profibus

[SpaceBee 0]

hey guys! i was wondering why did profibus adopted the master-slave exchange of data? it is the most efficient way? For example if i had a profibus-dp network with 5 stations (pc + 2 master + 2 slaves) all connected and with the termination resistors on at the end of the segment: What happens if some of the slaves devices turns off (say plc s7-200 shutdown)? this will interrupt the bus? I have read the profibus profiles and understand that is always a linear bus topology with token ring media access control, so if some trouble appears in any station obviously(because of linear bus) the bus will fail!......am i correct? How can i solve that? Did the master-slave method ensures some kind of safety data? Cheers to all!

[Behnam 0]

Hey Bee, Losing a slave on profibus will mess up the bus and will cause error. In a plc system you are not suppose to lose any slave because that will cause problem to your process too. In your case, if that slave is not important and it has to be turned off you can use a RS485 repeater. You will have rest of the line on one side of the repeater and that specific plc on the other side and this won't mess up your bus. Regards, Behnam www.sigmaprocess.ca

[Groo 7]

Not sure on the S7-200 range, with the 300-400 OB's can be used to stop the PLC stopping (OB82 I think), and SFB's can be used to determine node health and even switch off the node (SFC12) so the master will not fault. This is what I do with removable stations.

[SpaceBee 0]

Hey Behnam, thanks for the response! I understand the method including the rs485 repeater, it will assures me that the bus will not fail. But I was wondering what happens if a slave plc suddenly stop working, I mean: pc-----master (s7-300 or 400)------slave1------slave2------slave3-----slave4 ----: profibus cable The polling of the master will read first slave1, then slave 2, slave3 and slave4, repeat this cycle. So in the case that slave 2 suddenly shuts down, what happens to the access to the other slaves say: slave3 and slave 4. Will this interrupt the whole communication polling? or the master will ignores him and continue? Thanks for the help, Cheers!

[SpaceBee 0]

Hey Groo, thanks for the response! I will use s7-200 via em277 profibus dp module. So how can i check this node health in the master, to switch off the node? Do i have to configure it or it is automatically? Thanks for the help, Cheers!

[Behnam 0]

Bee, The CPU will keep polling and checking on the lost slave but it generates System Fault which causes the rest of the slaves to turn off their output till the issue is solved. Regards, Behnam www.sigmaprocess.ca

[JesperMP 2]

No no no no.If a DP slave is lost, then the master will get a bus fault, but it will not stop the bus. The master will continue to update the other slaves. In some cases, a faulty slave can cause the entire bus to fault. One such case is if the slave is physically at the end of the Profibus cables daisy-chain. If the cable is removed from this slave, or the slave is powered down, then it may cause the termination to fail. This may again cause the a complete bus fault. This is the reason why there are active bus terminators. This so the critical termination isnt dependent on any of the bus stations. Another method is to use Profibus repeaters. Repeaters are not necessary for DP slaves that are within the limits of one segment. The CPU of the DP master may repsond in different ways to a bus fault, depending on the functionality and configuration. A Siemens PLC as a DP master, a Profibus fault may or may not stop the CPU. If this is not the desired result (in most cases it isnt), adding the appropriate error OBs to the program will prevent the PLC CPU from stopping. Typically the error OBs OB82, OB85, OB86, OB122 are added to the program. See the online help for what each of these error OBs do. Edited 4 Oct 2010 by JesperMP