Industry Online Support
Technical Forum
Navigation
Rating 
Thanks 1
2 Entries
| 5/25/2009 11:55 AM | |
|
Posts: 45 Rating:
|
Dear all, I have problem with understanding SFC15 function. In RECORD parameterI specify data to be sent from PLC to the drive. These data, until they are passed to the drive,are written to memory space specified in LADDR. This memory space ( specified in LADDR)worksas a buffer. Am I right? In some topic in this post: /tf/WW/en/Posts/21895 there is a suggestion about writing control and status word to peripheral memory ( PQW256, PQW258). LADDR parameter allows using I, Q, M, D, L memory areas, so my problem is, data to be sent to the drive must go through PQ memory or no? Regards, gumis |
| 5/25/2009 12:37 PM | |
Joined: 9/27/2006 Last visit: 6/6/2023 Posts: 12036 Rating:
|
Hello gumis; To understand the use of SFC14 and SFC 15, you must understand the principle of "consistent data" and when you need to use it. Yes you can transfer a single word value to a drive over Profibus-DP without SFC15, using a Load/Transfer operation (MOVE instructionin Ladder) if this address lies in the PZD section of the drive (Process data). The drive will accept the new value and change the settings that the word point to (control word, speed setpoint). All PZD is defined as individual word units; data is "consistent over unit". However, if you are trying to write or read to a drive's parameters, in the PKW zone (Parameter Value), then you cannot write the parameter change word by word. This requires a telegram, built up on 4 words, as described in the MM4 manuals you can have access to. Without explaining the details of the words of the PKW contents, just understand that you must absolutely write those 4 words to the driveat the same time, as a single unit, for the drive to understand them; this is called consistent data transfer, "consistent over all"The drive will let you read the status of the parameters as a single unit of 4 words also, consistent data again. There is no Load/Transfer mechanism avaiilable for more than 4 bytes at a time, and SFC21 "BLKMOV" cannot operate in the Peripherical I/O addresses. SoSiemens providesSFC14 and SFC 15 to read and write consistent data over Profibus. Of course you may choose to use SFC15 to write to the PZD area of a drive; there is just the overhead cost of the SFC to take into account, but it matters little. I prefer to Load/Transfer the PZD valuers to the drive individually, I feel it guives me more flexibility. The question here is that you can choose, for the PZD, whether to use SFC14/15. For the PKW you have no choice. Hope this helps, Daniel Chartier |
This contribution was helpful to
|
|
Follow us on