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CPU goes into STOP Mode & Diagnostics Buffer says Cycle time Exceeded Twice

Created by: pavans at: 11/25/2014 2:09 AM (9 Replies)

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11/25/2014 2:09 AM	Rate (0)
pavans Posts: 81 Rating: (0)	Hello Friends, My CPU is getting in to STOP mode after some time &Diagnostics Buffer says Cycle time Exceeded Twice. I am confused with this error, My program has a lot of while statements . Is that a reason for this error Please help me out?? thanks pavan
pavans Posts: 81 Rating: (0)
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11/25/2014 6:15 AM	Rate (0)
William_B Posts: 5225 Rating: (1197)	Hi there. Careful in the way you write your programs. A PLC is not a PC. The PLC needs to run through all the code before a certain time period. If it does not do so, it will cause such a stop. This is to prevent the system to be crippled by endless loops. Look critically at your program. Remember the PLC needs to repeatedly scan through the whole program. Think of the program as a river for which you channel to your garden. You should have it flow continuesly. If the water gets held up, the banks of the channel will be over flow and flood. Greetings. w
William_B Posts: 5225 Rating: (1197)
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11/25/2014 7:44 AM	Rate (0)
pavans Posts: 81 Rating: (0)	Hello William , Thank you very much for your suggestion, I didn't do any complex programming. Here is what I did, Using for loop I incremented an array for 14 times. After this using while loop I checked for two conditions , based on the result I wrote four statements that needs to be executedI. Here is that code: "Row 1" := "Data_block_1".ROWS[0]; "Row 2" :="Data_block_1".ROWS[1]; "Row 3" := "Data_block_1".ROWS[2]; "Row 4" := "Data_block_1".ROWS[3]; "Row 5" := "Data_block_1".ROWS[4]; "Row 6" := "Data_block_1".ROWS[5]; "Row 7" := "Data_block_1".ROWS[6]; "Row 8" := "Data_block_1".ROWS[7]; "Row 9" := "Data_block_1".ROWS[8]; "Row 10" := "Data_block_1".ROWS[9]; "Row 11" := "Data_block_1".ROWS[10]; "Row 12" := "Data_block_1".ROWS[11]; "Row 13" := "Data_block_1".ROWS[12]; "Row 14" := "Data_block_1".ROWS[13]; IF "Data_block_1".Night = TRUE THEN FOR "Data_block_1".i := 0 TO 13 DO WHILE "Track_angle" - "Data_block_1".Angle > 8 DO "Motion Igniton" := 0; "Speed" := 0; "Forward (or) Reverse" := 0; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; WHILE "Track_angle" - "Data_block_1".Angle < 10 AND "Track_angle" - "Data_block_1".Angle > 1 DO "Motion Igniton" := 1; "Speed" := 1; "Forward (or) Reverse" := 0; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; END_FOR; Reverse Motion// ELSIF "DEFOCUS" = 1 OR "Data_block_1".Night = 1 THEN FOR "Data_block_1".i := 0 TO 13 DO WHILE "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] > -8 DO "Motion Igniton" := 1; "Speed" := 0; "Forward (or) Reverse" := 1; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; WHILE "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] < -10 AND "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] > -1 DO "Motion Igniton" := 1; "Speed" := 1; "Forward (or) Reverse" := 1; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; END_FOR; END_IF; Please let me know what is the wrong in my program. Thank you very much for your support, You really helped me a lot . Have a great day, Pavan.
