compact logix and scan SOLVE!!..Not scan time

Hey all...i know there are a thousand threads relating to how the logix series scan time works..my question is..how does it solve...after the scan..when its updating..does it update alphabetacaly?...or as it appeared in the program?..is there a way i can get the program to update as is solves??

I assume that by 'update' you mean transfering the internal status of an output tag to it's physical point. There isn't any way that I know of to trigger an output transfer outside of the configured RPI. That doesn't mean there isn't a way, just that I don't know about it if it exists.

If you are talking about internal tags they are refreshed when they are encountered in the plc program. This is where some people get into trouble as you can get an output update that occurs mid-scan, leaving some of your outputs in the last scan state and some of them in the current scan state.

Keith

Greetings darrenj ...



like my distinguished colleague Keith, I’m a little bit confused on what you’re asking here ...



just as a guess, take a look at the IOT (Immediate Output) instruction for starters ... if that’s not the answer that you’re looking for, please post again and give us some more detail about what you’re trying to accomplish ...



note that some of the words that you used in your original post (solve, scan, updating, etc.) mean different things to different people ... if you need more help, try phrasing your question in different terms ... that might help us understand the question better and get you the answer that you need ...

In the ControlLogix series of A-B controllers, the controller scans the user program, writing data to the tag database as it encounters each output instruction.

A secondary (background) process, "produces" output tag data to the outside world at the output module's specified "Requested Packet Interval" (RPI).

The connected output module then "consumes" the data from the communication network. If the output module is in the same chassis as the controller, the "network" is simply the chassis backplane (sometimes called ControlBus). If the output module is on ControlNet, Ethernet, or whatever, then a "bridge" module (a.k.a. Comms module), is put into the chassis to make the data available on to the external network.

This "producer/consumer" model for I/O data maens that I/O data is transferred asynchronously to the user's program scan.

FYI : Input data is transferred into the tag database in exactly the same fashion, and this asynchronous Input data can cause problems if an input is ispected more than once in a program scan (i.e. there is no guarantee that both reads of an input will read the same logic state). Often, you will see programs that have I/O data "buffered" in logic, so as to synchronise the data transferred to/from the I/O modules.