Regarding S7-300 PLC ladder diagrams
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I’m a beginner and have just started learning *programming. I came across a small program that controls the start and stop of a fan, with various external conditions incorporated into it; all of these are normally closed contacts. What’s unclear to me in the program is related to the operation signal of the fan: the normally closed contact of this signal (I74.4) is connected to the start coil (Q42.2), while the normally open contact of the same signal (I74.4) is connected to the stop coil (Q42.3). What I don’t understand is how a signal coming from the same contact can be both normally open and normally closed I would appreciate any further advice from everyone; thank you
In the PLC’s internal logic, when the same signal is used, it naturally comes in either normally open or normally closed form. It’s difficult to explain the specific programming without diagrams; it can be understood as using the normally closed point of the operation signal to interlock the start signal, and using the normally closed point to interlock the stop signal. In other words, once the device is already in operation, the start button will no longer be effective, and the same applies to the stop signal
The same signal can be either normally open or normally closed; you can think of it as the change in state when this signal is 1 compared to when it is 0. Whether to use a normally open or normally closed configuration depends on the situation – it’s quite simple
There is no problem with setting it this way. Connect the normally closed contact of the operation signal (I74.4) in series with the start coil (Q42.2). In this way, once the fan starts operating, the normally closed contact (I74.4) opens; consequently, pressing the start button anymore will not send a start command that causes the coil (Q42.2) to become energized, and thus no start signal will be generated. Similarly: by connecting the normally open contact of the operation signal (I74.4) in series with the stop coil (Q42.3), the normally open contact (I74.4) becomes normally closed only when the fan is running; it is then possible to activate the output of the stop coil by pressing the stop button (Q42.3). In both cases, a certain protective effect is achieved