Switching an Output Back and Forth After Given Time

Hello, 

Probably a basic question. 

I am looking to see how to program something in ladder logic. 

I have to fans controlled by one VFD's. For the first 30 days I want one fan to run and then switch over to the other fan for 30 days and then back and forth forever. I wasn't sure if a timer was the right approach or some other way or importing the date some how. Using S7-300.

I just need help with the switching between portions  of the fans code. My VFD's are controlled via a PID loop.

This is my sub for toggling in ladder:

Call this sub on a P_Trig once every 30 days. Use a memory retentive bit.

On the other question, about time. yes the cpu has a clock you can read. you can add 30 days to a memory retentive date tag. And there is a block for comparing two dates. Depends on what your using. Step7 V5.x  or TIA V1x...

But if it were me. I would use OB3x, and count every time it is called into a double integer. There are 2,592,000 seconds in 30 days. In the properties of OB3x set it to be called every second.

Hello,

next tips. I expect you initially set all on PLC to work and run. This is your day 1, I expect. Count days of working for switch as calendar isn't expected, correct? Then something can happen like you will need change something inside code or data and you need set all again initially. In this case you need to know how much days you already runned. This can be solved with some variable as parameter you set manually from Online table or Visualisation under some Service prottected level. It add to existing days counter some value to reach directly correct days counter. In case of all is correct (no needs do correction) you can have in this parameter simply 0. I think to this you need pay attention also. Look to S7-300/400 functions for work with Date and Time to have data for count. Be careful to set initially on your CPU correct date and time, via Online access inside CPU. After that you need due your internal counter to know +/- what date and time is and compare for sure what is inisde CPU, to do possible corrections to have correct count of days.

I am a little confused by your code. How does it toggle? Also when I my comparitor gets to 30 days do I reset it? because the counter will continue to rise?

Also I am using S7 V5 I think not TIA. The ladder logic shows an contact that closes a coil when the contact is closed. That would work for the first 30 day transfer but what would you do after that? Also I dont think there is a option in S7 manager move to a double integer only single integer and the value is to high for a single integer even if I used the 5 sec OB.

So I found the answer using some of the help.

If you call the Toggle subroutine with a one shot. it will write to the supplied bit the opposite of the current bit state. if call it every scan then it will change state every cycle..... not useful.

if you were watching the month.  only fire the toggle subroutine when the month changes...

Current Month                                                  Move                      ---(OneShot)---
       <>                                                    Current Month  ->
Remembered Month                       Remembered Month

I dont think I follow you tbh?

It doesnt fire every cycle it holds until it reaches the set point.