Logix 500 PID: Loop update longer than 10.23 seconds?

Just tuned a heating application where my natural period was 250 seconds. Everything I've read says that the loop update time should be 5-10 times faster than the natural period. This puts me at 25-50 seconds. The problem is, the PID instruction in the Micrologix 1400 I was using doesn't go above 10.23 seconds. I tried to change the STI value (S:30) to 25 seconds but something just kept writing a 0 back into it.

It's not that important now because the loop is tuned pretty well, but I heard that there might be a way to change the timebase of the Loop Update parameter. I've been looking through the help files and so far, nothing.

That "5-10" rule of thumb should be viewed as an upper limit to your update period. Said another way, it isn't going to hurt you to update more often than 10 times faster than the system time constant. It's just that you won't see much if any improvement in control capability if you do and it can be viewed as a waste of processor resources. About the only thing you DON'T want to do is update faster than your I/O capability.

Keith