It is generally believed that the number of input points is determined by the number of system input signals. However, in practical applications, the following measures can be taken to save on the number of PLC input points, which will be introduced below using the FX1N series PLC as an example.
(1) Combined input: For input signals that will not be turned on simultaneously, a combined encoding method can be used for input. As shown in the figure, the three input signals SB0~SB2 only occupy two input points.
(2) Grouped input, as shown in the figure below, the system has two working modes: "manual" and "automatic". X0 is used to identify whether to use the "automatic" or "manual" operation signal. In "manual" mode, the input signals are SB0~SB3. If following the normal design approach, then a total of 8 input points X0~X7 are needed. If the method shown in the figure is used in practice, then only 4 input points X1~X4 are needed. The diodes in the figure are used to cut off the parasitic circuit. If there are no diodes in the figure, the system is in automatic mode, SB0, SB1, and S0 are closed and S1 is open. At this time, the current flows out from the com terminal, forms a parasitic loop through SB0, SB1, and S0 and flows into the X0 terminal, causing the input bit X2 to incorrectly become on. After each switch is connected in series with a diode, the parasitic loop is cut off, avoiding the generation of errors. However, the strength of the input signal should be considered.
(3) Matrix input
The diagram below shows a 4x4 matrix input circuit. It uses four input points X0~X3 of the PLC to achieve the function of 16 input points, making it particularly suitable for situations where the PLC has many output points but insufficient input points. When Y0 is on, X0~X3 receive input signals from Q1~Q4; when Y1 is on, X0~X3 receive input signals from Q5~Q8; when Y2 is on, X0~X3 receive input signals from Q9~Q12; and when Y3 is on, X0~X3 receive input signals from Q13~Q16. Connecting the normally open contact of Y0 in series with X0~X3 results in input signals Q1~Q4; connecting the normally open contact of Y1 in series with X0~X3 results in signals Q5~Q8, and so on.
When using this method, it's important to note that in addition to wiring as shown in the diagram above, corresponding software is also required to enable the alternating conduction of Y0~Y3. Furthermore, the input signal width must be greater than the time required for one complete conduction cycle of Y0~Y3; otherwise, the input signal may be lost. The disadvantage is that it reduces the input signal sampling frequency to one-third of its original value, and the output points Y0~Y3 can no longer be used.
(4) Multifunctional input devices
In traditional relay control systems, a single command (button, switch, etc.) generates only one functional signal. In PLC control systems, however, an input device can generate different signals under different conditions. For example, a single button can generate both a start and a stop signal. As shown in the diagram, a single button controls the on/off state of Y0 via X0; that is, when X0 is first turned on, Y0 is on, and when X0 is turned on again, Y0 is off.
(5) Merging input and output contacts: Combining the inputs of certain digital input devices with the same function (normally closed contacts in series input, normally open contacts in parallel input). This method is often used in alarm circuits that protect computers.
If certain external input signals always appear in the ladder diagram in the form of a combination of "OR" and "NOT", some of their corresponding contacts can be connected in series outside the programmable controller and input as a whole into the programmable controller, occupying only one input point of the programmable controller.
For example, if a load can be started and stopped at multiple points, multiple start signals can be connected in parallel and multiple stop signals can be connected in series, and then sent to two input points of the PLC respectively, as shown in the figure. Compared with the method where each start and stop signal occupies the same input point, this simplifies the ladder diagram circuit.
PLC output control method one, the principle is the same as matrix input, the output points are made into 4*4 or 5*5, that is, 16 or 25 output points.
The names Y0 to Y7 are a0a1a2a3a4a5a6a7 respectively.
We arrange 4*4=16 output points.
a0a4a0a5a0a6a0a7a1a4a1a5a1a6a1a7, a2a4a2a5a2a6a2a7a3a4a3a5a3a6a3a7
In the wiring, we thread them in the order shown above.
In the PLC program, when A0 and A4 are both on, the first set of switches is energized.
When a0a5a are both on, the second group receives power.
Note that when multiple points output simultaneously, we need to exclude any point that is energized at the same time. Any combination of three points may result in duplicate outputs.
For example, if there are four output points simultaneously, we should try to use the first group.
Advantages: Can generate multiple points from a small number of points; Disadvantages: The program and wiring are slightly more complex.
