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s7 300 basics

Created by: ashwani at: 6/5/2010 10:47 PM (6 Replies)

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6/5/2010 10:47 PM	Rate (0)
ashwani Posts: 11 Rating: (0)	hi all, the program segment in stl is as follows network1: 1. a m 0.0 2. jc l1 3. a m0.1 4. jc l2 m0.0 =0 m0.1 =1 i don't know: 1.the value of rlo before step 1 2.the vaue of rlo after each step hlp aprctd thnx
ashwani Posts: 11 Rating: (0)
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6/6/2010 12:01 AM	Rate (2)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/16/2024 Posts: 12282 Rating: (2683)	Hello ashwani; RLO means "Result of Logical Operation". It is one bit of the Step 7 Status Word, and is used to evaluate every STL statement in your program (all code devolves to STL before becoming machine code for transfer to the PLC). When you write A M0,0 ... you are asking the CPU to proceed with a logicalAND between the RLO bit and the value of M0.0 and placing the value back in the RLO bit. You can manipulate the RLO bit with speciific STL instructions: if you wrote SET A M0.0 ... then you would have made sure that the RLO at the beginning of your code was equal to 1. Further reading suggestionsfor more information: From the STL manual: You can use one of the following instructions to change the result of logic operation (RLO): • NOT Negate RLO • SET Set RLO (=1) • CLR Clear RLO (=0) • SAVE Save RLO in BR Register http://support.automation.siemens.com/WW/view/en/18653496 See Frtiz' post in this discussion: https://www.automation.siemens.com/tf/WW/en/Posts/38369 　 Hope this helps, Daniel Chartier

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6/6/2010 12:26 AM	Rate (0)
ashwani Posts: 11 Rating: (0)	hi Daniel, your usual good self.my good luck. i have the manual with me. the other link that you gave is talking about some other confusion which was not mine. the following is from the def of A in the manual. "Achecks whether the state of the addressed bit is "1", andANDsthe test result with the RLO.":-stl programming manual the value of RLO at/after step 2 is 0 then as per the def cited above the value after step 3. should remain 0 even if m0.1 is 1 also what is the value of RLO at the start of a network.i think it should be 1 irrespective of the value at the end of previous network, if the analogy of a network in stl is a rung in lad and the power rail is the start point of a rung. there is a fault in my understanding but where? thnx agn
ashwani Posts: 11 Rating: (0)
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6/6/2010 3:02 AM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/16/2024 Posts: 12282 Rating: (2683)	Hello ashwani; I think that you understand the use of RLO pretty well. What you need to realize is that program flow is not controlled only by the RLO, but by all bits in the Status Word in combiation (it took me a while to understand this, documentation is not easily available with all details). Have a look at the article "What is the meaning of the BR bit in the status word in STEP 7?"in the following collective FAQ on creating Step 7program sequences, it might help: http://support.automation.siemens.com/WW/view/en/21554306 Hope this helps, Daniel Chartier

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6/6/2010 12:31 PM	Rate (0)
ashwani Posts: 11 Rating: (0)	hi daniel, thnx. i think the answer to my question lies in definition of jc "If the result of logic operation is 0, the jump is not executed. The RLO is set to 1, andthe program scan continues with the next statement.":stl programming manual so at step 3 the RLO starts with 1; your last post made me think in the direction of jc which i did'nt think will be fiddling with RLO,but it does. thnx for your guidance
ashwani Posts: 11 Rating: (0)	Last edited by: ashwani at: 6/6/2010 12:32 PM
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6/6/2010 3:53 PM	Rate (3)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 9/16/2024 Posts: 3020 Rating: (1054)	Hello ashwani allow me to addone supplementary piece of advice to Daniel's very usefulones and it relates to the "/FC" bit (First Check bit, which is bit0 inthe Status Word). The status of the "/FC" determines if - a new RLObegins (if /FC = 0) OR - if the instruction is to to be combined with the current RLO to form a new RLO (if /FC = 1). I know it sounds all a bit weird, but it isn't all that complicated and worthwhile trying to understand, so here are the rules: "=", "S", "R",all RLO and BR dependant Jumps (e.g. JC, JCN, JBI etc.)"close" the RLO (i.e./FC is reset to0), so that the next instruction creates a new RLO and does NOT depend on a previous RLO. Example: Assumend start condition: /FC = 0, RLO = N/A (because /FC=0) M7.0 = 1 M8.0 = 0 A M 7.0 //NEW RLO is formed, because /FC was zero // --> Result: RLO = 1 (M 7.0 is 1), /FC = 1 ("A" instruction sets it to 1) A M 8.0 //existing RLO combined with RLO of this instruction // Result:RLO = 0 (M 8.0 is0, which is "AND'ed" with existing RLO), /FC stays1 Asfor yourquestion of "the value of rlo before step 1", I must say that it will be whatever it will be (depending on the preceding logic). As you can see from the above though, it will NOTmatter as long as it was "closed" (i.e. /FC = 0). I hope this helps
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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6/7/2010 8:19 AM	Rate (0)
ashwani Posts: 11 Rating: (0)	thnx fritz, for the beautiful write up
ashwani Posts: 11 Rating: (0)
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