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CPU scan time imrovement with DP nodes numbers setting

Created by: motogateDP at: 11/10/2009 10:21 AM (9 Replies)

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11/10/2009 10:21 AM	Rate (0)
motogateDP Posts: 26 Rating: (0)	Hi guys, I have some doubts regarding profibus DP behaviour that I would like you to clarify me if possible. I had thougth that DP scan time could be improved using consecutive DP nodes for slaves because every DP master tried to exchange data with any possible slave from 1 to HSA address, so it took its time to get it so, but I have read in other FAQs that it does not behave so. Could you possibly explain me if this fact improve or not the CPU scan time? Regarding HSA, what is the significance of this setting on reducing scan time? Step 7 help says that this parameter is automatically calculated by itself and is not recommended to be changed. During my time using Step 7 (some years) I have never seen a value different that 126, if it is automatially fixed by Step7. Is it really important to be changed? Thanks in advance. Kind regards
motogateDP Posts: 26 Rating: (0)
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11/10/2009 12:49 PM	Rate (0)
Hulzhex Gold Member Joined: 8/25/2005 Last visit: 11/21/2022 Posts: 1161 Rating: (110)	Hello motogateDP, using consecutive adresses for the DP slaves has no effect on the scan time. Here some descriptions: Every active station (your DP-Master is an active station) at the bus checks wether there are other active station. Therefor it build internal some lists. The LAS (List of Active Station) contains all Adresses from the other active. The Gap between the one adress and the adress of the next active Station will be administrated within the GAP List. The station will check every adress within this list to check wether there will be a new activ station connected to the bus. How often this will be done could be defined within the bus parameters. A standard I know is 1 address within 10 bus cyclies. So reducing the HSA will reduce the addresses which must be checked to find new active stations. -> a new active station will get less time to start there communication. Within the profibus parameters you will get an idea how much time your bus cycle takes. This is a maximum! You could be shure that within this time the DP Master exchanged data with every slave minimum one time. Every CPU-cycle the CPU got once the actuall I/O data from the DP-Master. So this two cycle times are independent Best regards Sabine

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11/10/2009 5:29 PM	Rate (0)
motogateDP Posts: 26 Rating: (0)	Hello Sabine, If I understand you correctly, every new DP master checks, normallly every 10 bus cyclies, if there are adittional dp master within the DP master system. It checks the next possible active station with a node number equal to the previous attempt done and the Gap factor added to this figure. Performing this, it creates itself a LAS with all active statios with the master system. What I don't have clear is if every DP master starts communicating with its DP slaves when it has finished creating its LAS or it can make both opreations in paralell. You say that every bus cycle (it can last more than one CPU scan time), the DP master gets communication with every available DP slave, at least once. So every CPU scan time, the DP master has new data of some dp slaves and old data for the rest of slaves of the DP master system. Is that correct? Thanks.Kind regards
motogateDP Posts: 26 Rating: (0)
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11/11/2009 12:15 PM	Rate (1)
Hulzhex Gold Member Joined: 8/25/2005 Last visit: 11/21/2022 Posts: 1161 Rating: (110)	Hello motogateDP, hope that I could answer a part of your questions About the GAP list: An example: Your master got adress 2. The next active Station got PROFIBUS-Address 10. So your master will check the adresses 3 up to 9 wether there are new activ stations. The Station with the Address 10 will check addresses 11 and so on. About Starting communication: First of all the master listen to the bus to built his LAS (and his GAP list). Onlylistening. When this list is build he will be able to answer to a telegram where the active station with the lower address is asking about new stations. After this answer the station with the lower address will send him the "token". Now our new master will be able to start his communication with his slaves. About cycle time and so on: have a look at the typical cycle time at the bus and your CPU. Normally bus is faster than the CPU. So you allways will get very actuall data. The master will provide allways the most actuall data to the CPU which he got. Within this manual you will find a documention how it work. SIMATIC NET PROFIBUS Network Manual System Manual Maybe it is a little bit difficult to follow up my description. Sorry, I'm not native english speaker Best regards Sabine

