Timer

Has anyone ever made a timer without using a timer? The problem I have is that I am putting together a Control Logix redundant system and when the redundant processor goes online it all most doubles the scan time. My application is scan critical and I can not exceed a certain amount. One of the biggest issues with redundant processors is the crossloading. So everytime a timers value changes it cross loads about 250 bytes of data. One timer is 12 bytes of data. A dint is 4 bytes of data. So you can see that if a program uses numerous timers it crossloads 3 times the data of a dint. So if my math is right 100 timers will raise the scan time by 1 milliseconds. These timers do not need to be on the dot 100% to the actual timer. They will be just used to debounce photo eye signals. So it will not be an issue if I debounce +/- 60 milliseconds. Any ideas would be greatly appreciated. Thanks

Not clued up on Control Logix but on other systems I have used a counter with an internal clock pulse incrementing the counter and running a compare on the value or using the counter output.

Yes thats the way I would do it also. Why not use a single 1 sec timer. Using the Dn bit and a ONS increment a INT/DINT tag by one each time the timer expires. Then using your Compare and Mov instructions

Ignore the bytes it's the instruction time that's important.

you need to check how many ns it takes to execute a timer instruction compared to many instructions to increment an integer or a counter & then include the compare (this one is usualy the most time consuming), however not looked into the clx instruction set to see the times taken.

Answer

----[ ]----ADD_DINT
PE %R1_IN1 Q_%R1

1 const_IN2


----[/]-----MOV_DINT
PE
0 const_IN1 Q_%R1

This is not a timer but will work for DEBOUNCE. If you are using a N.O. PE.

Yep - used increment with a leading edge detector off an internal clock pulse many a time as well as a clock pulse into a counter. Have even used a counter as a compare function.

If time in the plc is that important, then you might want to consider a photoeye that has a timing function built into it. I've used these many times in the past as a standalone. It will work the same way as a debounce in a program. I've used an AB series 9000 timing Photoeye in the past.

brucechase said:

If time in the plc is that important, then you might want to consider a photoeye that has a timing function built into it. I've used these many times in the past as a standalone. It will work the same way as a debounce in a program. I've used an AB series 9000 timing Photoeye in the past.

Click to expand...

We banned these at one place as helpful operators would 'adjust' timings.