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Step 7 first scan bit ?

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#1 Guest_Arne_*

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Posted 20 January 2005 - 05:45 PM

I am using a plc tutor with some programming examples. (Not a Siemens book.)

The programming examples is using a "first scan bit" setting the state of the program at the initial start up. In this way bitmemories etc be set to the wanted values for a start up condition.

Is it possible to set up such a "first scan bit" for a s-7 300 / Step 7 5.2 ?

#2 Guest_Arne_*

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Posted 20 January 2005 - 05:54 PM

By the way - the book is this free e-book mainly based on ladder programming:

http://claymore.engi...ckh/books/plcs/

#3 Guest_Arne_*

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Posted 20 January 2005 - 06:10 PM

I found this info reg the first scan bit. Don't know if this is an ok answer ..

http://www.plctalk.n...pic/8412-1.html

#4 DesertDog

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Posted 22 January 2005 - 01:22 PM

There is a OB100 in Siemens that only runs on the first scan before starting to cycle OB1. This is intended for all such setup functions instead of using a first scan bit. I prefer the style used in most other PLC's so my OB100 only has:

SET
S     "First Scan"
S     "Always On"
R     "Always Off"
SAVE

and the last line in OB1 is

SET
R     "First Scan"
S     "Always On"
R     "Always Off"
SAVE

This can also be done in ladder but it is quick and easy in STL so it's one of the few instances of STL I use.

Now if there was a way to get Last Scan such as GE has.
We all live in a yellow subroutine
Glenn

#5 Guest_Arne_*

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Posted 22 January 2005 - 05:22 PM

Thanks a lot for your answer. I actually found out a part of it myself using a physical address and Ladder, but your soulution for the allways_on allways_off ant the first_scan and use of symbolic adress were fare much more elegant ! I will be using this from now :clap:

#6 HoaiThi

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Posted 28 January 2005 - 05:08 AM

Siemens is so difficult to use.

Omron is very good. You can do anything easily with CX-prorammer.
.


Thanks :-1

#7 Arne

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Posted 28 January 2005 - 06:01 AM

I do agree that my impression of Omron is that it is a lot easier to get started with.

One thing about Siemens, I think, is also that things are explained in a more difficult way than it had to be, trough manuals, books, etc. Siemens can also be easy if explained (My opinion).

Have tried Omron with Syswin, but never the new CX.

Does it contain a simulator, so you can train programming on your laptop without the PLC ?

Best reg Arne.




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