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Is it ok to use P_Trig instruction in an FC?

Created by: chriswu at: 1/24/2013 1:18 AM (2 Replies)

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1/24/2013 1:18 AM	Rate (0)
chriswu Posts: 8 Rating: (1)	I created an FC with the intention of calling it many times in OB1. One of the instructions in this FC is P_Trig. The edge memory bit of P_Trig is defined as a TEMP variable in the FC. My understanding is P_Trig compares the current signal state with the previous one. Therefore it needs an undisturbed memory bit to store the previous signal state. FC doesn't have memory so P_Trig will not work in an FC. However, simulation in S7-PLCSIM shows it works. On page 2089 of the system manual (Step 7 Professional V11.0 SP2 System Manual), it says "The address of the edge memory bit must not be used more than once in the program, otherwise the memory bit is overwritten. This step influences the edge evaluation and the result is therefore no longer unique. The memory area of the edge memory bit must be located in a DB (static area with an FB) or in the bit memory area." Can someone please tell me if P_Trig can be used in an FC?
chriswu Posts: 8 Rating: (1)
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1/24/2013 9:47 AM	Rate (2)
fritz Diamond Member Joined: 10/7/2005 Last visit: 11/1/2024 Posts: 3025 Rating: (1055)	Hello chriswu first of all, welcome to the Siemens Forum, let's see if we can help you: To start with, Yes, you can use P_Trigin an FC, just not the way you do it. The "Golden rule" for usage of TEMP variablesis: You must initialise them (i.e. WRITE to them before you READ fom them). If you use a TEMP variable as the memory forP_Trig, you do violate this rule. Depending on how you call the FC, it may though stillwork (I'd say unfortunately in this case as it can cause confusion) and you'll find the reasons for this explained in the thread Latching by temporary varibale inside FB or FC- recomended? The solution is to create an IN_OUT variable for the FC, assign an M or DB bit to it when you call the FC anduse it inside the FC as the memory bit for P_Trig. 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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1/24/2013 4:22 PM	Rate (0)
chriswu Posts: 8 Rating: (1)	Thanks, fritz! Now I understand it was just luck that my program worked because that TEMP bit was not overwritten by some other FCs or FBs. In the future I will stick to the "Golden Rule". I just started learning Step 7 this week and your detailed explanation is much appreciated.
chriswu Posts: 8 Rating: (1)
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