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How to clear previous data when modbus slave does not exist?? (Modbus RTU Protocol)

Created by: Srinath40 at: 12/31/2012 8:17 AM (17 Replies)

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1/8/2013 6:25 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Hello Sir, PLC receives Error 08 30H in Send FB Block Status. Example, As per Program, when 1st Query of 1st slave results error , PLC will send 2nd Query of 1st slave. Totally 32 Queries .Once 32 Queries completed ,PLC will send again 1st Query of 1st Slave.That time Done bit may come.Total time is approx 16 Seconds to complete 32 Queries. At Present I modified the program if error bit occurs continuosly for 60 seconds ,make that corresponding Slave data to Zero.It is working fine. Before 60 Seconds if Done Bit comes (Slave is ON),it will make reset the error bit (No need to make data zero). But Is there any way to reduce 60 seconds?? Because slave might be OFF by 7.00 PM. But till 7.01 Old data's are saved in SCADA. After 7.01 only it will go to zero .Our client wont accept this. Is there any way to make the communication accurately?? Warm Regards, Srinath
Srinath40 Posts: 31 Rating: (0)
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1/9/2013 12:28 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, error 0830 means, that the slave didn't answer within the monitoring time. There arethreereasons why this may happen: - the slave needs longer than the monitoring timeto generate the respond - the slave is in a state in which it can't answer - the request of the S7 was disturbed due to electrical noise Has the slave got a feature to check for errors which were recognized by it? If not, you need to do a trace of the telegram traffic to find out which of the three reasons mentioned above lead to the problems you have. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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1/10/2013 5:13 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Sir, I changed the Reply Monitoring time from 500 ms to 10000ms and checked it.Same problem persists. So I can say it is not because of reply monitoring time. The slave is in a state in which it can't answer The request of the S7 was disturbed due to electrical noise Regarding above highlighted two points I am not able to give confirmation .How can I justify the reason? Slave doesnt have facility to check for errors. I dont know how to trace the telegram traffic through Modbus Communication?? Can you Please suggest me any way to check? Thank You, Warm Regards, Srinath.
Srinath40 Posts: 31 Rating: (0)
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1/10/2013 5:54 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, with RS485 interface it is not that easy to trace the telegram traffic asmost PCs haven't got an RS485 interface on board. So you need a RS232/RS485 converter and a PC with RS232 interface (or RS232/USB). Then you can trace the telegram traffic with Hyperterminal. Did you connect a terminating resistor as in the manual described? (Appendix B in the Modbus Master manual) I can assist you in verifying the trace. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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1/10/2013 7:43 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Hello Sir, I referred the Appendix B what you told.It states that 330 Ohms resistor need to be connect in receive end (Slave). Actually It is not connected in Site. If 8 slaves are connected per CP , Where I have to connect 330 Ohm resistor? At present condition two wires with shield are connected to 1st slave .From there it is looped up to 8th slave. Warm Regards, Srinath.
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1/10/2013 10:47 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, just have a look at the attached picture. Regards, Kaulquappe Attachment rs485resistor.jpg (580 Downloads)
Kaulquappe Posts: 3093 Rating: (324)
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1/11/2013 4:58 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Sir, Thanks for your Information.I will check it and let you know. Can You please tell me the reason behind this ?? Is it to reduce electrical noise?? Thank You, Warm Regards, Srinath.
Srinath40 Posts: 31 Rating: (0)
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1/30/2013 11:27 AM	Rate (0)
Silent Bob Posts: 836 Rating: (100)	The resistor isto avoid undefined signal levels on the line due to electrical reflections. There is an article on Wikipedia about electical termination.
Silent Bob Posts: 836 Rating: (100)
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