what does the green mean? - part two
in the previous post, we discussed the basic question “what does the green mean?” in relationship to the highlights which appear around many of the ladder logic instructions ... in this current post, we’ll go into the basics of “what does the green mean?” in relationship to the vertical “power rails” which appear at the left and right sides of the ladder logic program display ... keep in mind that in the previous post we were working with an “offline” file without using a PLC processor ... in this next post, however, we’ll be working “online” and actually communicating with a PLC-5 processor which will be kept in the “Remote Run” mode ...
first let’s look at rung 0000 at the top of the program ... notice that the “power rails” are highlighted in green ... notice that the XIC is highlighted in green ... notice that the OTE is highlighted in green ... notice that there are NO “letters” located in the left margin of this particular rung’s screen display ... so from all of these green highlights, what can we definitely say, with 100% assurance, about rung 0000? ... specifically, is this particular rung being scanned/executed by the PLC processor? ... yes? ... or no? ... actually the answer is: “we can’t tell for sure” ...
now based on what we see before us on the screen, it certainly does LOOK like the rung is being scanned/executed by the PLC processor ... and, based on what we see here, there is nothing that would indicate that this rung is NOT being scanned/executed ... but personally, I would NOT bet the rent on this one ... in short, there are several dirty tricks that I could be playing on you here to keep this program from being scanned/executed by the PLC processor ... but I’m not ...
for the purposes of this discussion, let’s just assume (gosh I hate that word) that the sun is shining, the birds are singing, life is lovely, and the program shown in the figure above IS indeed being scanned/executed exactly the way it appears to be ... now let’s move on ...
next notice that rungs 0001 through 0004 all have letters located in the left margin of the display ... the presence of those letters indicates that those rungs have SOME type of “edit” operation associated with them ...
notice the capital letters “I” at the left of rung 0001 ... first of all, the letter “I” stands for “Insert” ... in other words, we’re interested in “inserting” this rung into the program ... and when you get right down to it, this particular rung is actually already present inside the PLC processor right now ... we can tell that from the fact that the letters “I” are CAPITAL letters ... we could talk much more about that - but not today ... for today, we’re only interested in “what does the green mean?” ... and in this case, the green “power rails” at the left and right sides of rung 0001 indicate that yes, this particular rung IS being scanned/executed by the PLC processor ...
caution! ... remember that we’re assuming in this discussion that this ladder program is indeed being scanned in its normal manner ... no “behind-the-scenes” dirty tricks are being played ... but out in the real world, we could NOT be so sure ... in other words, “green on the rails” does NOT (I repeat does NOT!) prove, beyond a shadow of a doubt, that a rung is actually being scanned/executed by the PLC processor ... but let’s move on ...
notice the capital letters “R” at the left of rung 0002 ... the letter “R” stands for “Replace” ... in other words, we’re interested in “replacing” this rung with the “insert” rung located immediately above it ... but at this point in time, this particular rung 0002 is actually still present inside the PLC processor too ... but let’s stay on course ... “what does the green mean?” ... and in this case, we notice that the “power rails” at the left and right sides of rung 0002 are NOT highlighted in green ... this definitely indicates that no, this particular rung is NOT being scanned/executed by the PLC processor ...
note that in this discussion, we are currently “testing” the edits that we have made ... more specifically, the PLC processor is “testing” the edits for us ... if we were NOT currently “testing” the edits, then the “I” rung 0001 would NOT be highlighted ... and the “R” rung 0002 WOULD be highlighted ... specifically, the PLC processor would still be scanning/executing the old “R” rung - and NOT scanning/executing the new “I” rung ...
now notice the capital letters “D” at the left of rung 0003 ... the letter “D” stands for “Delete” ... in other words, we’re interested in “deleting” this rung completely from our ladder logic program ... but at this point in time, this particular rung 0003 is actually still physically present inside the PLC processor ... but “what does the green mean?” ... again in this case, we notice that the “power rails” at the left and right sides of rung 0003 are NOT highlighted in green ... this definitely indicates that no, this particular rung is NOT being scanned/executed by the PLC processor ...
once again, note that in this discussion, we have already made some edits/changes to the program and the PLC processor is currently “testing” those edits/changes for us ... if we were NOT currently “testing” the edits, then the “D” rung 0003 WOULD be highlighted ... specifically, the PLC processor would still be actively scanning/executing this old “marked-for-deletion” rung ... but right now, rung 0003 is just “hanging-loose” and having absolutely NO effect on the operation of the system ...
we’re almost done ...
notice the lower-case (little) letters “e” at the left of rung 0004 ... the letter “e” stands for “edit” ... in other words, we’re interested in “editing” this rung and eventually adding it into our ladder logic program ... but at this point in time, this particular rung 0004 is NOT actually physically present inside the PLC processor ... in order to send this particular “little-letter” rung over the communication cable and into the processor, we’d have to “Accept” the rung ... and in this example, we still haven’t done that all-important step yet ...
but “what does the green mean?” ... and again in this case, we notice that the “power rails” at the left and right sides of rung 0004 are NOT highlighted in green ... and this definitely indicates that no, this particular rung is NOT being scanned/executed by the PLC processor ...
so keep in mind that we HAVE made some edits/changes to the program and the PLC processor is currently “testing” those edits/changes for us ... BUT! ... the edits/changes to rung 0004 are NOT (I repeat NOT!) being scanned/executed by the PLC processor ... think about it ... how could this rung possibly be scanned? ... it CAN’T be scanned because the little “e” letters tell us that this particular rung has still NOT been sent over the communication cable and into the processor ... as they say in baseball, “you can’t hit ‘em if you can’t see ‘em” ... and the PLC processor can NOT see this “unaccepted” rung ...
I hope this helps ...