DP cycle time is way too high comparing to theoretic value
Created by: DGD at: 1/5/2014 1:44 PM (1 Replies)
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| 1/5/2014 1:44 PM | |||||||||||||||||||||||||
Joined: 6/12/2012 Last visit: 8/22/2022 Posts: 1 Rating:
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Hi Guys, My current project is to log every telegram that passed between specific Slave node and Master. For this case Salve node is 30 and Master DP address is 2. In this DP Netwrok, there are 4 slave node including the 30 .All nodes are up and running. Nodes description is given below.
Bit rate was set to 1.5Mbit/s.The measured DP bus circulation time is surprisingly high as 20ms. I used a formula to estimate Profibus-DP cycle time (http://profibus.felser.ch/en/index.html?schatzformel.htm) , which gave reasonably ok result before, but not for this case. If I calculate DP cycle time from this formula it gives 2.9 ms. This is way too small comparing to measurement. I know that DP cycle time is independent of OB1 cycle time, which is measured to be 2-3ms. I use CPU 317-2 DP for this test, is this influencing the result ? Is that possible to see DP cycle time in hardware config ? I have attached my logging result to this tread which shows the same 20ms bus circulation time. I really appriciate if you could show me the problem why I see high DP cycle time. Thanks DGD AttachmentAtttachement.pdf (78 Downloads) |
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| 1/7/2014 9:00 AM | |
Joined: 10/7/2005 Last visit: 3/18/2025 Posts: 3042 Rating:
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Hello DGD I agree with yoru calculation, you should expect a DP update time (Ttr) of less than 3 ms in your application and have a few questions for you: 1.) Apart from the CPU and the 4 DP Slaves, do you have any otherdevices on the bus like HMI's? 2.) Are you doing any acyclic reads from your S7 program (e.g. usage of SFC48/49 or SFB52/53)? 3.) How exactly do you measure your DP "circulation time"? For example, do you set a DI on the DP Slave that causes a DO to be driven by the PLC program and thenmeasure the time it takes for the DO to go high after you switch on the DI? If so, you are not measuring the bus update time but intead the overall reaction time (have a look atChosing Decentral Peripherie for time critical application for more on what's come into effect then) Lastly, while Step7 will NOT tell you the actual Ttr for it's onboard DP port (it will if you use a Profibus CP), it will tell you what its estimates that the Ttr will be (see Profibus parameteres in HWconfig). One tool that I use regularily and that will tell you exactly what is going on the bus is PROCENTEC's ProfiTrace. I hope this helps |
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