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NTP mechanism

Created by: Quike at: 7/31/2014 8:36 PM (4 Replies)

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7/31/2014 8:36 PM	Rate (0)
Quike Regular Member Joined: 11/22/2009 Last visit: 10/7/2024 Posts: 11 Rating: (0)	Hello, After a deep search... anyone could explain to me the NTP mechanism. I know how to configure it and seems that works OK, but when analysing the time adjustements I've seen that there are times when the new time is larger than 300 ms from the PLC time. This happens when the NTP is configured as being 10 seconds (maximum rate). I'm a bit puzzled about the 300ms drift which happens every 50 seconds!! I would expect that the PLC requires a new time every 10 seconds and then updates it every time if different from the current time but the strange thing is those 50 seconds.... Well, any expert on that?? Thanks in advance, Qk

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8/1/2014 10:25 AM	Rate (0)
eisenpferdreiter Posts: 1058 Rating: (93)	Quike Hello, After a deep search... anyone could explain to me the NTP mechanism. I know how to configure it and seems that works OK, but when analysing the time adjustements I've seen that there are times when the new time is larger than 300 ms from the PLC time. This happens when the NTP is configured as being 10 seconds (maximum rate). I'm a bit puzzled about the 300ms drift which happens every 50 seconds!! I would expect that the PLC requires a new time every 10 seconds and then updates it every time if different from the current time but the strange thing is those 50 seconds.... Well, any expert on that?? Thanks in advance, Qk Hello Quike, the NTP protocol and algorithm is defined in RFC 5905 You will find all details there. regards
eisenpferdreiter Posts: 1058 Rating: (93)
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8/1/2014 10:30 AM	Rate (0)
eisenpferdreiter Posts: 1058 Rating: (93)	here is a quite easy, but not so detailed expanation: Network Time Protocol regards
eisenpferdreiter Posts: 1058 Rating: (93)
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8/1/2014 10:42 AM	Rate (0)
jklm Platinum Member Joined: 12/22/2006 Last visit: 1/25/2022 Posts: 1742 Rating: (309)	Hello, Here an interesting FAQ about NTP in automation context. http://support.automation.siemens.com/WW/view/en/17990844 I would recommend to configure more than 1 server if possible. Also asymmetric routes and network congestion can cause errors of 100 ms or more. Where do you take the NTP time from? Local source on the local LAN? May be some events in the network (automatic topology changes using STP, or other redundancy)? Could it also be that you get the time from a second source (via CPU PN port + via a CP)?
	Best regards jklm Suggestion was helpful? Feel free to rate it.
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8/1/2014 5:14 PM	Rate (0)
Quike Regular Member Joined: 11/22/2009 Last visit: 10/7/2024 Posts: 11 Rating: (0)	I have two external servers which work with other hundred of PLCs and FECs without problems. They are in the same network and other machines do not suffer any issue. I was reading deeply about the NTP mechanism and my conclusion (seems obvious) is that the client requires the time to the server and the server answer under demand but not unsolicited time is originated in the server. Network experts have corrborated this, and they see that from time to time the PLC requires the time and its own time (included in the requirement) is drifted. I've tried several PLCs and surprise, the only ones failing by now are the ones with integrated comms in the CPU. I've tried a S7-416 with external CP and everything works fine. The asumption now is that the PLC is bugged and sets a time which is not the one received by the NTP every 50s (quite regular when having 10s as NTP request) To this point I'm blocked and to continue I may require a SIEMENS expert looking at this. No further diagnostics in the PLC are available ...
	Last edited by: Quike at: 8/1/2014 5:15 PM
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