SLC 5/04 - add floating point as online edit

Greetings,

PLC: SLC 5/04
DCS: DeltaV ver 9.3

Originally at our plant, the diffential pressure across our baghouse came from a single Rosemount differential pressure transmitter as I:25.0.

That differential pressure came into the PLC and was sent to the DCS throught an SCP block as O:28.0.

It was decided to change the differential pressure transmitter to 2 pressure transmitters and subtract the difference in the PLC, then send it to the DCS.

Someone did the change for us and did not change I:25.0 as the input for the SCP block. So I was trying to fix things.

Here are some screenshots of what I am trying to do and what I have explained above.

http://webpages.charter.net/drw46/examples/5_Hamon_PLC-b.jpg

Question 1. Can I accomplish what I want to do as an online edit?

Question 2. Is what I am doing essentially correct?

I am in the position of being a PLC novice but knowing more than anyone else in the plant so by default I am the expert.

Thanks for any help,
Doug

The primary question is - does F26:0 already exist in the running process data table? If not then I believe adding or expanding a data table cannot be done online.

Doug,

Bernie is right. During an online edit, you cannot do anyting that causes any data table to have to become larger or smaller, and you cannot add or subtract Inputs or Outputs.

However, you can manipulate the data with the memory locations already in use. You could do away with the all the SCP instructions except one. First Subtract I:25.0 from I:25.1, then do the scaling with SCP, then send it out to the original Output for the DCS system. You could use the old F25:2 or F25:3 for your SCP input, but probably not the new F26:0. There could also be other "spare" addresses already in the program that you could reuse.

Online Editing
RSLogix 500 - Copyright Rockwell Software 2000

The online editing function lets you monitor and correct your ladder program when your programming terminal is connected to a SLC 5/03, 5/04 or SLC 5/05 processor. Only one programming device at a time can perform online edits of the program.

Online editing functions consist of inserting, replacing, and deleting rungs in an existing ladder program while online with the processor. You can also document your program while performing online edits.
The upper left portion of the RSLogix 500 window can let you know if edits exist in the online ladder program. Observe the No Edits or Edits Present list box.

The MicroLogix controllers do not support online editing.

Warning! Before beginning an online editing session, make sure you fully understand the possible results of the edit to the system under control. Physical injury or equipment damage may result!

Online Editing Restrictions
During an online editing session you cannot:
resize data table files
create or delete program files
change program file protection
change index across file boundaries flag
reconfigure the I/O
select Force protection

RSLogix 500 - Copyright Rockwell Software 2000

Click to expand...

bernie,

The primary question is - does F26:0 already exist in the running process data table? If not then I believe adding or expanding a data table cannot be done online.

Click to expand...

No it does not exist. Those were changes I made offline in the office, just to make the screenshots. I then closed without saving.

Lancie,

However, you can manipulate the data with the memory locations already in use. You could do away with the all the SCP instructions except one. First Subtract I:25.0 from I:25.1, then do the scaling with SCP, then send it out to the original Output for the DCS system. You could use the old F25:2 or F25:3 for your SCP input, but probably not the new F26:0. There could also be other "spare" addresses already in the program that you could reuse.

Click to expand...

I will look around for spare addresses. I hope I know one when I see one.

I am at home now, away from the PLC but in light of your idea of looking for unused data files:
F25-AI_SCLD does exist. I will look through that to see if there are any unused addresses on Monday. If not, I guess we wait for our late October shutdown to do this.

Thanks to both of you for replying,
Doug

The way I explain this limitation is "if you can't see it online, then you can't use it". If you do not see F26:0 (or F25) when you go online, then you can't use it.

When you open your data files, click on the Usage button. Anything that is being used will have an "X" in it. Look for something without the "X". There are gotchas to this, but it is a starting point.

OG

When you get the chance to make your changes, do yourself a favor and expand/add any and all data tables that you think you might need in the future.

I like to make sure I have plenty of extra files of various types so that I can add new things online and still keep the program organized. I also like to add spare ladder files.

When you do this offline, you can check your memory usage by verifying the program after any changes and then looking at controller properties.

To verify the program, right click on the Program Files folder icon in the tree in the left pane. Or, go to the Edit menu and click Verify Project. That will update the memory usage.

Always leave some free memory space to accommodate adding program logic. instructions too.

Operaghost and OkiePC,

Thanks for joining in here. I am one of the electrical guys in a power plant maintenance team and although I have been around PLCs since the early 80s, taken some PLC programming classes and done some minor ladder logic edits to existing programs I did not think that I would ever be the "expert" by default. For years I managed the CAD department in small electrical design groups of small engineering firms.

