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endless loop

Created by: Marciel at: 1/15/2016 1:56 PM (20 Replies)

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1/15/2016 1:56 PM	Rate (0)
Marciel Posts: 21 Rating: (3)	What is the best way to implement loop functions using STL instructions (jump) preventing the occurrence of endless loop and at the same time get confirmation of my loop code was executed completed? Code example: L #ZERO T #AUX_IDX OPN "Group_CMD" Loop: L P#DBX 0.0 L #AUX_IDX LAR1 L #ZERO T DBW [AR1,P#0.0] L #AUX_IDX L 1 +I T #AUX_IDX L 50 L #AUX_IDX <=I JC Loop
Marciel Posts: 21 Rating: (3)
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1/15/2016 2:09 PM	Rate (1)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365)	Hello , Marciel What is the best way to implement loop functions using STL instructions (jump) preventing the occurrence of endless loop and at the same time get confirmation of my loop code was executed completed? Code example: L #ZERO T #AUX_IDX OPN "Group_CMD" Loop: L P#DBX 0.0 L #AUX_IDX LAR1 L #ZERO T DBW [AR1,P#0.0] L #AUX_IDX L 1 +I T #AUX_IDX L 50 L #AUX_IDX <=I JC Loop Your piece of code with jump may work if you replace "JCN Loop" at the last line but Implementation of loops is easy in STL .It is compulsory to follow a syntax to avoid endless loops and putting the CPU in the danger of excessive cycle time and then entering the STOP mode.The syntax and structure you are using should be like: L 10 // initialize your loop LP: T # LOOPCOUNTER //Your task to be done iteratively L # LOOPCOUNTER LOOP LP If you fail to load the loop counter again before the LOOP instruction , this is the main cause of the endless loop.I suggest checking the samples in the online manual of the software you are working with. L L#1 //Load the integer constant (32 bit) into ACCU 1. T MD20 //Transfer the contents from ACCU 1 into MD20 (initialization). L 5 //Load number of loop cycles into ACCU 1-L. NEXT: T MW10 //Jump label = loop start / transfer ACCU 1-L to loop counter. L MD20 * D //Multiply current contents of MD20 by the current contents of MB10. T MD20 //Transfer the multiplication result to MD20. L MW10 //Load contents of loop counter into ACCU 1. LOOP NEXT //Decrement the contents of ACCU 1 and jump to the NEXT jump I hope this helps, hdhosseini
	Last edited by: hdhosseini at: 1/15/2016 2:11:25 PM Last edited by: hdhosseini at: 1/15/2016 2:25:39 PM Demo Channel on Youtube
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1/15/2016 4:17 PM	Rate (0)
Marciel Posts: 21 Rating: (3)	Thanks for your answer. I'm asking about this because one of our S7 300 CPU went to stop mode as you can see at the diagnostic buffer attached and this CPU is in operation (running without this similar fault) for over 9 years. Talking to same friends and siemens technical support by phone, this problem maybe cause by endless loop, according of the time set at the CPU properties . Analyzing the program we found at least 4 loop's like the example of the first post. Because this I'm thinking the better way to eliminate the probability of endless loop to these 4 cases found. Should not have at least more one event at diagnostic buffer advising about the "cycle timing" and than just at second cycle time exceeded the CPU should have stop? The OB80 is inserted at the project. Taking the opportunity, could you give an idea how can I identify what is wrong according to event 2...99 of 100 (diagnostic buffer attached) ? I checked if the program code looking for DB address (DBW 0 in this case?) used at the code and not fit at one DB and not found anything. Are there more things can I look for? Thanks for attention . Br, Marciel Fernandes Attachment Diagnose.txt (334 Downloads)
Marciel Posts: 21 Rating: (3)
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1/15/2016 5:37 PM	Rate (1)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365)	Hello again, Adding OB80 and downloading it to the CPU will not solve your problem because it only masks this error only once. It is a bit strange to find out such an endless loop occurred after a long time.My suggestions would be : Identifying all loops in your CPU and check if they are executed all the time or conditionally . Execute the loops and all related conditioned manually by hand on a printed version or use simulating module .When the error is identified, try to find a solution or share the code for our checks here. Event ID 16# 2522 that filled your diagnostics buffer , is an indication of a non existing address inside a global DB.It is a common problem while using indirect addressing .In can be identified easily by checking the "Stack" tab in properties of CPU after CPU entering the STOP mode. Other suggestion would be sharing your user program with more clues from your online test for more helping suggestions. I hope this helps, hdhosseini
