This programming technique is known as feedback, where an output variable of a function (in this case, the feedback variable is OUT contactor) is also an input to that same function. What would be truly strange is to label a coil with an input bit address or tag name, since the PLC is not electrically capable of setting the real-world energization status of any input channels. In reality, contacts and coils are read and write instructions, and thus it is possible to have the PLC read one of its own output bits as a part of some logic function. Note 3 : A very common misconception among students first learning PLC Ladder Diagram programming is to always associate contacts with PLC inputs and coils with PLC outputs, thus it seems weird to have a contact bear the same label as an output. The purpose of this contact, of course, is to latch the motor in the “on” state after a human operator has released his or her finger from the “Start” pushbutton. The coil labeled OUT contactor writes the status of that bit, while the contact labeled OUT contactor reads the status of that same bit. At first it may seem strange to have both a contact and a coil in a PLC program labeled identically (Note 3 ), since contacts are most commonly associated with inputs and coils with outputs, but this makes perfect sense if you realize the true meaning of contacts and coils in a PLC program: as read and write operations on bits in the PLC’s memory.
Note the seal-in contact bearing the exact same label as the coil: OUT contactor. To illustrate, we will imagine the construction and programming of a redundant flame-sensing system to monitor the status of a burner flame using three sensors. Similarly, discrete output channels – referenced by coil symbols in the Ladder Diagram – must also bear some form of address or tag name label.Īlso Read : Mis-conceptions of PLC Ladder Logic
In modern PLC systems, each discrete input channel has a tag name created by the programmer which is applied to the contact(s) within the program. In legacy PLC systems, each discrete input channel has a specific address which must be applied to the contact(s) within that program.
The most elementary objects in Ladder Diagram programming are contacts and coils, intended to mimic the contacts and coils of electromechanical relays.Ĭontacts and coils are discrete programming elements, dealing with Boolean (1 and 0 on and off true and false) variable states.Įach contact in a Ladder Diagram PLC program represents the reading of a single bit in memory, while each coil represents the writing of a single bit in memory.ĭiscrete input signals to the PLC from real-world switches are read by a Ladder Diagram program by contacts referenced to those input channels.
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