How to "Control a contact" online

Can someone help me that how can I change a "contact" in ladder logic (siemens step 7) while I am Online. During right click on to a contact, it pops up with the option "modify to 0 or modify to 1". But it does't work. Is there any check box to customise it so that I can control contact online.

Thanks for your reply!

If you have to ask, should you really be forcing a bit then?

No, really, you need to ENABLE the bits (along with toggle/force) update table.

you cannot force internal memory in step 7, just change state, the logic can always change it back.

You can force I/O, to do this you have to create a force table, a bit like a VAT table.

And not all CPUs have the possibilities to force I/Os. As you see, there are a lot of BUTs.

More information from you would enables us to give a more to the point answer.

Regards,

Here's the Siemens help that applies to Modify to 0
(You can click on the contact or coil)

This menu command immediately modifies the selected Boolean variable to 0 once only.
Immediate modification means the value is assigned as fast as possible:
· With S7-300 this implicitly occurs at a cycle control point.
· With S7-400 this occurs at any point in the user program.

So this means I can not debug online in step 7. I am use to with RS Logix 5000 that have features to chage contacts online. Is this feature included in step 7 400 series (To change the condition of binaries??
Thanks
Dhillon

PeterW said:

you cannot force internal memory in step 7, just change state, the logic can always change it back.

You can force I/O, to do this you have to create a force table, a bit like a VAT table.

Click to expand...

no, this means you can do anything you want to - as long as you know how.
you can stop CPU and still be online and set values all you want.
or you can leave cpu running but don't let it scan affected code or I/O you want to test.
or you can write program that is more flexible and mimic AB style force etc.

dhillon said:

So this means I can not debug online in step 7. I am use to with RS Logix 5000 that have features to chage contacts online. Is this feature included in step 7 400 series (To change the condition of binaries??
Thanks
Dhillon

Click to expand...

Why can't you de-bug?

Are you talking Forcing or Changing, you can make changes on-line.

Are you talking S7-200, S7-300 or S7-400? The 200 range is completely different to the other two.

I am using S7-300. I know i can force Outputs but can i change binaries mode ( NO to NC) in the network inputs while online?? I have to go off line, make changes into logic and download the program again in order to debug and to look into status of the network. Is there anyway that i can do it while i am online?

Yes you can make changes on-line, what you don't have is the ability to test edits, so once the change is made, there's no quick return, you have to change back if necessary.

If you wanted the ability to test and return quick, then you would have to make your change in a different manner, like perhaps putting the changes in parallel with the existing and place a temp flag n/c in one branch and n/o in the othere and switch which branch runs by toggling the bit (you can do this where there is no coil).

If you did modify off-line, you would only need to download the changed block or blocks, you can also go on-line if the blocks are different and guess what, you can also compare the off-line block with the on-line block (each block indiviadually if you want) and the comparison is done within the Step 7 environment no external resources needed.

You don't need external programs to go on-line and it doesn't matter if the PLC program was written by an older version of Step 7, you newer version can be used no problem.

There are things in AB that are better than Siemens but there are also a lot of things better in Siemens.

Thanks very much for all your support. I guess I am feeling lot better now.

Thanks
Dhillon

Forgot to mention you can download changes to a running PLC, but you have to be careful if loading multiple blocks with the order.

For example if you have created a change that includes a new datablock or extention to a datablock then ensure the datablock is loaded first, otherwise the program could crash if you loaded the FC/FB first and it tried to access the datablock area that doesn't exist.

If you extend a DB and download that, then the values in all the datawords would be changed to what you are downloading.

You can stop the PLC crashes in case of program changes by ensuring OB120 is in the PLC, and no PLC stop block called from within it. Don't leave OB120 in there afterwards though.

Yes you can make changes on-line

Click to expand...

i'm not sure i can agree with this statement. please correct me if i'm wrong:

In step7 (300/400) you can make code changes ONLY offline (never online). you may download changed block without stopping PLC...

panic mode said:

i'm not sure i can agree with this statement. please correct me if i'm wrong:

In step7 (300/400) you can make code changes ONLY offline (never online). you may download changed block without stopping PLC...

Click to expand...

I suppose you are correct there, you can be on-line but when you make the change you go into edit mode, where you make the change and download and automatically are back in on-line mode to view the change(I think that's right).

But you can make changes without stopping the PLC, which is how I would interperet the original question, I maybe wrong.

I've always seen the above as on-line changes although technically its not the same as the changes being made whilst monitoring.

The Allen Bradley method is doing the same although in a different manner, the changes are not instantly made, you have two rungs, one with the change and one without and until you switch the changes are not running.