one shot animation
In the example above, each time the operator pushes button 0000 the difu instruction turns on(becomes true) FOR ONE SCAN. Notice that it turns on(becomes true) at the rising edge of input 0000. In other words, when input 0000 intially turns on(becomes true) the difu instruction is true. When an input initially becomes true its called the leading or rising edge. When it initially turns off(becomes false) its called the trailing or falling edge.

If after viewing the animation you still find it difficult to understand, don't feel bad. You're just an idiot and should give up!!! No not really. Just try to remember how its scanned by the plc. Think it through one instruction at a time from top to bottom, then left to right. (i.e. in this order: NO 0000, difu 1000, NO 1000, NO 1001, NC 1001, NC 1000, OUT 1001, NO 1001, OUT 500) While executing each instruction is it true or false?

Rung 2(it has 5 instructions) is the confusing part for most of us. To make it easier lets break it into sections. NO 1000 and NO 1001 are in parallel. In other words they form an "or" circuit. Call this section 1. If 1000 OR 1001 is true then this mini section is true. Next we have another section in parallel. This section is NC 1001 and NC 1000. Again these 2 are in parallel. Call this section 2. If NC 1001 OR NC 1000 is true then this section is true. Finally, those 2 parallel sections(sections 1 and 2) are in series with each other. This means that if section 1 AND section 2 are BOTH true then the rung is true and output 1001 must be true.

Now its much easier to understand, isn't it? We also just learned about OR and AND logic, commonly called parallel and series logic.

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