A similar naming convention is used for the command and state words as well, substituting instead the letter “C” for command, or “S” for state.

Events are named symbolically, the symbol constructed as the concatenation of the subsystem mnemonic “CV1” (for Conveyor #1), followed by an underscore “_”, an “E” for “event”, and finally the event number as specified in the state transition diagram. The output of each rung triggers an event bit. Listing 2 depicts the ladder logic required to implement the event conditions for the conveyor. The use of this bit will readily become apparent. This is desirable so as to prevent the actions of a state from ever being skipped. Secondly, it facilitates the synchronization of state changes, within the state transition ladder logic, so that only a single state change for a given module can occur within a single scan. Firstly, it provides a means for ladder logic to be written in such a way that a state action occurs only once upon entry into a state. This bit is “false” during a scan in which a state change has occurred within that module, otherwise, it will be “true”. Bit 0 within each state word has a special meaning and is referred to as the “one-shot” bit. In most cases, a single 16-bit word is sufficient for each of these, however complex modules may require additional words to contain the states and events. Alarm and scaling calculations are examples of non-sequential or “continuous” logic.Īs mentioned earlier, associated with each module is a set of command, state, and event bits. Additional sections containing ladder logic associated with the module, but non-sequential in nature, can appear above or below the sections listed above. This organization is motivated by the natural sequence of first evaluating conditions leading to state changes, potentially performing the state change, then acting on the outputs affected by the current state.

State Actions - Implements the logic specified by the “States” section of the module documentation. State Transitions - Implements the state transition logic specified by the state transition diagram.ģ. Event Conditions - Implements the logic specified by the “Events” section of the module documentationĢ. Each module may be contained within a single PLC program routine and consists of at least the following three ladder logic sections:ġ. Given a well-defined set of module documentation, it then becomes a relatively straightforward task to produce a ladder logic implementation satisfying that module’s requirements. This article will build upon that existing information to discuss module implementation, and the potential for advanced users to construct software tools which are capable of automatically generating the appropriate ladder logic from a high-level state language description. A previous article discusses state module definition and documentation.

Plc ladder logic program