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I'm sure this must be done quite a bit: Have a customer upgrading a PLC-5 to a CLX. They want to continue to run the PLC-5 in the process, so it uses existing inputs to control outputs. At the same time, they want to use the same inputs as the PLC-5 to demonstrate the converted CLX code... how the outputs would react to the same set of inputs. (This could be demonstrated in a SoftLogix as well.) A hardware solution would be to wire up the same inputs as the PLC-5 to the CLX, but that would be prohibitively expensive and labour-intensive. We are looking at a soft solution, probably using OPC in one way or another. Suggestions? |
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I'll make the assumption that both are on ethernet and they can communicate with each other. First of all, to truly simulate a PLC/5 with a ControlLogix, you need to setup synchronous input mapping and output mapping at the beginning and end of your ControlLogix scan, since Allen-Bradley went from synchronous I/O to asynchronous I/O when they created the ControlLogix. That actually helps here. So if your ControlLogix already has a bunch of ladder files that copy the inputs bit-by-bit over to internal coils at the beginning of the scan, you could potentially short out those routines and have those bits driven by some other source. In this case, why not use a message instruction in the ControlLogix read the input words from the PLC/5 and write them into the intermediate input bits. Also have it read the output words from the PLC/5, and then do a comparison between the PLC/5 outputs and the ControlLogix outputs you're testing. The ControlLogix is much faster than the PLC/5 so you should have lots of scan time for doing comparisons, etc. You could even setup another task for this. To do the comparison, I see two alternatives:
That prevents a lot of manual effort with rewiring because you only need an ethernet connection. I have never seen this done before, but a great idea to prove that the upgrade works!
(Nov 20 '09 at 01:34)
Jeremy Sluyters ♦♦
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The asynchronous updates with ControlLogix can definitely become problematic when upgrading older platforms. Another option that you could use would be an OPC linkage between the PLC5 input and a ControlLogix tag that would be used to test the new program. Kepware offers a product called LinkMaster that could be used for this shadow testing: http://www.kepware.com/Products/products_linkmaster.asp |
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Great Answer Scott! Just in case it wasn't obvious... To make the comparing quick, you could use the XOR (bitwise exclusive OR) to do it slot by slot. If the result is not equal to zero, a bit is different. With some clever indirect addressing, you may be able to do it with a loop in only a few rungs. |
