mov and compare

hi I'm trying my first program, I kind of know what I need to do but little help would be great.

I have 16 bits of or a word I would like to monitor, is the best way to put the 16 bits into a data memory. use the move function to move this data memory and then use a compare function with a equals or less than 0 so if any of these bit change at all I will be aware

cheers

What is it you are trying to do?

If you are trying to see if any of the bits change keep in mind that you would have to store what the bits were, and what the bits are now. Then compare to see if they are the same or different.

OG

I want to monitor all 16 bits and compare them so when they change state I can bring on an alarm

Welcome to the forum!

The easiest way to see if any bit changes is to look at the 16 bits as an integer.

Each bit that turns on or off will change the value of your integer. So, compare the value of your integer this scan, to it's value last scan. If it's different, a bit has changed, so set your "alarm detected" bit and then copy the new value into your "last scan" value for the next check.

Of course, there's a gotcha here - if all of your individual bits are "1=alarm", then it's not enough to just detect that one has changed. To detect that you have a new alarm, you need to detect that a bit has changed from zero to one, but not from one to zero.

One way around this is to detect if your integer is larger than on the last scan - so you would use the same logic, only use a "greater than" comparator, instead of a "not equal to" comparator.

Of course, there's a gotcha in that method too, in that if you have one alarm turn on and one alarm turn off in the same scan, this logic will only detect it if the alarm that just turned on is assigned to a more significant bit than the alarm that just turned off. You'll have to decide how likely that is in your application, and whether it's a problem if it does occur.

Alarming is just one of those things - everyone has their own way of doing it, and probably none of them are perfect (except mine, of course ). Search the forum for threads on the topic and you'll find dozens of ideas on how to do it - try them all out and see which one works best for you!

thank you for your help I will have a go at this

A = alarm word this scan
B = alarm word last scan
C = A xor B. (C = bits which changed)
D = C and A ( D = bits which changed going high)
B = A ( save current for next scan)

That's neat. I like that.

bernie_carlton said:

A = alarm word this scan
B = alarm word last scan
C = A xor B. (C = bits which changed)
D = C and A ( D = bits which changed going high)
B = A ( save current for next scan)

Click to expand...

Great tip! Thanks

bernie_carlton said:

A = alarm word this scan
B = alarm word last scan
C = A xor B. (C = bits which changed)
D = C and A ( D = bits which changed going high)
B = A ( save current for next scan)

Click to expand...

Well done!

bernie_carlton said:

A = alarm word this scan
B = alarm word last scan
C = A xor B. (C = bits which changed)
D = C and A ( D = bits which changed going high)
B = A ( save current for next scan)

Click to expand...

I will add this to my repertoire

Note: if you do use these word based logic manipulations be sure to document them well. It may not be apparent to a tech (or even you in a few months) what is going on.