pavans Posts: 81 Rating: (0)
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11/25/2014 8:13 AM	Rate (1)
William_B Posts: 5225 Rating: (1197)	okay.. let us extract a piece of code: WHILE "Track_angle" - "Data_block_1".Angle > 8 DO "Motion Igniton" := 0; "Speed" := 0; "Forward (or) Reverse" := 0; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; The trouble with this particular piece of code is the neither "Track_angle" nor "Data_block_1".Angle is updated within the WHILE-loop. This will constitute an endless loop when the first condition is made. I am sure you meant to use an IF statement. w
William_B Posts: 5225 Rating: (1197)
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11/25/2014 8:50 AM	Rate (0)
pavans Posts: 81 Rating: (0)	Thanks Yes, In the beginning I tought of using IF statement but if I use " IF" the statements inside will be executed only once & it will go to next condition. But I need to execute those statements until the conditions in IF structure are invalid. So I chose while loop. Please suggest me is there anyway that I can program without that error. Thanks, pavan.
pavans Posts: 81 Rating: (0)
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11/25/2014 5:18 PM	Rate (0)
William_B Posts: 5225 Rating: (1197)	pavans Thanks Yes, In the beginning I tought of using IF statement but if I use " IF" the statements inside will be executed only once & it will go to next condition. But I need to execute those statements until the conditions in IF structure are invalid. So I chose while loop. Please suggest me is there anyway that I can program without that error. Thanks, pavan. A WHILE loop only makes sense if the conditions to that loop is updated. Tell you what.. why don't you make a diagram like a flow diagram that explains what you want to do. At this moment we are (or at least I am) clueless to what your intentions are. Make us understand per diagram what is needed. A hand-phone picture of a hand drawn picture is okay by me. w
William_B Posts: 5225 Rating: (1197)
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11/25/2014 6:04 PM	Rate (0)
pavans Posts: 81 Rating: (0)	Hello William, Thank you very much for your assistance, Please find the picture for the Flow chart. Basically what I wanted to do is, I need to rotate a motor which rotates a panel forward & Backward to required angle. So I used while loop to check whether the difference between actual angle & required angle is less. I included relay outputs in its statements. I introduced another While loop just below that to rotate the panels slowly & followed by two other while loops for Reverse motion. So I tought that statements will be executed unless the condition becomes invalid. And I wanted to do this for 14 panels, So I used a for loop above the while loop. IF "Data_block_1".Night = TRUE THEN FOR "Data_block_1".i := 0 TO 13 DO WHILE "Track_angle" - "Data_block_1".Angle > 8 DO "Motion Igniton" := 0; "Speed" := 0; "Forward (or) Reverse" := 0; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; WHILE "Track_angle" - "Data_block_1".Angle < 10 AND "Track_angle" - "Data_block_1".Angle > 1 DO "Motion Igniton" := 1; "Speed" := 1; "Forward (or) Reverse" := 0; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; END_FOR; Reverse Motion// ELSIF "DEFOCUS" = 1 OR "Data_block_1".Night = 1 THEN FOR "Data_block_1".i := 0 TO 13 DO WHILE "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] > -8 DO "Motion Igniton" := 1; "Speed" := 0; "Forward (or) Reverse" := 1; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; WHILE "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] < -10 AND "Data_block_1"."Static Angle" - "Data_block_1".Angle["Data_block_1".i] > -1 DO "Motion Igniton" := 1; "Speed" := 1; "Forward (or) Reverse" := 1; "Data_block_1".ROWS["Data_block_1".i] := 1; END_WHILE; END_FOR; END_IF; Please let me know what should be done to execute the above conditions. Thank you very much for your support Have a great day pavan Attachment DSC_0225.JPG (410 Downloads)
pavans Posts: 81 Rating: (0)
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11/25/2014 7:49 PM	Rate (1)
William_B Posts: 5225 Rating: (1197)	There is a gap in the foundation on which you wish to build the code on. I suggest give the extract of the S7-200 PLC a read. Then return to me and tell me which while loops are required. w Attachment EXTRACT_S7200_System manual_200808.pdf (402 Downloads)
William_B Posts: 5225 Rating: (1197)
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11/25/2014 8:19 PM	Rate (1)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 12/12/2024 Posts: 15494 Rating: (2456)	pavan, W.B. is spot on suggesting you rethink how PC versus PLC work. In the PC world, you are the only thing running that you currently care about and to the exclusion of all other code running. Windows doesa good job of stealing time away to update the screen, capture mouse movement and keyboard entry, communicate over the network, and all manner of other things. In the PLC world, the PLC "scans" your program, top to bottom, then processes the inputs and outputs that changed after that scan. Next it scans your program top to bottom, and processes the inputs and outputs that changes after that scan. It continues this process until you stop the plc or remove power - or it thinks it has gotten into an infinite loop and then watchdog faults (cycle time exceeded). A while loop that waits for real-world time events to happen is certain to generate watchdog faults. All other I/O handling is halted during a while loop, and it is not appropriate for Siemens ( and most other ) PLCs. If you are doing PC based control, the while loop would have to be run ina separate process thread or it would have similar problems. I hope this makes sense. The PLC will run your "IF" conditions thousands of times during a real-world event like motion control or physical motion like pneumatic cylinder extend/retract which gives it time to notice it is moving and to notice it has reached its destination (assuming cylinder switches of some sort) - as an example of where a while loop mentally makes sense but "IF" makes more sense for a PLC.
