Siemens S7 Special Bits - PLC ON/ First Scan/ PLC Clock HH-MM-SS/ etc

Created by: DanAtLogan at: 11/12/2013 12:55 PM (2 Replies)

11/12/2013 12:55 PM	Rate (0)
DanAtLogan Posts: 1 Rating: (0)	Hi, I am fairly new to Siemens S7 but have experience with Mitsubishi and Allen Bradley systems. At present i am taking a few weeks to look into the Simatic Manager S7 software (I also have been given TIA Portal V12 to look at). I am creating simple logic just to make sure i can use all of the symbols, naming and simulation software parts. I am looking for the 'Special Bits' IE;- the name i can give to a N/O contact to allow it to read for the first scan only or a PLC healthy bit, does it have a specific internal memory bit that i can assign to a contact to use? I am trawling through manuals at the moment but thought this was worth a shot too. I did search the FAQ's for the post but didnt see much, if i have repeated then i apologise. Thank you in advance Regards Dan
DanAtLogan Posts: 1 Rating: (0)
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11/12/2013 1:44 PM	Rate (0)
miami Platinum Expert Joined: 8/31/2005 Last visit: 3/24/2025 Posts: 2198 Rating: (331)	The SIMATIC does not use special bits. Instead of this, OBs are called. OB100 (S7-400 has additional OB101 and OB102) is called after the CPU has started. It is the first cycle the CPU runs. After this the CPU calls the OB1 with the normal program. Several error OBs (OB8x) are called when the corespondending error occurs. The CPU clock can be read and set with SFC 0 and SFC 1.

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11/12/2013 2:14 PM	Rate (0)
hdhosseini Diamond Member Joined: 1/28/2009 Last visit: 3/29/2025 Posts: 6867 Rating: (1367)	DanAtLogan ... I am looking for the 'Special Bits' IE;- the name i can give to a N/O contact to allow it to read for the first scan only or a PLC healthy bit, does it have a specific internal memory bit that i can assign to a contact to use?... Hello, For S7 300/400 ,We can consider the bit inside the status word as special bits.They are activated on special occasions as explained in the manuals.If you have experience with smaller CPUs such as S7 200,There are special memories to indicate the first scan or memory for cyclic behaviors.These options are not available explicitly but can be created if necessary.Please, check the following discussion as a sample: power up bit I hope that helps Hamid Hosseini
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