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11/11/2009 2:25 PM	Rate (0)
motogateDP Posts: 26 Rating: (0)	Hello Sabine, ok, I got it. Last question. If there are 2 DP slaves (nodes 3 and 99) and 1 DP master within a master system, this master , Will it only try to access nodes 3 and 99 or it will try every possible dp slave between nodes 3 and 99? Kind regards.
motogateDP Posts: 26 Rating: (0)
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11/11/2009 5:22 PM	Rate (0)
J_Bell Posts: 5822 Rating: (411)	Hello, may I answer this question too? The point is, if you do not configure a DP Slave the DP Master, as an active station, will not try to configure this address. But this does not mean thatthis addresswill beexcludedform the GAP List. So it will be diagnosed and if there is an active or passive node this node will be entered in the LAS as soon as it is available. So every address between the local Address of the DP Master or any other active nodeand the next active station will be polled at a certain time, this time is defined by the GAP factor and the TTR. I hope I could make my point clear. Best regards J_Bell
J_Bell Posts: 5822 Rating: (411)
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11/11/2009 8:56 PM	Rate (0)
motogateDP Posts: 26 Rating: (0)	Hello J_Bell, thanks for your prompt answer. I guess I don't get the point yet. Please correct me if I am wrong. Every DP master (active station), when it is connected to a master system, waits till it gets the token from another active station (if exist), and after that, it polls the next node (gap factor) to check if it exists (active or passive) or it doesn´t. If it exists and is an active one, it is included in its LAS.If it is a passive one, it is not included. And so on till the HSA. The Gap list has to ve involved but..how?? Once it has its LAS created, it is ready to communicate to its dp slaves because it has the token at his moment (If there is only one master within the system, I suppose it does not lose the token as long as it does not find another active station so as soon as the LAS is created, it does communicate to all slaves connected ).About how it speaks to dp slaves , I am not sure if it goes straight to its projected dp slaves (by means of a certain list) or it scans every possible slave node. The DP master checks for a new active station every several bus cyclies (as Sabine said). If this is so, What if the HSA is 2 and the DP system is comprised of 1 DP master and 6 DP slaves (3,4,5,60,78,90)? Does the master in this case try to poll nodes only to HSA? How does it poll slaves nodes if their nodes are bigger than HSA? I hope you understand me about my doubts. I know profibus works, but sometimes you have to deal with some technical hitches and it is useful to know how it really works. Thanks for your time Kind regards
motogateDP Posts: 26 Rating: (0)
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11/12/2009 8:39 AM	Rate (1)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 3/26/2023 Posts: 2962 Rating: (1022)	motogateDP I know profibus works, but sometimes you have to deal with some technical hitches and it is useful to know how it really works. Hello motogateDP have a look at this (free) Web based animated training course, I have a feeling you will like it (it is well done and detailed): Profibus International Web based training 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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11/12/2009 10:15 AM	Rate (1)
Hulzhex Gold Member Joined: 8/25/2005 Last visit: 11/21/2022 Posts: 1161 Rating: (110)	Hello motogateDP, think you have to decide two things. Checking for active stations, LAS, ... This is a mechanism every active station do again and again. If your HSA is 2 and there is only one active Station at the Bus it will check addresses 0 and 1. The time the station hold the token is limited (sorry don't remember the exact english word at the moment ). After this time the active station will send a telegram to the next acitve station to "transfer" the token. Only 1 active station -> Telegramm to its own. During the time the station got the token he will communicate with his partners. In your case it will exchange date once with every of his slaves (adr. 3,4,5,60,78,90) . This is given by the hardwareconfiguration of your master. He wouldn't check any other address to exchange date via PROFIBUS DP. Waiting for your next question Best regards Sabine

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This contribution was helpful to 2 thankful Users J_Bell motogateDP

11/12/2009 12:27 PM	Rate (0)
motogateDP Posts: 26 Rating: (0)	Thanks a lot guys, now I got it clear. The profibus web training looks very nice.
motogateDP Posts: 26 Rating: (0)
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This contribution was helpful to 1 thankful Users fritz

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