AB Controllogix 5000

Hi,

I was wondering about the timing of the scan in a 5000 system:
Usually,
1/ plc reads inputs
2/ ladder processed
3/ plc updates outputs

So during 2, the ladder sees inputs that are not changing.
I heard that this is not the case with the 5000 series ??

Correct or not ? who can give me confirmation.


Thanks

As I understand it in the Logix system the "Read Inputs" and "Update Outputs" are in their own process which runs asynchronously to the scan of the logic. So - compared to previous AB lines an input could change in the middle of a scan. And outputs could change at any time after you set the bit.

To make the Logix scan work more like previous lines then at the beginning of the scan copy the current inputs into some type of internal table. (Exactly how this is done I can't tell you. I haven't directly worked with these.) In processing, write "outputs" to an internal table. Only at the end of the scan copy the internal "outputs" table to the real outputs.

I'm sure others will be able to help more with AB Knowledgebase references and actual first hand experience.

The way CLX does I/O exchange is different than PLC5:
For descreet modules see chapter 2 of the user manual:
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um058_-en-p.pdf
page 2-9:

You should read whole chapter, here are some notes:

Input Module
Operation In traditional I/O systems, controllers poll input modules to obtain their input status. Digital input modules in the ControlLogix system are not polled by a controller. Instead, the modules multicast their data either upon Change of State or periodically. The frequency depends on the options chosen during configuration and where in the control system that input module physically resides.
An input module’s behavior varies depending upon whether it operates in the local chassis or in a remote chassis. The following sections detail the differences in data transfers between these set-ups....

Output Module
An owner controller sends output data to an output module when
either one of two things occur:
· at the end of every one of its program scans (local chassis only) and/or
· at the rate specified in the module’s RPI
When an output module physically resides in a remote chassis (with respect to the owner-controller), the owner-controller sends data to the output module only at the RPI rate specified for the module.
Updates are not performed at the end of the owner-controller’s
program scan.
Whenever the module receives data from the controller, it immediately multicasts the output commands it received to the rest of the system.
The actual output data is echoed by the output module as input data and multicast back out onto the network. This is called Output Data Echo. The Output Data Echo also may contain fault and diagnostic information, depending on the module type.

The Logix5K uses a producer/consumer data model and IO modules are not slaves to the processor, thus you can have multiple processsors in the same rack or have IO in one rack that responds to a processor in another rack, or even have multiple processors acting on the same input data (but an output card should be assignend to only one processor). Input cards are data producers. Output cards are data consumers. A processor is both a consumer and a producer of data. IO cards with built in diagnositics are both producers and consumers. Because there is no master/slave relationship data is multi-cast by the modules asynchronously on the control-net backplane. However, on the CLX platform this update time is user configurable and is often many times faster than the IO update time of traditional PLCs.