I appreciate any help I can get here and am going to print a lot of this stuff out.

There are certain areas of the plant where I have refused to attempt any changes. I have told management that they need to employ outside firms for those changes mostly involving anything that I believe is a safety issue or anything involved with the burner management system.

Another question involving F25 which does exist:
From the attachment you can see that F25:7 does exist but without symbol or description assigned. Would the addition of symbol and description represent a database change?

Thanks,
Doug

No, adding comments and symbols doesn't do anything in the PLC. Those items are stored on your PC with the .RSS file. You can edit those to your hearts content at anytime without affecting the operation of the PLC. Just be sure to save the file when you are done editing them.

What you want to do to find out if F25:7 is in use:

Click on the Usage button at the bottom of that window. Then, you can see what addresses are actually being used in the PLC program. The Usage view of the data file will show an X where an address is used, and a period where they are not directly addressed by the PLC program.

Side note: When viewing the data with a binary radix, it may also show an X for individual bits, a W for individual words, and a F for groups of words.

There are certain things that won't show up in the Usage view, like indirect addressing. So, it's a good idea to search for the item in the logic also.

To search for indirect addressing, click on the ladder window (anywhere) and hit CTRL + F. Then type "F25:" in the search box (without the quotes). Hit "Find All" and you should get a list of every F25 address used in the program including indirect addressing. Indirect addressing will look like: F25:[N7:0]. The contents of N7:0 determine which address within file F25 is being used, so RSLogix can't know what all the possible values of N7:0 will be at runtime, therefore the Usage view can lead you to believe an address is free when it isn't.

Earlier versions of RSLogix had other problems with the reliability of the Usage view, so I have stayed in the habit of using the Find function which hasn't let me down.

Once you have done that search, you can be sure the address is free and available for use.

Hope this helps,

Paul

OakiePC,

I did hit that usage button when I was at work and Xs were displayed at every address for F25 so I guess that means we wait for the October shutdown. I will be sure to take a closer look on Monday morning to see if I missed anything.

At that point I will follow your suggestion about making data ready for online editing in the future.
I appreciate the tips.

Thanks,
Doug

Look for other data files that might have free addresses. The letter designates the type of address, so you want a file that starts with F. F8 is a file that is there by default and might have some free spots available...

Post your .RSS file ( zip it first) we will help you look.

Adding on to OkiePC's recommendation just a little bit here.

In addition to his recommendation to expand any data files you might need in the future, I always find it a good idea to create extra data files with 50-100 elements in them as sort of a "just in case". I usually create one extra N (integer), one extra F (Float), and one extra B (binary).

Programmers often organize their data into specific data files. So rather than have 100 elements in a single data file they might instead use 5 files each with 20 elements. If it makes sense to keep the data separated then separate data files can be used.

The problem can be if you (or someone else) starts expanding those files and store random unrelated data then that organization breaks down. That is why I prefer separate files.

Now, I usually only need one each of the Timer, Counter and Control files. So, I am more inclined to expand those files rather than create new ones.

Just food for thought!

OG

OkiePC

F8 is a file that is there by default and might have some free spots available...

Click to expand...

There are, in fact, spares in there.

Operaghost

When you open your data files, click on the Usage button. Anything that is being used will have an "X" in it. Look for something without the "X". There are gotchas to this, but it is a starting point.

Click to expand...

I take it that one of the gotchas is with analog inputs which appear with a W followed by . . . .?

Thanks,
Doug

The PLC programming for this is pretty straight forward, and the help you have received so far is good.

However, just a comment on the basic idea of using two pressure transmitters to replace a dP transmitter. It's often tried and rarely sucessful. Just as an example, say you are operating at 50 psi and you are using 0-100psi transmitters. And the dP range you are interested in is 0-10psi. If the transmitters are good for 1% accuracy, then your 0-10psi dP range is only good for 10% at best. I think you are going to measure error as much as dP.

I know a baghouse doesn't run at that high a pressure, the numbers were simply easy to do. The same principle applies. Use a dP transmitter for dP.

mellis,

I can't explain to you why we went to 2 pressure transmitters. The decision was made higher up the food chain. I am not even sure what issues brought about the idea so I am just carrying it out but thanks for the perspective. If there are issues up the road I will bring up your point.

You are correct in that we are dealing with a differential generally of 0 to 10pdi.

Thanks,
Doug