Returning Student - Help Please

Hello people, I'm a returning student (over 10 years out of school) and I need help with a couple questions please. I was able to figure out 2 out of 4 of the questions, the last 2 I couldn't figure out. Can you guys please provide answer for 2c and 2d along with explanation?

2. Redraw each of the following programs corrected for the problem indicated: a. Problem: nested programmed contact (Fig. 5-44).

b. Problem: vertical programmed contact (Fig. 5-45).

c. Problem: some logic ignored (Fig. 5-46).

d. Problem: too many series contacts (only four allowed) (Fig. 5-47).

Umm, should there be some pictures attached?

are there any jumps, subroutines, or MSRs in 5-46?

keshik said:

Umm, should there be some pictures attached?

Click to expand...

Was wondering the same. Maybe I just don't know enough about the subject matter but it seems to be short on details haha

Here are the 2 problems I need help with:

I mean... I guess I still don't really know what's going on. For D, could you just draw a branch?

----|A|----|B|---|C|---(X)-----
|.................................|
--|X|----|D|---|E|---(Y)--

(Ignore the periods, they were just to keep the branch rungs from crowding together on the left side)

Here are the 2 answers to the first 2 questions if anyone would like to see them:

there got to be some context missing. The questions makes no sense to me.

At first I thought this was a pretty bad exercise. But then I re-read it. The basic premise here is you can do things with hardwired relays that do NOT translate easily to PLC code. If you have a job where you have to replace hardwired relays with PLC code, you should be careful.

For Fig 5-46.
I would "fix" this one by rearranging it like this:

A B
A C D
E D
E C B

Four branches with these patterns leading to Y should perform the same function as the hardwired circuit. The main difference is that PLC code has limitations on allowed arrangements. Hardwired circuits allow current to flow through any path available to the coil.

For Fig. 5-47

LoganB already answered this one, but in case it is helpful I'll add my own explanation.

Some PLCs have limits on how many elements can be placed in a single horizontal row. This is usually only old hardware or very cheap hardware. In this situation, the limit is 4 contacts, then a coil. (That is how I read the question, LoganB read it slightly different)

A B C D Y1
Y1 E Y2

To solve this you need to add another coil or output. This lets you solve the first 4 conditions and then add on the fifth.

Finally, a comment on the answer to Fig. 5-45.
The last row of contacts A B D is not necessary and the program will produce the same results without it. This is because if A is true, the coil will turn on and the state of B and D don't matter.

Good luck

P.S. The key to understanding the problems is the answers that were given. It is possible that it was clearly explained that these were hardwired relay problems, but it wasn't on the text we saw. Also, the problems didn't make sense as pure PLC problems.

Okay I found out the answer guys, thank you guys.

I agree with #3, but the logic for #2 is not correct in my opinion.
you changed the logic from the original rung.

2d.

---|a|---|b|---|c|---|d|-----
.................................
---|y|---|e|-----------------

Steve