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drusso

ControlLogix Processors

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I was researching using multiple processors in a single Control Logix rack. Has anybody found a need and what did you use the extra processor power for? I have a project where the outputs,up to 128, are pulsed on for 6 ms every 3 seconds. The pulse train would run for hours and the on time would be user adjusted from 6 ms to a full on state. I was looking to see if the extra processor would be used to guarantee the scan times. Thanks, Drusso

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I have a unit where we have a 17 slot rack and three CPU's a L64 and two L63s. This is a recipie intensive machine. The L64 is the batching engines and the L63s are the equipment phase sequencers.

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Bob, What type of scan times and how much code does the 2 L63's handle? I was looking at using a Siemens FM352-5 High Speed Boolean Processor to control the outputs. It is a parallel processor that has a resolution of 10 us, over kill for my application. I have not found a compatible product from Allen Bradley so my answer was multiple processors to keep the scan times within requirements. Thanks, drusso

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What you are describing sounds like an ideal application for the "scheduled outputs" feature of ControlLogix. It's described in the User Manual for the discrete I/O pretty well. To examine the mechanism, create an Output module connection in RSLogix 5000, say for a 1756-OB16E module. Select the second option in the Connection type: "CST Timestamped Fuse Data - Scheduled Output Data". You will see that the software creates two Output tags for the module: Local:s:O.Data[x] Local:s:O.CSTTimestamp What you do in your program is add to the current CST timestamp and place that future time in the .CSTTimestamp tag, and put your output values in the .Data[x] tag. When the CST time you assigned arrives, the Outputs will change to their programmed state. You can write this CST time to multiple modules and they will all turn on or off simultaneously. Because the CST is a microsecond resolution clock propagated to all modules in the chassis, your resolution even faster than the RPI of the module, because the modules always know the CST even between updates from the controller. I don't think you need the special OB16IS modules for this application; those are good for synchronizing outputs with motion events. You just need the clock. If your controller isn't also running a large application, you should be able to run your output pulsing application in a fast Periodic Task and hit those timing requirements easily. CST clocks do not propagate over ControlNet networks. You need to have these modules in the chassis with the controller, or in a remote chassis with a 1756-SYNCH module or 1756-EN2T running version 2 firmware with the IEEE 1588 precision time protocol. 128 outputs can be run with eight 16-point modules or just 4 32-point modules, so you should be able to do all of these in one chassis.

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Ken, I read the manual 1756-um058_-en-p.pdf Thanks for pointing it out to me. The part that reads, " Your control becomes much more deterministic even under changing loads. For this synchronization to work properly, the 10ms delay must be long enough to account for any controller, backplane, and network delays." Is that statement saying a pulse width less than 10 ms cannot be controlled? Or is it depending on the amount of code and I/O combined? Thanks, drusso

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Great question. I think that statement in the manual could have been phrased better, and it is based on some old system assumptions from 2001 (when that manual was last edited). The example has an input module that is checked for a CST timestamp change to detect change-of-state, so that detection depends on the delay of the input module, any network the input module is on, and the controller's continuous task execution time. In the modern ControlLogix, we have Event Tasks and faster CPUs, so you can run applications with CST Scheduled Outputs that are significantly faster than 10 ms. In your application, I expect you would us Periodic Tasks with high priority settings to schedule your 6+ millisecond pulses. To schedule an On event and an Off event within 6 milliseconds of one another, you need to be able to execute your control task faster than 6 milliseconds, which is easy with a 1756-L6x controller.

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Ken, I appreciate your time in answering my question. We are trying to size the project $$ wise. We will try to make a test and use a single L63 processor that is used in another system. The $$ difference when adding more than 1 processor per system is large because we need to build up to 12 systems. Thanks, drusso

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