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I/O addresses of freq. drives on profibus


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Posted
Hi all, I have 12 units (G120C) that I want to control via profibus. They are addressed from 2 to 13, with the CPU 315 at address 1 as master. I use the "standard telegram 20" for the communication. (Read 6 words, Write 2 words) Since I'd like to keep the IW and QW start addresses similar and at a low range, did I start with QW100 and IW100 for the first unit. The I/O addresses for the second unit starts at QW112 and IW112, but when I try to set the I/O addresses for the third unit will the software correct my typed IW124 to IW128. Four bytes higher. Same with the next two units, but not with the rest of them. All other I/O addresses for digital and analog I/O's are below IW100 and QW100 Can anyone please explain to me why this happens?
Posted (edited)
Possibly because you were "crossing the boundary" of the fixed Process image - see below (extracted from the manual) - The process image often does not comprise the whole I/O address area of the CPU. So, for example, CPU315-2DP (6ES7315-2EG10-0AB0) has a total I/O address area of max. 2048 bytes of inputs/outputs, but only a fixed process image of 128 bytes. If I/O address areas of a module are outside the process image, you must access these areas via "Peripheral addressing". The peripheral addresses are accessed immediately after addressing the addresses in the user program. Thus with "Peripheral addressing" it is always ensured that the actual value is read in immediately or an output is implemented immediately. This is particularly important for analog values and process alarms.. Edited by Moggie

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