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gromit

PLC for first out service...with 8 mSec scan time

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I am looking to specify a PLC that can be used for first-out service whereby it is wired across all contacts in a hard wired trip string in order to latch in the first event that caused the trip.

Does Rockwell have a PLC that could achieve this functionality while operating within an 8 mSec scan time?

The typical number of inputs would be from 9 to 12, with an additional PLC input to serve as a trip reset.

Additionally, an output to identify which input caused the trip would be needed...so if there were potentially 9 potential contact wired up in the trip string the PLC would need 9 digital outputs to illuminate the respective tile associated with the input that caused the trip.

Thanks.

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I would expect any of Rockwell's controllers to easily monitor a dozen inputs while maintaining an overall scantime of 8 ms.

Any Logix-family controller could be set up with a Periodic task to monitor this function quickly.

Any MicroLogix or Micro 800 series controller could do it too.   If everything is 24V DC, probably the least expensive controller with the A-B brand name on it would be the Micro 830;  the 24-point version has 14 inputs and 10 outputs.

While I don't like the Micro 830 for general purpose automation, for a very simple cost-limited, function-limited process like the one you describe, it would be fine.

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Thanks Ken.

What software package is needed to program the Micro 830, and does it have a similar instruction set as the SLC500 and PLC5?

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Ken,

I just researched and found the software, and am downloading the "Connected Components Workbench Software"  now.

I was not able to find the specs for the input impedance or the scan rates.

Edited by gromit

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What is  "event input interrupt support" and would that be how I would set up the inputs for the first-out trip detection?

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I'm hesitant to recommend the Micro 800 series to somebody who is comfortable with MicroLogix and SLC-500.    While it does have an A-B sticker on it, it's fundamentally a different instruction set, different editor, different everything.   It's a little ISaGraf controller, with no Allen-Bradley heritage.

But it's very inexpensive.    And if you take the time to remind yourself that it's not a MicroLogix, it does some nifty things with trigonometry and structured text that you can't do in a MicroLogix.

The Micro 830/850 User Manual (2080-UM002) says the input impedance is 3k ohms for inputs 0-3 (they are wired to a high speed input subsystem but can also be used for general purpose I/O) and 3.74 k ohms for the rest of the inputs.

The default input filter is 8 ms (on AC.. I think also on DC) but the on/off times for the Micro 830 inputs are under 0.1 milliseconds.  

The Micro 830's Event Input Interrupt feature is described in Appendix D, but I don't think it will be necessary for a system whose minimum time threshold is 8 ms.

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If you did want to use the Event Input Interrupt, the Micro 830 would just exactly fit your needs.

You would wire up the 12 inputs to Inputs 1 through 12.   Wire up the Reset input to Input 13.

Then wire all the 12 inputs in parallel to Input 0.   That will be your Event Input Interrupt trigger.   The moment it goes true, the EII subroutine will execute and examine all the other inputs to determine which one is true, and store that state in memory.

The performance will be very fast... I haven't found a documented speed for reaction to an event input interrupt, but I would expect it to be under a millisecond.

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Thanks Ken, very informative.

After speaking with a local Rockwell automation specialist, and expressing concerns about the Micro830 & 850 indicating a COMmon terminal for all inputs rather than individual returns, he suggested the following.

Rather than wiring across each of the contacts in the trip string, to instead wire the downstream terminal of each contact to the input of the PLC digital input card.  He suggests this, instead of the other scheme, because the 830 & 850 don't have isolated digital inputs which might impede the trip circuit from actuating otherwise.

I got a quote for the Micro850 and a 7" PanelView.

What are your thoughts on a preferred wiring scheme.

Is the attached wiring scheme what you had in mind?

Thanks!

ct_trip-string_plc_triptrap_sketch.jpg

Edited by gromit

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Maybe I am missing something and please tell me if I am but......

the way you have this drawn out wont all the inputs go true when any 1 input is true since you are tying them all together at terminal #2?

Seems like if this is 24VDC that you would need diodes to keep from back-feeding every input at the same time.

BCS

Edited by Bering C Sparky

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Great point Bering, on both counts. 

Wiring all of them up in parallel was recommended to trigger an event input interrupt and I kinda included it in the diagram without enough thought or consideration.

I will either not wire them up in parallel,  or will consider incorporating diodes as you suggest.

Thanks for your input! 

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