	Last edited by: huggy_d1 at: 11/25/2014 8:20 PM science guy
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11/25/2014 9:42 PM	Rate (1)
William_B Posts: 5225 Rating: (1197)	@huggy_d1: My thanks to you for contributing and backing me up on this one. @pavan: After some thought I wish to explain something by an example (simile?). The following however does not negate my last post.. it enhances it. A PLC can be compared to high speed photography.. also called stop motion photography. High speed photography uses a very special camera that can take photos at a very high speed but is also able to record these for later analysis. A high frequency strobe (flashing light) is also essential for supplying the light. The overpowering light of the strobe has the effect of "freezing" the movement for the camera. In a way the PLC works like this. The "flash" for the PLC is at the start of the PLC scan. At that moment all the external devices (inputs) are read. That moment in time is frozen for that one PLC scan. In the science world where high speed photography is used, the photos are recorded an analysed photo for photo. You might have seen the movement of a bullet (fired by a gun) moving towards an apple. Because we know how quick the strobe flashes, we can measure the speed of the bullet by measuring the distances of travel from one photo frame to the next. In the PLC world it is a bit different. We do not have to record for later analysis. The PLC can process the data between each "flash". Inside the cycle, the PLC can make measurements and make decisions based on the "freeze frame" readings. The action of the repeated "strobe" (repeated PLC scan cycles) ensure that we can make decisions without needing to force repeat loops onto the coding. Practical example: [code]IF movement_needed AND NOT(forward_move) AND NOT(reverse_move) THEN direction := ( ask_target - read_position ) > 0 forward_move := direction == TRUE reverse_move := direction == TRUE END_IF IF forward_move OR reverse_move THEN distance := ask_target - read_position IF reverse_move THEN distance := 0 - distance END_IF slow_move := distance < small_distance IF distance < stop_distance THEN movement_needed := FALSE END_IF END_IF IF NOT(movement_needed) THEN forward_move := FALSE reverse_move := FALSE slow_move := FALSE END_IF[/code] The example does not include.. - how the cycle is started movement_needed := TRUE - handling of non-scheduled stops - security - corrections in target position while already in motion to handle direction change. What the code does show is the ability of the PLC to repeatedly scan the code while the movement is taking place. The above can also be repeated in a FOR loop to read an array of values, if required. Handling of overshooting the target is not handled in the strict sense. Be careful in how that is done as it may cause an endless zig-zag motion. --- I hope sincerely hope that you can recognise the value of this posting. Now please review your own work. I can see you have potential, but I feel you have jumped the gun a bit and is missing some baseline informations that is going to cause major issues later on. Please take a step back for a bit before continuing full steam. Greetings from the teacher.. w
William_B Posts: 5225 Rating: (1197)	Last edited by: William B. at: 11/25/2014 9:42 PM NOTE: expect error in the coding
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