Time Sync

[gleblanc 1]

I've got a couple of Mitusbishi PLCs on site here that I need to keep synchronized a bit more closely than their RTCs will allow. They tend to drift a few seconds a month, which gets a bit annoying when I have to go out on the first of the month with my laptop and try to re-sync their clocks with the time.gov server. I can generally get these down to within half a second by hand. I'd like to get to within, say, 0.1 seconds, but I haven't found a good way to do this yet. The best thought I've come up with is to buy a couple of those "atomic wall clocks", hack them open, and set up the alarm function on them to sound every day, and use a simple ladder rung to write that time of day to the PLC clock. I expect that this will work fine as long as the clock doesn't drift too far, but I wondered if anybody else had a better/easier idea. My biggest worry is that the atomic clock won't be able to pick up a time signal inside of the giant faraday cage that we call a plant. Thoughts? Greg

[kaare_t 155]

Do you have a server on site? Set up the server as Win32TimeServer (NTP server), and register the PLC's NTP server function against the time server....

[gleblanc 1]

None of my PLCs have ethernet cards, nor IP.

[Ken Roach 194]

The longwave clock idea isn't a bad one. The fact that it's a super-long-wavelength signal makes it easy to receive indoors where GPS or cellular systems can't reach. If you don't have an NTP client feature in the controller, nor access to a network with an NTP server or access to the Internet, you're not going to be able to get an elegant solution. I've been investigating this with a different platform; PanelView Plus and a couple of CompactLogix controllers. So far the "best" solution I've found is a $1500 serial GPS box, but that needs an outside antenna. My challenge is that the system needs to be put in a box for three months; unpowered, that's the outside range of how long the PV+ or the CompactLogix can keep up the Real Time Clock. Upon repowering, I need to be able to automatically get the actual time, synchronize the two CompactLogix and two PanelViews, then subsequently keep them all from drifting. I saw that the Red Lion Crimson3 has a serial driver for GPS units... I wonder if it can get time from a GPS clock. Edited 6 Feb 2012 by Ken Roach

[kaare_t 155]

Great idea! I would definetively get someting that can communicate via serial connection, either a pure ascii interface, or maybe NMEA interface. Shipping industry uses NMEA a lot, and they also use serial NMEA GPS systems so there shouldn't be a problem finding those....

[drforsythe 15]

Interesting topic. I will watch to see what you come up with. Regarding the atomic clocks- we used to use them in our plants, but most of them had trouble getting the signal. This could have been due to cheap clocks, I am not sure. The one we had in our office never could sync after Daylight Savings Time changes unless we either took it outside or set it on a window sill. The clock was mounted on a wall about 30 feet from a wall of windows. This could point to the cheap clocks theory... Good luck, please tell us your solution when you get it.

[gleblanc 1]

I don't know if you've made any progress on this... I've been searching half the morning trying to find something decent. So far, I haven't found anything great for my application. I did find a WWVB receiver with an EIA-232 interface. http://www.beaglesoft.com/radsynreceiver.htm Looks like it would be quite a bit cheaper than the GPS box, and shouldn't need an outside antenna (maybe). It's a little more than I want to spend, since I'm only looking for 1 second accuracy. I've not found any cheap WWVB alarm clocks that plug in to the wall. Looks like my first test will be with something I can get for $20 shipped that runs on batteries. Depending on how much of a PITA it turns out to be, and if I can get reception, buying that receiver might turn out to be the cheapest solution.