Basic Question about Subroutines

Hi everyone, I am fairly new to programming subroutines and interrupts. I am used to dumping evrything on the main program. But with the Siemens S7 200 I am currently using now, I wish to use subroutines and interrupts. My question is -

When a subroutine/interrupt is called (on say instruction 1 of the main program) and executed, does it come back to the next instruction on the main program (instruction 2)?

Or Does both the subroutine and the main program run simultaneously?When the subroutine is being executed, what is the status of the main program?

May be this is a very basic question, but please help me understand the sequence. Thanks for your time!

hi preeya,
this is regarding the interrupts & subroutines in s7-200.....
the basic difference in subroutines & interrupts is interrupts as name indicates interrupts the main program scan...executes itself depending on the priority assigned & then returns back to parent coe starting from the next instructions from its occurance as u asked...
whearas subroutines just coexecute.....infact in siemens the microwin takes care of everything....u don't even meed to write the simple RET or END instructions as u do in Allen Bradley & other PLCs...
if u read the help of step 7 microwin u'll find everything u need.....i guess u r using CPU 22x.......i hope this is useful to u

i remain,
Mandar Joshi.

Thanks Mandar. Your reply was really useful.
If I understand the coexecution right, while the sub is being executed, the main program also runs parallely..?

And yes I am using CPU 222.
thanks again.

preeya,

I have to admit, I don't have any experience on the Siemens S7-200 series. However, I also have to say that I am doubtful about lucifer's answer. I would be surprised to learn that the S7-200 can simultaneously execute a subroutine and the routine that called it. I expect that when a subroutine is called, program execution shifts from the calling routine to the subroutine, proceeds to the end of the subroutine, and returns to the statement following the Call.

If I'm incorrect, my apologies to lucifer.

Steve is correct. The difference in subroutine/interrupt calls, the subroutine is called from somewhere in your program scan at the same place everytime. The interrupt is called from a hardware input, or a timed event that is defined as an "Immediate Interrupt", which is called whenever needed without predetermining in the program scan. Yes, both return to the point in the scan, where it happened, to execute the next command.

Thanks for all your reply. I might use a subroutine and a timed interrupt for my program. I now understand the flow better.

steve is correct
a subroutine is a program called from a higher program.
wife is knitting on bench
when she has to take a new bale she uses a subroutine, and she stops knitting until she has the new bale.
when you ask for coffee she is interupted and will make coffee (if you ask nice) and after she will continue knitting again

so she stops here main program and she will continue.
the danger of interuupts is that when you interrupt her a lot she will never finish knitting.

as knitting is slowed down it is not realtime anymore.
and you will get cold as your sweater is not ready yet.