In between your logic and the count-up input to the counter, add a P-TRIG or similar leading-edge contact so the counter is only enabled for one PLC scan for each count requirement.

The 258 means the count up input was on for 258 PLC scans.

AutoGen_1988357

Hi

I am using a S7 1214 and a Basic KTP 400 to set a PV value into a cu counter using %MW (int). On monitoring the counter in operation it reads the entered pre-set value ok until the 1st count input then the PV value jumps to 258?

Any ideas on what I am doing wrong?

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Split from Counter value to db.

While online with the PLC showing the network with the counter, take a screenshot and post in a reply here so we can see what is really going on. 

AutoGen_1988357

Hi thanks for the reply, but I have tried a leading edge and falling edge contact and had the same result. The strange thing is the figure is always 258.

As already pointed out, you do have overlapping tags.

In case of Simatic MW6 is not some separate entity no 6. It's a memory word starting at byte 6. Since a word is 2 bytes long, it also uses byte 7. There are few conclusions out of this, if you create a tag at MW6:

• bits M6.0 - M7.7 are occupied
• bytes MB6 and MB7 are occupied
• MW5 and MW7 are partially occupied (other story is that you should avoid using odd addresses)
• MD3 - MD7 are partially occupied (and same as above)

So creating next tag with address in the above scope will cause overlapping, i.e. unwanted (usually) changes of value of a tag after changing value of another tag.

This is why I use data blocks for global variables....

addressing is done by system

This is another reason why Siemens recommends using DBs for tags.

Also, here's a "never" for TIA Portal and S7-1200/1500 PLCs:

NEVER create a tag using the address first.

Instead, create tags by giving them a symbolic name that gives meaning to the tag and what it represents to us humans.

After that, you can right-click on the name and select "Define Tag".  Portal will look at the required datatype and automatically suggest the next free and available M address that will not overlap another existing tag.

It will help you prevent these types of problems should you choose not to use DB tags and instead allow your foot to get shot every so often - like it did this time.

AutoGen_1988357

Thanks for the info on occupied tags and the double word, changing the tags to 2 apart cured the problem.

Thanks everyone for your help.