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Old 08-11-2006, 05:02 PM
frostee frostee is offline
Join Date: Mar 2002
Posts: 29
Time Synchronization of PLC to time server

Are any of you doing this? I have found that the times on the PLC's are different from our loal SQL server.
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Old 08-11-2006, 05:24 PM
PaulEngr PaulEngr is offline
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Join Date: Jan 2006
Posts: 172
Yes. It can be tricky to do.

First make sure you have all your servers synchronized. Windows is getting better about this but I still recommend going with some sort of NTP service since they still seem to work better. Watch out that your slew rates are not excessive or it will cause lots of issues with redundant servers. In fact with redundant servers, make sure you only synchronize ONE of them.

Once that is established, there's the hammer approach...purposely reset the clocks on the PLC's via a script running in the basic control engine periodically.

If you really want to be careful about it, then write the server time clock to PLC memory. Have the PLC slowly slew it's internal time clock to match the server such as making a +/-1 second adjustment every minute. This minimizes "clock jitter"...clocks jumping by huge amounts suddenly as they "catch up". You will need to also implement a "change detector"...something that verifies whether Cimplicity is in fact updating the readings and then not make any changes if the data is out of date (not changing).

Regardless of your approach, on the PLC side, be very careful of the implication of daylight savings time changes. Also consider "traps". For instance, set up something that resets once per hour to cache the last time it reset and then to specifically watch for an increment in the hour or create a one hour timer as opposed to synchronizing with the "real time" clock. The reason for this is that if you have an event fire off at say the top of the hour triggered by "minutes=0, seconds=0", then clock jitter can cause it to fire too many times or never at all. Better to do something like (in symbolic code):

If last_reset_hour <> current_hour then
(do my once an hour action)
Set last_reset_hour := current_hour
end if

As long as your slew rate is less than one hour per hour, it will always work correctly.

You may also have to use a trick if you do control slew rate. If the clock adjustment you need to make is very large (say hours or days), then execute a special routine that simply sets the clocks equal to each other and then execute your "power failure" routines since obviously the PLC has lost battery power or main power or something which threw the clock so far out of whack that small adjustments are pointless.
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Old 08-11-2006, 06:24 PM
dagudelo dagudelo is offline
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Join Date: Jun 2002
Posts: 133

Another question, I'm working with several PACs RX7i in a Ethernet Network and I need to synchronize them to a SNTP Server for a SOE application, I understand that every PAC Ethernet Interface has its own built-in clock (different to CPU clock) and that is the clock you can synchronize over a Ethernet network .. Am I all right?

Regarding the above idea, I'm trying to send the Ethernet Interface clock data to the PAC CPU clock via EGD, in fact an ETM001 Ethernet module is "producing" data and the CPU is "consuming" data with the timestamp (8 bytes) option, so I'm trying to set up the CPU clock with this info, but this has not been possible, Do you have experience working with this themes ?? I'd appreciate your help

Thx in advance
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