two ladder logic questions

Hi!
I'm learning ladder logic programming and I have two questions about the oil tank example (http://www.plcs.net/chapters/scan13.htm)

first question:
scan 1 -> second rung, the internal utility relay goes true because the program flow goes from top to bottom, isn't it?

and the second question:
If I use OTL instead of OTE in rung 1, the internal utility relay goes true at the first scan too?

thanks in advance

Greetings hannibalevivo ...



first let’s make sure that we’re talking about the same thing ... I think that you might be using the word “rung” incorrectly ...







we often hear that type of construction called a “seal-in” branch ...



now then ... if we’re on the same line of reasoning, then the book is correct ... the XIC in the “seal-in” branch does NOT become “true” until scan #2 ...



your confusion is VERY common (even for some technicians who have successfully worked with PLC for years) ... you’re looking at the ladder logic program like an electrical circuit ... it is NOT an electrical circuit ... instead the ladder logic program is a set of “instructions” (commands) to a computer (the PLC processor) ...



the best way to actually “SEE” the order of what is happening is to display the rung in the ASCII (“text”) mode ... in RSLogix, this is usually quite easy ... we simply edit the rung - and then double-click the rung number ... the text listed in the little popup window shows the EXACT ORDER in which the PLC processor executes the instructions on that rung ...








note that BST stands for “Branch Start” ... NXB stands for “Next Branch” ... BND stands for “Branch End” ... you should already be familiar with the rest of the instructions ...



point of the story: notice that by the time the processor gets to the OTE at the end of the rung, then it HAS ALREADY PASSED the XIC located in the “seal-in” branch ... secret handshake: while on that same scan, the PLC processor can NOT “go back upstream” and recognize the XIC as “true” ... and so the XIC will be evaluated as “true” on the SECOND pass through the logic, but NOT on the first pass ...



hope this helps ... you are NOT alone in your confusion ...


finally, if you understood my answer above, then you should have little trouble figuring out your second question for yourself ...