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S7-300: analog card values during first plc scan

Created by: John25 at: 11/26/2014 5:16 PM (3 Replies)

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11/26/2014 5:16 PM	Rate (0)
John25 Posts: 12 Rating: (0)	Hi all, I have an analog card (331-7KF02-0AB0) and a 315-2 PN/DP cpu. I'm reading my analog value with a pheriperal address (PIW) and i'm noticing that the digital value is 32767 during the first real plc scan (the first scan that OB1 is executed). One scan later the analog value is measured correctly (e.g. 27680). Can somebody explain why this is happening and if this is avoidable? Since i'm using a pheriperal address, i would assume that the plc would immediately have the correct value but it seems to be one scan later. I'm generating an alarm in my code when the analog value overflows (32767) so each time the plc restarts (e.g. after a power cut), i get an alarm on all my analog values being out of range. Is there any elegant way to solve this. I want to be sure that the plc is working with the correct analog values when it goes through OB1 for the first time. Many thanks for any helps.
John25 Posts: 12 Rating: (0)
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11/28/2014 1:49 PM	Rate (0)
jpapu9 Posts: 50 Rating: (2)	Hi, Please see attached, this information is in the manual for this module. I also noticed in the manual that there are configuration plugs on the side of the module that you must set to match the input voltage being feed into the module. Regards Attachment Capture ai table.JPG (216 Downloads)
jpapu9 Posts: 50 Rating: (2)
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11/28/2014 2:37 PM	Rate (0)
Kahl Platinum Member Joined: 12/16/2012 Last visit: 1/14/2025 Posts: 655 Rating: (131)	Hi John! I do not know the exact reason of your problem. According to the manual, after power on the data is available after the first conversion. Do you have a lot of analog input channels? If it happens only during the first cycle, you could disable your alarms for the first cycle using a First Scan bit. Attachment influence of the supply voltage ....pdf (231 Downloads)
	ZEISS SMT
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12/2/2014 12:46 PM	Rate (0)
John25 Posts: 12 Rating: (0)	Kahl Hi John! I do not know the exact reason of your problem. According to the manual, after power on the data is available after the first conversion. Do you have a lot of analog input channels? If it happens only during the first cycle, you could disable your alarms for the first cycle using a First Scan bit. Thanks for the replies. I assume it is because the analog card needs a bit of time to calculate the first value (= first conversion done). I thought that this was done during startup of the cpu itself (the time before the cpu actually goes into run after power is applied) but apparently it doesn't. The easiest way to solve this would be to jump over the "main" program (where all the analog value processing is done and alarms are generated) for about a second after plc startup.
John25 Posts: 12 Rating: (0)
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