1 Basic Principles of PLC Ladder Diagram will be open loop (Off), so Y0 will be Off. When X0 is pressed, the contact will be On, so Y0 will be On. Row 2: Using a normally closed (NC) switch X1 (“B” switch or “B” contact). When X1 is not pressed, the contact will be On, so Y1 will be On. When X1 is pressed, the contact will be open loop (Off), so Y1 will be Off. Row 3: The combination logic of more than one input devices. Output Y2 will be On when X2 is not pressed or X3 and X4 are pressed. 2. Sequential Logic Sequential logic is a circuit with "draw back” structure, i.e. the output result of the circuit will be drawn back as an input criterion.
Therefore, under the same input criteria, different previous status or action sequence will follow by different output results. Examples of traditional ladder diagram and PLC ladder diagram for sequential logic: Traditional Ladder Diagram PLC Ladder Diagram Y3 X5 X6 Y3 X5 X6 Y3 Y3 When the circuit is first connected to the power, though X6 is On, X5 is Off, so Y3 will be Off. After X5 is pressed, Y3 will be On. Once Y3 is On, even X5 is released (Off), Y3 can still keep its action because of the draw back (i.e. the self-retained circuit). The actions are illustrated in the table below.
From the table above, we can see that in different sequence, the same input status can result in different output results. For example, switch X5 and X6 of action sequence 1 and 3 do not act, but Y3 is Off in sequence 1 and On in sequence 3. Y3 output status will then be drawn back as input (the so-called “draw back”), making the circuit being able to perform sequential control, which is the main feature of the ladder diagram circuit. Here we only explain contact A, contact B and the output coil. Other devices are applicable to the same method. See Chapter 3 “Basic instructions” for more details.
1.2 Differences Between Traditional Ladder Diagram and PLC Ladder Diagram Though the principles of traditional ladder diagram and PLC ladder diagram are the same, in fact, PLC adopts microcomputer to simulate the motions of the traditional ladder diagram, i.e. scan-check status of all the input devices and output coil and calculate to generate the same output results as those from the traditional ladder diagram based on the logics of the ladder diagram.
Due to that there is only one microcomputer, we can only check the program of the ladder diagram one by one and calculate the output results according to the program and the I/O status before the DVP-PLC Application Manual 1-2 Device status Action sequence X5 X6 Y3 1 No action No action Off 2 Action No action On 3 No action No action On 4 No action Action Off 5 No action No action Off