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Profibus message frame

Created by: gumis at: 6/19/2009 3:25 PM (15 Replies)

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6/19/2009 3:25 PM	Rate (0)
gumis Posts: 45 Rating: (0)	Dear all, I connected CPU315-2DP to Simoreg 6RA70 via Profibus DP. The converter is only slave to the bus.Both probes of oscilloscopeare connected between 3 and 8 pins of the plug made in RS-485. They are on both ends of the cable. On the screen of oscilloscope there are cyclic portions of bits. These portionshave equal lenghts and the interval between them is constant, more then less. According to my knowledge all data in Profibus DP are transferred without gaps between indyvidual characters. I use PPO3 (2 words of data). There should be also addreesses of master and slave, starting delimiter etc. The problem is, why I these portions are not longer? One portion is captured and shown in attachment. Best regards, gumis Attachment frame_38.JPG (470 Downloads)
gumis Posts: 45 Rating: (0)
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6/19/2009 4:47 PM	Rate (0)
Paresh Posts: 116 Rating: (1)	Hi, Can you tell me at what baud you are communicating between DP master and slave? Regards Paresh
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6/19/2009 10:39 PM	Rate (0)
gumis Posts: 45 Rating: (0)	Hi 12Mbit/s. Regards, gumis
gumis Posts: 45 Rating: (0)	Last edited by: gumis at: 6/19/2009 10:40 PM
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6/20/2009 2:24 AM	Rate (1)
Anchobi Posts: 139 Rating: (25)	The image of the data you posted looks like a token exchange message.
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6/20/2009 9:12 AM	Rate (0)
Paresh Posts: 116 Rating: (1)	Hi, I also thought in same manner. At baud rate of 12 Mbps it is not posible to capture frames or bytes on scope. Another point is in Profibus, there is continious token passing between master. If there is single master within network, it will continiously pass token to itself and this frame is very small comprises of three bytes only. If you want to really analyse Profibus bytes or frames, you need to use Profibus analyser such as Profitrace. Thanks and Regards Paresh
Paresh Posts: 116 Rating: (1)
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6/20/2009 10:58 AM	Rate (0)
gumis Posts: 45 Rating: (0)	Paresh Hi, If there is single master within network, it will continiously pass token to itself and this frame is very small comprises of three bytes only. Does it mean the master passes token to slave without influencing the slave and thenthe tokencomes back to the master? Do you think I would be able to observe something more for 9.6kbit/s? Regards, gumis
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6/20/2009 11:24 AM	Rate (0)
Paresh Posts: 116 Rating: (1)	No, Master passes token to itself if there is no any other master present within network. Master never passes token to slave. Please find the attached image of Profitrace software in which I have highlated such frame. As far as 9.6 Kbps speed is concerned, I still doubt whether you will be able to see exact frames or bytes and I do not know whether Profibus communication is possible at 9.6 Kbps Thanks and Regards Paresh Attachment Profitrace.zip (397 Downloads)
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6/20/2009 1:35 PM	Rate (0)
Anchobi Posts: 139 Rating: (25)	At 12Mbps one bit is about 83.3 nanoseconds. So from your image Gumis you could roughly work out what the actual bits are (at that scale idont know how acurately though). But it would help to slow the bus down some. When decoding, remember also that a profibus byte is 11 bits long. But i dont see why you want to do it so painfully this way. As paresh said earlier, Procentec's Profitrace makes all this so easy and for the time you commit to decoding it would be cheaper to get this kit. what is it that you are trying to acheive anyway?
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6/20/2009 2:22 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 1/24/2025 Posts: 3032 Rating: (1059)	Hey guys 9.6 Kbps is a supported Profibus DP baudrate. The only difference thatgumis would see at this speed though is a change in the time base of his oscilloscope recording (apart from possibly aslightly "cleaner"square waveform). For your info gumis you will "see" at least 4 messages in your setup per bus cycle which are (am I correct in assuming that this still comprises of one PLC and one PPO3 drive and nothing else?): 1.) SD2 (request) message from PLC to drive (outputs to drive, this will be 13 Bytes long all up with 2 Words of net data for PPO3). 2.) SD2 (response) message from drive toPLC (inputsto PLC, this will also be 13 Bytes long all up with 2 Words of net data for PPO3). 3.) SD1 message (aka FDL status request, PLC "pings" for another master, 6 Bytes long). 4.) SD4 message (token passing, in this case PLC passes token back to itself, 3 Bytes long) For completeness, every so often (e.g is SYNC or FREEZE commands are used or Master enters CLEAR state or periodically at a rate of 3*WDT) you'll also "see" aso called Global control message on the bus (coincidentally also 13 Bytes long and of SD2 type) Nothing else to add since Anchobi said it all and good on him for recognising the captured (Oscilloscope) message as a token passing one (I concur with his assertion as it is too short to be anything else). Cheers Fritz
	Last edited by: fritz at: 6/20/2009 3:43 PM 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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6/20/2009 5:25 PM	Rate (0)
gumis Posts: 45 Rating: (0)	My researches are not commercial. I am student and I try to learn as much as possible. Maybe it will help me to find better job. The univ. has not got the Profitrace. Yes, there are just CPU315-2DP and Simoreg 6RA70 on the Profibus DP bus. I do not understand two problems: 1. How the PLC passes the token back to itself? If PLC sends the token on the bus it has to go to slave in my case. I can not imagine this. 2. I have seen the cyclic frames with equal lenghts on oscilloscope. Maybe,I will make the tests again for 9.6kbit/s on Monday and I will try to confirm this. Best regards, gumis
gumis Posts: 45 Rating: (0)
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