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Forcing IO from an external software

Created by: mukeshm at: 6/18/2009 8:17 AM (14 Replies)

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6/18/2009 8:17 AM	Rate (0)
mukeshm Posts: 7 Rating: (1)	Hi, We're planning to use a simulation software called PICS which is used to simulate devices and use this to test our PLC logic. But for the simulation software to communicate to the S7-400 over DDE, we've had to use a wonderware driver built for this purpose. So far, we are able to force memory bits from the simulation software but are unable to force inputs as it seems that the PLC keeps forcing that to 0. Using Siemens forcing table, that same bit can be forced but we need a way to force this externally. I've declared a test input module in the Hardware configuration to ensure that it had no issue with not having addresses attached to a module. Does anyone have any experience with this or any ideas? Thanks!
mukeshm Posts: 7 Rating: (1)
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6/18/2009 8:33 AM	Rate (0)
naah Posts: 76 Rating: (8)	Hi, You should know that the inputs of PLC are read into "PLC input image" part of memory and the "PLC output Image" is transferred to outputs of PLC. It means, you can't force the inputs and outputs of the PLC, because every cycle the "input image" is read from physical inputs and "Output image" is transferred to physical outputs. So, if your software writes somethings in "Input image" or"Output Image" the memory is overwritten next cycle and you can't see the result. Hope this helps! Good luck!
naah Posts: 76 Rating: (8)
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6/18/2009 9:00 AM	Rate (0)
mukeshm Posts: 7 Rating: (1)	Thanks for your reply naah. It's fair enough what you're saying but the simulation software is continously writing a value to the Input so it should keep that value for subsequent scans of the PLC shouldn't it? The Siemens forcing table can bypass Actual IO, so there should be a way of another program doing it. All I'm trying to find out is if there is a user function or method that we can use to do the same thing.
mukeshm Posts: 7 Rating: (1)
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6/18/2009 9:23 AM	Rate (0)
naah Posts: 76 Rating: (8)	Hi, As you know, your software can't write the values faster than the PLC OS, isn't so? The cycle time of PLC is sometimes shorter than 1ms, and your software can't write values faster than it! So you should expect, your values to be overwritten. You have a choice. Use PLC memory Flags(M) instead of Inputs(I) and Outputs(Q) in your PLC program. In first lines of OB1 call conditionally an FC to read from Inputs into these flags and at the end of ob1 call conditionally an FC to write the values to Outputs. In the PLC program you must use the assigned flags instead of Inputs(I) or Outputs(Q) If you reset the call condition to every FC, you may simulate the inputs or Outputs to PLC program from your software Good luck!.
naah Posts: 76 Rating: (8)
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6/18/2009 9:52 AM	Rate (0)
mumbles Posts: 160 Rating: (16)	This might be a bit far fetched but you could try writing to inputs that exist outside the process image of your CPU. For the S7-400 the size of the image could be changed. Perhaps this would then stop the input image from being updated.
mumbles Posts: 160 Rating: (16)	Last edited by: mumbles at: 6/18/2009 9:58 AM
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6/18/2009 10:07 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 6/3/2023 Posts: 2966 Rating: (1025)	Further to mumbles suggestion you could also untick "Update OB1 process image cyclically" in the "Cycle/Clock memory" tab of the CPU properties (this"feature" is only available in S7-400's). Cheers Fritz
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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6/18/2009 1:16 PM	Rate (0)
Sydney Gold Member Joined: 1/4/2008 Last visit: 3/8/2023 Posts: 703 Rating: (96)	Hello, good suggestion is from Fritz, sorrynaah but your solution is a pretty complicated . At least it would be in oppositedirection - from simulation write memory bits andat start of OB1 copy them to input area(not rewriting the programm code to flags). Another possibility results from the fact that the CPU writes the process image only from assigned addresses in the HW config, so if you haven't any I/O module in your HW configuration of the simulation project then you could change the input area valueswithout problems. Regards Sydney
	Last edited by: Sydney at: 6/18/2009 1:22 PM Last edited by: Sydney at: 6/18/2009 1:17 PM
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6/18/2009 2:14 PM	Rate (0)
Krish Silver Member Joined: 5/24/2007 Last visit: 6/1/2023 Posts: 284 Rating: (27)	Hi Mukeshm Is there a particular reason why you are using PICS. The other solution would be to use WinMOD or SIMIT (from Siemens). These 2 softwares can simulate directly your profibus input and you can make the behaviour exactly like your input/outputs. The advantage with these softwares are that you can see exactly howthe processor will run in the field. You don´t have toorce any I/Os or any scan cycles. They also provide facilties to connect through Ethernet/OPC etc. It is worth a try in case you have not finalised on PICS. Krish

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6/18/2009 2:45 PM	Rate (0)
Paresh Posts: 116 Rating: (1)	Hi, According to me, any simulation software deals with reading inputs and writing outputs to internal memory of PLC only. They do not reach up to physical inputs and Outputs. Please correct me if I am wrong. Thanks and Regards Paresh
Paresh Posts: 116 Rating: (1)
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6/18/2009 10:31 PM	Rate (0)
Krish Silver Member Joined: 5/24/2007 Last visit: 6/1/2023 Posts: 284 Rating: (27)	Hi Paresh Sorry, no, the packages I am talking about (and there are more in the market) are much more efficient. You have to plug your profibus cable into the profibus card provided by these products in the computer slot. You have to import the hardware configurations from the respective PLC and the start simultating the input signals on the profibus based on the output signals you get on the profibus from the PLC. The system is pretty neat because you don´t have to change anything in your PLC software. Krish

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