Program cycle operating flow

[ghid]

Hi, From what I know about PLCs program cycle, the inputs are readed in the order that are positioned in the program, and at the end of the program flow, the outputs are actuated. And in my opinion, if a memory it is positioned as a condition befoire it's actuation, the output that is deppending on this memory condition never should be actuated. In the following example, the H10.0 memory will go high when I will set the bit 0.02 to high, and will stay high just until at the end of the cycle (because of the DIFU instruction ), in those conditions, the H5.04 souldn't go on high, but will go, but why? because when the cycle is starting the H10.0 is scanned as low and H5.04 showld not go on high, then I set the 0.02 to high, H10.0 goes to high and at the start of the next cycle should be low, and H5.04 should be low. Please help me to understand. Thank you.

[PMCR]

Here is what is happening: The DIFU instruction turns a bit on for an entire scan, not just to the end of scan. So, the bit will be on from the DIFU instruction, through the end of scan, back to the beginning, and back to the DIFU instruction. When it reaches the DIFU 1 full scan after it was turned on, the bit is turned off.

[ghid]

Thank you for your response. So, you are saying that the DIFU instruction extends during two PLC cycles?

[PMCR]

Yes. If your PLC program looks like this, bit x will turn on at the DIFU instruction, remain on for Ladder 5,6, END, Ladder 1,2,3,4, and then turn back off when it reaches the DIFU again. <LADDER> 1 <LADDER> 2 <LADDER> 3 <LADDER> 4 <DIFU> bit x <LADDER> 5 <LADDER> 6 <END>

[Michael Walsh]

In other words, EVERY use of bit X will be in the on state for exactly one scan.