48hrs timers

crram

Guest
C
I have an application which requires 48 or still more hours timer to work in a sequence to open valves so that the temperatures gets controlled to a point. but i just want to know how that should be done in the rung with consideration of limitation of timer limits
 
Cascading timers

Six timers with presets of 28800 seconds each. If you are using a PLC processor with a numerical limit of 32767 then that limit gives you a little over 9 hours for a single timer running and using a 1.0 second time base. So, using six with the preset above will require the cascading. Cascading is just when one timer finishes and is done, then that done bit is used to start the next timer and so on until you have the entire process complete. You can reset the whole thing by using the done bit to turn off the first timer in the sequence and that in turn will drop all the others. There may be other ways but this will work. One caution, timing this long makes room for error so if you need really precise 48 hour time markers, then there may be a better approach than this using a PC or some other non-plc device that you can use to signal the PLC to do its work. Good luck!
 
If this is an A-B processor I have used an STI set to 1 second that fired counter logic to record days, hours and minutes. This was one of the more accurate (& easier, without having to get in to date math) ways to keepup with the elapsed time for the current cycle. If I remember I think the jitter on the STI is something like 50ns on a SLC, which if I remember right, allowed me to keep better time than the system clock.

It is used for controlling an extreamly long ramp/soak cycle on an aging oven.

Darren
 
I'd go with jimbo3123's option it's much easier to debug/follow.

Or pehaps change the preset of the timer to 3600sec (1 hour) and the counter to 48, but it really depends on the application if you need to adjust the time in whole hours or part thereof.

C
 
Last edited:
Kim, that's one way to count hours but, as Randy pointed out this is not a precise counter so if precision is needed you may need to use a separate device. One way to get this precision is to get a separate, more precise timer that has the ability to send an input to the PLC when a minute or an hour has expired, and has remote reseting. Then use this input in your logic to count the minutes or hours and a PLC out put to trigger the timer to reset.
 

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