination of the three points may be repeated.

Such as: 4 points output at the same time, we try to use the first group

Advantages: Multiple points can be obtained from small points, insufficient: the program and wiring are slightly more complicated.