Seconds since 8am - best way to do this?

I have created a task that executes every 10ms, it uses a GSV to determine the time and set a flag once 8am has occurred. I use a periodic task to not flood the CPU with GSV commands more often than I need to.



In another routine I then run a timer and count the number of seconds, when this flag is true I reset my counter.



Is this the best way to do this? Will this be reasonably accurate over the course of a day? Obviously if they change their clock it will break the routine but I'm not fussed by that at the moment.

Why not just compute how long it ha been since 0800 from your localdatetime udt tag?
I imagine that a timer resetting every second is not going to give you great accuracy.
It will end up quite a bit slower than it should be over a whole 24hours.

I guess because of the roll over at 12:00 it seemed like it would be a pain in the ***. But you are right, it will be more accurate.

edit: Actually it is pretty easy to do, I'll just do that.

Yeah ok this was way easier heh

I think you may have a discontinuity in your equation. In the less than 8 case you want to add 16 to the hour, not 8. 23:59 - 8 is 15:59.

Keith

Periodic task with a 1000 ms interval.
ADD SecsSince0800 1 SecsSince0800

In your main or continuous task, check for the time to pass 0800 and oneshot CLR SecsSince0800.

There are a ton of other ways, you could probably just count milliseconds with a single timer that resets at 8 a.m. and DIV the ACC by 1000. I just woke up so I am not sure how many milliseconds there are in a day and if that fits in a DINT...coffee...coffee...

86,400,000 milliseconds in a 24 hour period. That will fit in a DINT (-/+ 2.14 billion). KISS.

kamenges said:

I think you may have a discontinuity in your equation. In the less than 8 case you want to add 16 to the hour, not 8. 23:59 - 8 is 15:59.

Keith

Click to expand...

Thank you, I had only tested the equation on paper and forgot to use 24 hour time.

Cheers.

Each time you reset a timer, you stand to lose a few microseconds. Don't reset your timer any more often than absolutely necessary. Time does not stop ticking while the CPU goes out and wipes away what it had stored, and once the timer is DN, you can be sure a few microseconds up to a few milliseconds have passed you by. Resetting the timer will erase that timer overrun from your intended accumulation over a long period.

If you want to keep your existing work and the one second timer, rather than RESet it, when the acc >= 1000, subtract 1000 from the accumulator and let it keep rolling. At the same time, fire off a oneshot to act as the "DN" bit. Then set the preset to a number it will never reach and let it roll on...

Different platform, but the same principles apply unless there is some magic in Controllogix that circumvents scan time...
http://www.plctalk.net/qanda/showthread.php?t=87079&highlight=timer+overrun

OkiePC said:

Periodic task with a 1000 ms interval.
ADD SecsSince0800 1 SecsSince0800

In your main or continuous task, check for the time to pass 0800 and oneshot CLR SecsSince0800.

Click to expand...

Why not reset the SecsSince0800 at 08:00:00 in the 1000mS periodic task, you only need to to check the Hour and use a oneshot.

GSV WallClockTime LocalDateTime LocaDateTime.Year
ADD SecsSince0800 1 SecsSince0800
EQU Hour 8 ONS Oneshot CLR SecsSince0800

There's no need for a faster GSV than 1 Second, since the WallClock Seconds anly changes once a second...

daba said:

Why not reset the SecsSince0800 at 08:00:00 in the 1000mS periodic task, you only need to to check the Hour and use a oneshot.

GSV WallClockTime LocalDateTime LocaDateTime.Year
ADD SecsSince0800 1 SecsSince0800
EQU Hour 8 ONS Oneshot CLR SecsSince0800

There's no need for a faster GSV than 1 Second, since the WallClock Seconds anly changes once a second...

Click to expand...