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M Flag Behaviour - How Does The Program Scan?

Created by: RandomCoder at: 4/13/2011 8:50 PM (6 Replies)

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4/13/2011 8:50 PM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 7/8/2024 Posts: 50 Rating: (8)	Hi LOGO guru's, I have a problem that I hope someone can explain. It might be a simulation bug but I've not got the hardware available to test it on. Basically I'm working on a TD menu structure that I intend to post so that other users can benefit from it. The problem appears to be that the M flagcoil is set but the contacts for the M flag don't get updated until a full scan later. However for functions likethe latch, thecoil is set and the contacts are also updated in the same scan. This is causing me a problem. Attached is a sample program that demonstrates this. The output Q1 should not come on until F1 is pressed and released for a second time. Is this just a simulation error or does it actually work like this in practice? I have a smaller menu structure workingusing a work around but wondered if I am actually doing something wrong? I have previous experience with Allen Bradley PLC's and have recently started with the larger Siemens S7-400 which work slightly different in terms of how the program scans and updates I/O and flags. Thanks, RandomCoder Attachment M Flag.zip (473 Downloads)

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4/14/2011 7:52 AM	Rate (0)
Andres [Str] Posts: 153 Rating: (16)	Hi_RandomCoder Cursors work as the pushbutton in a reality. Therefore it is required to make changes to the program. I have made cursors as it will work really. Look att- Attachment M Flag_change.zip (497 Downloads)
Andres [Str] Posts: 153 Rating: (16)	Last edited by: Andres [Str] at: 4/14/2011 7:53 AM
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4/14/2011 8:47 AM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 7/8/2024 Posts: 50 Rating: (8)	Good morning Andres, Thank you for your reply but I'm afraid that you has misunderstood the problem. The problem looks as though an M flag contact takes one scan more than a function block to change. My program was just an example to demonstrate the strange behaviour. It takes just onepress on F1toturn output Q1 on. However it should wait until the F1 key is released after the second press. Attached file is my work around showing how I intended itto work, but the question is why do M flags operate differently? Is it just a simulation bug or does it also occur on the LOGO? (Hopefully I will get chance to test later today on a proper device). Kind regards, RandomCoder Attachment Example 2.zip (431 Downloads)

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4/14/2011 10:55 AM	Rate (0)
LOGO!Barbarian Posts: 173 Rating: (95)	Hi RandomCoder! From what I understand above, you are expecting the M (marker) flags to behave slightly differently to what you are used to on AB PLC's and even the bigger Siemens PLC's Essentially, the M flags in LOGO! are intended to 'hold' the status that was input into them in one cycle for one cycle longer before they will change. They are useful if you have some program functionality that it recursive (an output goes back to a previous block input). Here you would put an M in after the output and then go back to the input. A good way to demonstrate how the M works is to simply put down an inverter (NOT block), followed by an M and then 'recurse' back to the input of the NOT block. This will cause the M to toggle between on/off on each 2nd cycle, becuase it is delayed by 1 cycle as it is supposed to. If you put an output Q on this 'loop' on a physical LOGO! 24, 24o, 24C or 24Co (with transistor outputs), you could watch the cycle speed of the program on an oscilloscope. As you added other program funcionailuty & more blocks, you would see this cycle speed slow down. The highest I ever saw with no other functionality, was about 853 Hz On the matter of simulation..... When simulating on a PC, this is done in a Java environement and its speed (and sometimes behaviour) is dependent on the PC platform & performance. The great test of all LOGO! programs is when they are in a physical device! Let us know how it goes when you do get around to doing a download! Cheers
LOGO!Barbarian Posts: 173 Rating: (95)
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4/14/2011 4:01 PM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 7/8/2024 Posts: 50 Rating: (8)	Hi, thanks for the post. I can confirm that the simulation works the same as the device and it is nota bug. I'm still a little surprised by how it operates as this is different to any PLC I have programmed before (I've also worked on Mitsubishi and Omron PLC's to a lesser extent). Usually when programming in ladder the flow works from left to right and top to bottom updating the logic as it completes each rung, Memory bits and I/O contacts are updated as the scan moves from one rung to the next. the physical inputs and outputs are then refreshed at the end of each scan. The function blocks in theLOGO appear to obey this rule but the M flag appears to lag behind by one scan as shown in my screen shot captured whilst running in step mode. I'm not stuck as I will simply use another latch instead of the M flag but it just seemed like a strange thing to do and so thought that I would ask the question. Kind regards, RandomCoder (PS. I'll try and post my TD menu template in the next couple of days ) Attachment Screen shot of M flag behaviour.JPG (574 Downloads)

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4/14/2011 9:17 PM	Rate (0)
Toeternietoe Platinum Member Joined: 12/27/2006 Last visit: 8/18/2024 Posts: 1197 Rating: (113)	On the bigger PLC systems you can a M (Marker) or an Output etc several times in use. P.E: Line 1 M1 = ON Line 2 M1 = OFF Line 3 M1 = ON Line 4 M1 = OFF Result, the last line is working: M1 is OFF Thats not possible on the LOGO! If u want to use a M... several times, you have to use a AND or OR etc. blocks

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4/14/2011 9:37 PM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 7/8/2024 Posts: 50 Rating: (8)	Thanks Toeternietoe, As you have seen, I managed to work out what was occurring but it just seemed a little strange.Now that I know this is the way a LOGO works I will obviously bear it in mind when programming. There are often times when I program that rely on the way in which the program scans, and as can be seen by my example I fell foul due to how the LOGO updates its flags. It's all a learning curve and everyday is a new challenge. I'm certain that I'll be back asking more questions in the not to distant future. Regards, RandomCoder

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