Internet PLC Forum
General => Technical support => Topic started by: bongofury on May 14, 2009, 05:42:46 PM
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::) Hi All, we have successful bi-directional comm. with the f2424 via Modbus RTU. However when I change a value in DM[n] it holds for one scan and then reverts back to the original value. I see the value change on the LCD and in the monitor mode. I would like to use DM's as set point placements for PID control. Wrong application of DM's?
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You can use MODBUS RTU to change DM data, but if your program will write back an original value after every scan then what you see is only good for 1 scan!
Please check your program to make sure that it is not writing back to the same DM. To verify this you can pause the PLC and then make changes to the DM, then observe it using online monitoring. The data should be changed to what you have assigned it and stays there. If the moment the PLC exits the PAUSE mode and the data change back then you know that your program is responsible for this behavior.
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Support, you were right, it was something in my program. I simplified the program to a single input firing the {dCusf} with DM[1]=[n]. Worked perfect. Good call........Dave :)