Communication / Networks
Speed up profibus communication
Thread Starter: Oscar_83PL Started: 8/2/2012 8:40 AM Replies: 5
8/2/2012, 8:40 AM
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Dear siemens fans,
My quesstion is what You do if You want to communicate (ex. VFDs) 30 slaves with one master in profibus on 9.6kb?
In my math calculation I get the transmit cycle ~2sec.
Correct me if I am wrong. However If I undarstand right When I want start 5 slaves in the same time in fact there can be 2 sec delay between each other.
How do You cope with such problems in Yours projects?
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8/2/2012, 11:47 AM
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Hello Oscar_83PL
Correct me if I am wrong. However If I undarstand right When I want start 5 slaves in the same time in fact there can be 2 sec delay between each other.
Each Profibus scan starts by the DP Master sending the outputs to the DP Slave with the lowest node address, receiving back its inputs, then doing the same for the next DP Slave and so forth until all DP Slaves have been polled.
If you have a bus scan of some 2 seconds there will indeed be around 2 seconds difference between the first DP Slave and the last DP Slave being polled.
How do You cope with such problems in Yours projects?
You don't, instead you remove the problem by changing your Profibus baudrate to a speed that gives you the update time your application demands.
Question: Why do you want to use the slowest Profibus baudrate (9.6kbps), instead of the more typical ones of 500 kbps or 1500 kbps?
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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8/2/2012, 12:31 PM
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The baud rate is limited by profibus segment length.
I can rise baud rate but then have to use repeater PB to shorten the segment length. Am I right? No other solutions?
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8/2/2012, 12:53 PM
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What are the profibus segments lenght?
BR Hristo Mihalev
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8/2/2012, 3:13 PM
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Segment length is something about ~500m
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8/2/2012, 3:17 PM
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Hello Oscar;
Look up the table correlating segment length and bus speed in the following FAQ:
What should you watch out for when using the RS485 repeater in PROFIBUS and MPI bus systems?
If your segment length is more than 400 m (max. distance for a bus speed of 500 kB), you can still use speeds of up to 187,5 kB, which should be an improvement on your present situation.
If however you need higher bus speeds for the same total distance then you should think about using RS-485 repeaters. So for 500 kB (in this network) you would need 2 segments (1 repeater).
We have used them in many projects without problems and they work very well; the only hastle sometimes is to provide them locally with 24 VDC.
Hope this helps,
Daniel Chartier
=== Edited by dchartier @ 8/2/2012 10:01 PM [GMT ] ===
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