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S7-200 with TD 200 - SM0.0

7/8/2009 9:07 AM	Rate (0)
jamil_amin Posts: 3 Rating: (0)	I was monitoring the status of the program in one of our S7-200 PLC system. SM0.0 N/O contact showed as "open" in some parts of the program, while at other parts it showed as "closed". I checked the same in another PLC which was running normally without any problem and also found the same. Can anybody explain why at some parts of the program thebit has a value "1" while simultaneously at other parts it has value "0" ???
jamil_amin Posts: 3 Rating: (0)
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7/8/2009 9:17 AM	Rate (0)
naah Posts: 76 Rating: (8)	Hi, When you are watching the status of a memory, you are watching the current status of the memory after execution of the instruction. For example,consider this situation, in first line you set the bit M0.0 and in 5'th line you reset it, and again in 10'th line you set it. So, when you watch the status of M0.0 in line 1&10, you will see the "1" as ststus and in line 5 you see the '0" as status. Hope it helps! Good luck!
naah Posts: 76 Rating: (8)
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7/8/2009 9:39 AM	Rate (0)
Bilal Moolla Posts: 133 Rating: (9)	This is definitely a strange problem. I agree that this could occur if your are trying to reset M0.0 Could you attach a copy of you program?
Bilal Moolla Posts: 133 Rating: (9)
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7/8/2009 11:00 AM	Rate (0)
jamil_amin Posts: 3 Rating: (0)	Naah & Bilal - Thanks for your posts. As I understand, SM0.0 is "Always on" as soon as the PLC is powered on. So it really doesn't matter at what instant of scanwe are monitoring it's state. Nevertheless, the status of SM0.0 shown in two different segments of the program (one in SBR_0) and the other in (MAIN) are different. I was thinking that as SBR_0 may be called only initially from the Main program, could that be the reason that explains this ?
jamil_amin Posts: 3 Rating: (0)
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7/8/2009 12:59 PM	Rate (0)
Robster Platinum Member Joined: 5/28/2008 Last visit: 1/15/2026 Posts: 1344 Rating: (141)	Hi, if SM0.0 is in SBR_0 then you can only watch it, if SBR_0 is calling at this time. I think SBR_0 is OFF. How do you call SBR_0?? And it is like you wrote. SM0.0 is always on. if you use SM0.0 as negation, then it is always off. regards

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7/8/2009 1:22 PM	Rate (0)
jamil_amin Posts: 3 Rating: (0)	Robster, Thanks. It confirms my hunch. SBR_0 is called based on some conditions in OB1.
jamil_amin Posts: 3 Rating: (0)
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