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9/12/2012 12:10 PM | |
Posts: 1547 Rating: (129)
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Hello, I guess this link will help you: http://support.automation.siemens.com/WW/view/en/36748706 " Consistent data: Data that belongs together with respect to content and which describes a process state at a specific point in time is known as consistent data. For data to be consistent it must not be changed or updated during processing or transfer." |
9/12/2012 12:54 PM | |
Posts: 50 Rating: (2) |
I read them befor but i stil don't know when and why to use every one of them |
9/12/2012 3:31 PM | |
Joined: 9/27/2006 Last visit: 5/2/2024 Posts: 12257 Rating: (2666) |
Hello mohamed; Let me try to give you a concrete example of when "consistent" data transfer is required in an application, based on Profibus-DP exchanges qith a MMaster drive: Operating Instructions: MICROMASTER PROFIBUS Optional Board There are 2 areas for data exchange described in the manual, one is the PZD (process data area) that includes the Control Word and the Speed Reference command word, and values you want to read from the drive to your PLC program (current, voltage...). The other is the PKW (parameter values) area that allows the user to read/write the drive parameters, one at a time. When using the PZD, you can first setup different lengths of the PZD area, minimum 2 words (Control word and Speed Reference, always required), maximum 10. You can write the Control Word alone, to send start/stop commands to the drive, or you can change the speed setpoint by writing a different value to the Speed Reference command at any time. You do not have to read/write these multiple words together, as a single unit, "consistently". But when accessing the PKW area, you have to build a 4-word telegram that will contain the read/write command, the parameter number, the parameter index (if required) and the parameter value (if this is a write operation). Once this telegram is built-up in the CPU (as part of a DB, for example), you must send the telegram as a single 4-word unit to the drive, or the drive will not recognize the command. You cannot write one word, then the other...it must come as "consistent " data over the 4 words. in this case, to write to the PKW area, you must use SFC15/SFC14 to write/read consistent data to the drive. I hope this example will be clear enough, Daniel Chartier |
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