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Modbus communication problem

Created by: Vedran_Bakic at: 5/31/2010 4:35 PM (6 Replies)

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5/31/2010 4:35 PM	Rate (0)
Vedran_Bakic Posts: 9 Rating: (0)	Hi, For aquisition of data on 12 powerstations I used S71200 CPU 1212C as a slave with communication module CM1241 RS485(3 and 8 pin) Modbus, for all 12 stations . As a Master we used on side of master RS485/Ethernet converter and then switch - SCADA. Communication cable is profibus. We have set the communication parameters to Baud rate: 9600 parity: 0 Response time : 100ms Flow control: 0. 150-200 meters distance from the farthest slave station to the master. Problem is that slave doesn't answer, I can't see any mistakein monitoring. I also used 120ohm termination resistor on both Master side and last slave side from the manual. Can someone advice me what to do?
Vedran_Bakic Posts: 9 Rating: (0)
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6/1/2010 1:34 PM	Rate (0)
little holmes Experienced Member Joined: 12/9/2009 Last visit: 4/5/2022 Posts: 156 Rating: (11)	Hello, under the following link you can find the S7-1200 manual. http://support.automation.siemens.com/WW/view/en/36932465 In chapter 6.3 you can find some information about modbus. Maybe it helps you. Best regards Little Holmes

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6/2/2010 8:57 AM	Rate (0)
SSE Posts: 45 Rating: (8)	Hello, Do master and slaver communicate wth a short cable? If so, then you must re-look your wiring topology and possibly use RS485 repeaters (6ES7972-0AA01-0XA0 for instance) to boost your modbus line. I would recomment to use native profibus connectors with terminators, not hand-made terminating resistors. The main point is 120ohm may not match profibus cable resistance better thanprofibus connectors do. Best regards
SSE Posts: 45 Rating: (8)
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6/3/2010 12:39 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 7/28/2023 Posts: 1525 Rating: (527)	Do the slave devices have any diagnostics you can access? Such as error counters, receive buffers. You need to clarify if the slave devices are receiving the modbus resquests correctly, as if they do not understand the requests, they will remain silient. The requests may be being corrupted en route, especially if the interface route is complicated (as in your case).
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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6/4/2010 9:17 AM	Rate (0)
Vedran_Bakic Posts: 9 Rating: (0)	Thanks for your reply, We have established communication, but we have another problem. We didn't use 120ohm termination resistors because network behaved in same manner when we use and when we don't. On one network where length is 150 meters, we have no problem with communication. But, on another network where length is 250m, there is problem in some cycles when reading measurements. In first two cycles there is reading parameters, but in third cycle we lose connection, and error is enabled with message like "slave is not recognized". Thanks in advance!
Vedran_Bakic Posts: 9 Rating: (0)
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6/4/2010 10:04 AM	Rate (0)
SSE Posts: 45 Rating: (8)	Hello, If you are not inclined to by additional active components (active RS485 repeaters), you have to make love with cables, topology and terminating resistors. Practically it's a hard task sometimes. Look here for SIMATIC NET PROFIBUS Networks manual, where you can find cable impedance on your data transfer speed to select terminators correctly. Be sure you are using the same cable through all the network. I would no recommend star topology in this case. Here is the last thread you also would like to observe: PROFIBus End of Line Resistor Value Hope this will help
SSE Posts: 45 Rating: (8)
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6/4/2010 12:58 PM	Rate (2)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 7/28/2023 Posts: 1525 Rating: (527)	Vedran_Bakic We have established communication, but we have another problem. We didn't use 120ohm termination resistors because network behaved in same manner when we use and when we don't. Terminating resistors are not strictly necessary. They are fitted to increase electrical noise tolerance and reduce refections on the bus. They are important if:- i) The transmission speed is high. ii) Poor standard cable is used. iii) The environment is electrically noisy. iv) The cable run is long. v) All/Any of the above. We are very lucky that the RS485 designers made the standard very noise tolerant and it is testiment to their skills that it works as well as it does without line terminators. Vedran_Bakic On one network where length is 150 meters, we have no problem with communication. But, on another network where length is 250m, there is problem in some cycles when reading measurements. In first two cycles there is reading parameters, but in third cycle we lose connection, and error is enabled with message like "slave is not recognized". Not sure what the exact cause of the "slave is not recognized" error is. What / where is this error being reported? Is it the master of the slave device that is reporting this error? It looks to me like something is corrupting the request. A possible cause of why it only fails on the third cycle might be that a buffer overrun is occuring and the slaves receive buffer is getting swamped. Remember, RS485 has no flow control, so the slave has no way of holding off the master when the buffer becomes full. You could try slowing down the masters poll rate (how often the request telegrams are sent). The fact that it works at 150m but not 250m might suggest that there is a noise issue. As a test, you could try slowing down the baud rate to see if that improves things. At that sort of distance, you certainly ought to be fitting terminating resistors. Pull-up / pull-down resistors might also help by returning the line to a known state before / after transmission. Here is a RS485 spec for information.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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