add instruction question.

[josesaucedo]

Do I have to use a reset instruction for an ADD instruction?

 

[TConnolly]

Greetings Jose.

I'm going to assume that since most of your questions have been about AB's SLC platform that this one is also about the SLC platform.

You would not use the RES instruction after an ADD instruction. The RES instruction is for use on timers and counters. There may be times however when you want to adjust the PREset of a counter or timer using an ADD instruction and then RESet the counter or timer. In this case that is how you would reset the counter or timer.


That said, there may very well be times when you might need to reset other things in the PLC after and ADD instruction. You could use the CLR instruction to reset any one of the words addressed in the ADD instruction to zero (this will be applicaiton dependent and most of the time unnecessary).

After an ADD it may be necessary to reset certain minor error bits. These bits are the overflow trap S:5/0, the overflow bit S:0/1. These bits indicate that the result of your ADD is greater than or less than what can be accurately represented in a 16 bit word. If this is a possibility you should check the bits right after you ADD and respond to the overflow with appropriate logic. And you need to clear the bits with an OTU insruction before an END statement is scanned or the processor will fault.

Thats a shotgun answer. A little more information on what you are trying to do will be helpful for getting a more precise answer.

 

[josesaucedo]

Thanks Alaric.

 

[Ken Moore]

Another shoot gun answer

If you are doing some addition and then need to clear the total, you can use the MOV instruction to move a zero to the register used in the ADD instruction.