Keeping Time With AB PLCs

I have a question on keeping the same time through out multiple PLCs. A conveyor line has multiple sections to do different things (IE box erector, filler, sealer). The customer wants to track uptime and available time for each section. What would be the best way to have all PLCs keep the same TRUE time. I have used timers and the minute status bit but neither one could keep exact time 18 hours later in comparison to each other. I am trying to display the time in tenths of a hour. I am counting six minutes then using a count of one to indicate tenths. In this conveyor line there is: PLC5, 2X SLC5/05, MLX 1400, 2X MLX1200. I have found that the PLC5 and SLC tend to keep the same time but when the same logic is in the MLX the times come out different. I have thought of keeping the time in one as they are all in a network but this would be little more time consuming than I would like at this point. Any ideas?

If you are using the real time clock for your information collection, then I recommend running the lock sync utility from Rockwell. It is advertised as a tool for Logix5000 controllers, but it works with the PLC5 and SLC and I am pretty sure it worked on the ML1100, not sure about the others in your list.

https://www.software.rockwell.com/support/download/detail.cfm?ID=3842

The clock sync utility runs on a PC and works over your RSLinx network and will measure the error before and after and is really easy to use, once all your controllers have been picked for it to operate on. I am pretty sure you can schedule the clock update tool to run on your PC but I always did it manually since I was just being picky about the touchscreen and alarm times, not for any calculations.

My experience with the PLC5 and SLC was that the PLC5 had more accuracy but was off time in the opposite direction from all my SLC controllers, by up to about 30 seconds a week wrong, so I would run that utility at least once a week to stay accurate within a rounded minute. If you want them sync'd better than that, you might want to run it daily, perhaps right at shift change or during a known daily or nightly down period.

If you want your seconds to match up across all those different models of PLC after 24 hours without intervention ... you are probably not going to get there, minutes, on the other hand, should be pretty easy.

I already looked into the real time clock. The RTC for the 1200 requires an extra module. The engineers do not want to buy it. I am still attempting this route though. Thanks for the suggestion.

Ah, then if you are using RTO timers, you must not lose the overrun each DN sequence.

I do this by never resetting the timer, rather, I subtract the PRE from the ACC, make sure it is greater than or equal to zero, then OTU the DN bit. This will keep your hundredths of a second carried over to the next period you are measuring. Use the finest resolution available in each controller ( 0.01 timebase or 0.001 if available).

If your PLC is busy doing other things, the acc value when evaluated at the timer instruction, may often be greater than the PRE, so don't lose those one or two hundredths of a second each minute, if you can capture thousandths, even better.

First, let's discuss how you're measuring time in your logic.

Are you using Timer instructions, like TON and TOF and RTO ?

Or are you using logic that references the Status files (PLC/SLC) or Function Files (MicroLogix) of the controllers ?

You mentioned not having a Real-Time Clock module in your MicroLogix 1200 controllers. The RTC is modular on the MicroLogix 1200 and 1500, and built into the MicroLogix 1100 and 1400.

But there's something funny about the clock on the 1100 (and I think also the 1400) when you read it with the Function File. It only updates in logic on even numbers of seconds ! If you are trying to capture elapsed time using the Function Files, and your time intervals are small with respect to 1 second error, you might find a lot of inaccuracy.

Anyhow, let's discuss how you're measuring time.

f1retank80 said:

I already looked into the real time clock. The RTC for the 1200 requires an extra module. The engineers do not want to buy it. I am still attempting this route though. Thanks for the suggestion.

Click to expand...

Of course, the old "I absolutely, positively, 100% MUST have a brand new Cadillac! But I will NOT pay more than $500 for it!"

Ken, I tried both methods but neither was correct after a 24 hour time period. The status/Function was off by twenty minutes but some of the time being recorded will last a week so by then it would be off by more than an allowable amount.

I'm just wandering if the enclosure temp will make a difference in the time.

Thanks OkiePC that method with the timer worked perfectly. I never realized how much time was lost over a short period with timers.