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1/15/2016 6:44 PM	Rate (0)
Marciel Posts: 21 Rating: (3)	Thank you so much for answering me. I'll check all things that you said. Attached is the program that I upload at same day when CPU went to stop mode (the diagnostic buffer is this day too). If can you check too I'll be very thankful. Again thank you so much. Br, Marciel Fernandes Attachment program.zip (426 Downloads)
Marciel Posts: 21 Rating: (3)
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1/16/2016 4:28 AM	Rate (0)
Marciel Posts: 21 Rating: (3)	Hi hdhosseini, thanks for your attention. I started the code simulation using S7-PLCSIM, first I removed the OBs (80 and 121) that mask programing errors and then I started simulate the same situation of the day failure.I simulate sixth times, the first five were ok, but at the six the CPU went to STOP mode because of "Event ID 16# 2522". Different of the previous diagnostic buffer that I sent, now it shows the FC where the erro ocurred. Attached follow the piece of code appointed by the Stacks properties and the diagnostic bufferof the simulation. Please, give me some ideas about what is wrong and maybe how can I improve the program. At the same time I'll study more here to try to solve this problem and looking for more. Thanks a lot. Br, Marciel Fernandes Attachment 1º ERROR.rar (317 Downloads)
Marciel Posts: 21 Rating: (3)
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1/16/2016 6:39 AM	Rate (1)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365)	As I guessed before this is a conditional loop i.e it is executed in a certain condition and this is the reason why you have it recently.I have checked FC53 and I think your loop implementation is the main cause of the problem we are discussing.To find out more check #temp1 and DB205.DBW92 in uploaded version (negative value generation in some cycles). To solve the problem , we should know what exactly should happen there and try to write it again in a way that CPU will run without trouble.
	Last edited by: hdhosseini at: 1/16/2016 6:40:32 AM Demo Channel on Youtube
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1/18/2016 12:51 PM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 10/21/2024 Posts: 1547 Rating: (537)	You are now realising that loops in PLC should be treated with extreme care. All loops in PLC code MUST repeat MUST have a valid loop exit condition that is GUARANTEED to be satisfied. The normal way this is achieved is having an exit condition that is a counter greater than / less than a limit and the counter incremented / decremented within the loop. Be very careful of having an exit condition that is linked to a external / IO value (e.g loop until temperature > 36.4) as the condition may not be satisfied in a reasonable period of time (or indeed at all) and the result will be a CPU stop. The number of interations of the loop should also be considered as too many iterations = too much time in the same block = CPU stop. If you have a loop that requires a great number of iterations (for example setting default values into a large array), then break the loop up into smaller chunks and process the chunks in separate scans.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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1/19/2016 1:33 AM	Rate (1)
zzVss Gold Member Joined: 5/30/2013 Last visit: 9/17/2024 Posts: 112 Rating: (37)	Hi Mr, smiffy. smiffy The number of interations of the loop should also be considered as too many iterations = too much time in the same block = CPU stop. What exactly means? Many time repetitions only one the loop in the block , many the loop in some the block or one the loop but large the array. Where find some data for restriction for number loop in block. What are You large experience suggestion. Best regards zzvss

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1/19/2016 9:22 AM	Rate (2)
Piotr.M Platinum Expert Joined: 6/24/2013 Last visit: 11/18/2024 Posts: 1652 Rating: (426)	Ref loop time PLC monitors how long does it take to do one scan (i.e. execute OB1). Should it take more than time pre-set in HW Config PLC goes to STOP. This means the total execution time must not be more than pre-set threshold (regardless if it is causes by one loop or many loops executed one after another). The limit of iterations depends on - how many instructions are within loop (and time needed for one loop pass) - what operations (floating point needs longer time than bit operation) - CPU - maximum scan time watchdog setting

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This contribution was helpful to 2 thankful Users smiffy zzVss

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