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MODBUS RTU (MB_SLave error - Bad CRC)

Created by: Starka at: 7/4/2014 9:02 AM (11 Replies)

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7/4/2014 9:02 AM	Rate (0)
Starka Posts: 8 Rating: (0)	I have problem with MODBUS RTU RS485 communication. I use S7 1200-1214C as Master with one modul CM 1241 RS422/485. On Slave side are PAC3100 and S71200-1212C with CM 1241 RS422/485. Every 500ms I send message to PAC3100 and PLC 1212C. Communication with PAC3100 is perfect, there is no error.After some time (2-3 hour) there is an error at communication with PLC 1212C, (MB_Slave_DB, Bad_CRC_Count=1) and Slave PLC do not Respond on Master request anymore.Communication with Slave PAC3100 is still ok. Does anyone know, what the problem could be here? Must I use 2 modul CM 1241 RS422/485 on Master side (for every Slave one modul)?
Starka Posts: 8 Rating: (0)	Last edited by: Starka at: 7/4/2014 9:51 AM Last edited by: Starka at: 7/4/2014 9:05 AM
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7/4/2014 10:00 AM	Rate (1)
Kaulquappe Posts: 3093 Rating: (324)	Hello Starka, it is possible to use up to 31 slaves with a RS485 network. Did you insert the terminating resistors? If not, I recommend to add them. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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7/4/2014 10:07 AM	Rate (0)
Starka Posts: 8 Rating: (0)	HiKaulquappe, Yes I am insert the terminating resistors. 10K Ω to +5 V on B, PROFIBUS Pin 3 10K Ω to GND on A, PROFIBUS Pin 8 Best Regards
Starka Posts: 8 Rating: (0)
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7/4/2014 10:46 AM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 11/20/2025 Posts: 1561 Rating: (540)	I think Kaulquappe was referring to the terminating resistors on the RS485 modbus interface, not the profibus interface. The terminating resistors should be placed between the A and B transmission lines at both slaves that are at the end of the physical extremes of the network. Their value should be the approximate resistance of the cable, but NEVER to exceed 200 Ohm. I generally use 120 Ohm resistors.
	Last edited by: smiffy at: 7/4/2014 1:22 PM Spelling and grammar 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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7/4/2014 11:28 AM	Rate (0)
Starka Posts: 8 Rating: (0)	I tried with this option resistor between A and B. I am not understund why only reset Slave PLC can enable comunication if comunication ones stoped?
Starka Posts: 8 Rating: (0)	Last edited by: Starka at: 7/4/2014 11:33 AM
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7/4/2014 11:46 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Starka, please tell us the error numbers of master and slave. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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7/4/2014 12:02 PM	Rate (0)
Starka Posts: 8 Rating: (0)	MB_Slave Status=0000 NO ERROR...?!?!? Only change when communication is not ok is that it MB_SLAVE_DB atribute Bad_CRC_Count=1 MB_Master Status=0000, Master still successfully communicat with PAC3100
Starka Posts: 8 Rating: (0)	Last edited by: Starka at: 7/4/2014 12:04 PM
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7/4/2014 2:12 PM	Rate (0)
Starka Posts: 8 Rating: (0)	I think I found and solved problem... 8083 Buffer overflow (Point-to-point (PtP) communication Status) The CM or CB returned a received message with a length greater than the length parameter allowed I
Starka Posts: 8 Rating: (0)	Last edited by: Starka at: 7/4/2014 2:13 PM
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7/10/2014 8:28 AM	Rate (0)
Starka Posts: 8 Rating: (0)	Can I reset MB_SLAVE_DB and SLAVE RX Buffer ???
Starka Posts: 8 Rating: (0)
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7/10/2014 10:52 AM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 11/20/2025 Posts: 1561 Rating: (540)	Receive buffer overflow is not normally an issue that would expect to see as modbus slave devices are normally quite slow in their data transmission and the data packets are usually quite small. Could you try requesting a smaller amount of data and see if that fixes the issue? That would at least demonstrate if the issue really is a receive buffer overflow. If it is a receive buffer overflow that is the issue then you could try a few things to fix it. 1) Lower the baudrate. 2) Reduce the data packet sizes (for example request the data in multiple smaller chucks instead od it all in one lump). 3) Run the receive blocks in a faster OB. 4) Convert the interface from RS485 to RS422 and then use the hardware flow control handshaking (RTS / CTS). Note this will only be possible if the interface is a point-to-point interface not a multi-drop network.
	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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