SLC500/05 <L> and <U>

Hi all, great site here and looks like quite a brain trust for PLC's.

I have a question and have looked through ALL FAQ's and did not see any reference to this particular item although I did learn a lot of other things. I am actually a DCS programmer but my fellow PLC programmer is in IRAQ (Nat. Guard) and I am trying to modify his code.


r--------------------< L1 >----r
a | a
i | i
l -< U2 >-----l

First let me explain L1 and U2 designations above;
L1 is actually B16:4/3 <L> (designated as stop pushbutton)
U2 is actually B16:4/4 <U> (designated as start pushbutton)
I just could not get these numbers easily printed in Email.

Like I said I am a novice at PLC code. I guessed
that the above code would keep L1 latched at all times except when the "stop" pushbutton was pressed on the attached Panelview and U2 unlatched at all times except when the "start" pushbutton was pressed.
There are no < U1 > or < L2 > used in the ladder subroutines.

The "stop" functions ( <L1> XIC and XIO ) and
the "start" functions ( <U2> XIC and XIO )
work fine throughout the program as written. I
attempted to add a latching contact to latch a "stop" signal.

r r
a----------|/|------------< SL >---a
i i
l l

-|/|- is actually an XIO of <L1>
<SL> is actually B10:3/2 <L>

Although all XIC and XIO of <L1> throughout the
program work, my added attempt at a latch <SL>
never latches. I added the attempted latch one rung below the rung which goes to the pushbuttons in parallel (<L1> and <U2>)


It's almost like the pushbutton is instantaneous
instead of writing to the table and holding for a full cycle.

Anyone see what I am doing wrong?

I hope this all lines up and makes sense as I am writing it offline and uploading it.

PS: SLC 500/05
I forgot to say that I made other non-related changes and everything verified and worked except the attempted latch.

Thanks in advance for any and all help!

Trying to correct original message

Although I edited the message above twice it never showed as being
changed.

Sorry

The two attempts at showing a line of code are:

first one;
<L1> and <U2> are in parallel and always
connected directly to the rails.

second line;
uses an XIO of <L1> to latch <SL>

1. I'm guessing that the first latch/Unlatch rung is somewhere near the end of the program (that is, near the end of file 2 or at least after the last reference to the XIO or XIC condition of these bits). This would allow the effect of the 'Stop' and 'Start' signals from the Panelview to be used through only one pass. Can you comment on my assumption?

2. How are the buttons in the Panelview for the actuation of these signals defined (Momentary, Maintained - which is 'toggle', Turn On or Turn Off)?

3. If the first Latch/Unlatch is near the end then your attempt to Latch the NOT of the 'Stop' bit should be near the start of the program.

4. Finally, use the 'LADDER' button to start and the 'Close Current Tag' to end a ladder drawing. This will retain your spacing formatting.

hi scottsull

If you take a closer look on the buttons above editor window
you will notice one labeled "LADDER". It will help a lot if
you press it once you start editing ladder diagram and once
after you are done with ladder.

About latch and unlatch coils...
There are number of instructions that can change state of
one bit or whole word. Latch and unlatch are just two of them.
Using FLL, MOV, COP, ADD, SUB, MUL and just about any other
instruction, you will change value in some memory location.

Tip:
Go to B16 data table so you can see B16:4/3 and B16:4/4.
Click on button "Usage" (it's in the bottom of this window).
Bits that are not used individually will be presented by dot "."
while bits that ware addressed at the bit level will be marked
with an "X". Now take a look to the left of the B16:4/15.
If you see "W" it means that whole word B16:4 was used.
If you see "F" it means that word B16:4 was used in block
instruction such as FLL or COP (instructions capable of writing
to more than one word at a time).
But there is more to it... Usage and crossreference will show
only that certain address was used by ladder program. You will
not see a trace of information if the memory is used (and modified)
by some other device (touchscreen, other PLC or whatever...).

regards

1. I am at home as I write and do not have a copy of the code here, but the latch/unlatch across the rails is in sub 19 and XIC XIO are used in subs both before and after sub 19.

2. Panelview has only 1 start and 1 stop pushbutton. Each I believe must be momentary.

3.I thought the table values were written at the end of the scan and stayed the same for a full scan - until written again at the end of the next scan.

Panic Mode

Yes, Bernie_Carlton mentioned the use of ladder - Sorry, first post.

B16:4 is not a word or block write.

Doesn't the fact that all XIO/XIC's associated with B16:4/3 changed state (all pumps stopped, all valves closed etc. indicate that B16:4/3
changed for one full scan? If so why would not the latch I added using only an XIO of B16:4/3 also work?

I know I am missing something, I just cannot put my finger on what.

You may be confusing 'table values' with the interface to 'real world' I/O. The outputs are written from the internal output table (O:? ...) to the real cards at the end of the scan. The input table (I:? ...) is updated from real cards just before the beginning of the scan. Now this part I'm hazy on but I believe that bits affected by the Panelview, or any other communication, only change outside the scan in the SLC world. Of course if the bits are then overwritten within the ladder then that change is immediate and useable for all rungs following. If you view any of these bits with a programming program (RSLogix) you are viewing the status at the end of a scan.

Bernie_Carlton

You probably hit it on the head.

I told you I knew VERY little of PLC code. I assumed all I/O was written at the end/beginning of a scan but I was also using this same logic with the ladder code.

Are bits changed at the end of each rung scan? Seems like this would be the case and makes sense. I just was not thinking.

Thanks

Thank you to Bernie_Carlton and Panic Mode!

I will try to figure out how Panelview logic in the ladder logic scheme of things works.

But right now it's the Packers 27 - Seahawks 27 with less than 20 seconds to go.

Thanks again.

Into overtime - it's tough trying to answer questions in the middle of a game.