Writing a string value without clearing the previous data??

RSlogix5000 - Here is what I would like to accomplish and what I am running into. Currently this is a scanner ASCII gateway to PLC set up.

A) String A IS NOT equal to String B outputTest is energized.

B) String A IS equal to String B outputTest is unlatched

2nd Scan is needed. This will take the ASCII string to fill String A, but it won't unless I clear the 1st Scan with a CLR instruction. Here's the catch, scenario A goes true and the outputTest us latched on. I want to avoid this clearing issue to accept the latest string and avoid the outputTest from getting energized.

Is there a way to write a string on top of a string without clearing it first. I am sure it is something simple I am missing. Help this newbie out.

Thanks in Advance!!

How about a power up delay timer? Say 5 seconds.

Ah yeah... that may just work. Thanks a bunch!

HJTRBO said:

How about a power up delay timer? Say 5 seconds.

Click to expand...

Lets say I put in a 5 sec delay. I guess I am still stuck on how and when I should clear out this string so that the next scan can be written to the plc. If I clear it out and it's longer than 5 secs I am in the same boat.

Don't do the compare if the string is null?

What instruction would do this? Or do I just go right into structured text?

randomFire said:

Don't do the compare if the string is null?

Click to expand...

Yup. I figured it out. Something small like this helps.

Thanks!!

randomFire said:

Don't do the compare if the string is null?

Click to expand...

Good! What did you do in the end? Post it, you never know when it'll be useful for others

I just used a compare instruction and because the string I was dealing with is always a 10 digit value I made the output true if that was the case.

The clear instruction set my string back to a zero value. As long as my string has a index value of 10 (True) it will continue to do a NEQ String A to String B and latch on the output.

Then I used an existing timer to execute the clear instruction on String A.

I don't know if that was the best approach but I am learning.