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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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12/31/2012 8:17 AM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Experts, I am doing commissioning for Chiller Plant Automation.PLC System is S7400- H.8 chillers are communicating through CP341 as a Modbus Master.Totally 4 CP's are used for 32 Chillers.FB7 and FB8 blocks are used .Each chillers contains 4 query.So totally 8*4 = 32 Queries for Once CP.Communication is done and found OK.All datas are correct. If any one of the chiller goes to shut down,I need to clear data of the corresponding chiller. But now DB's showed old data's.(Last value stored) How can I clear data during Chiller is in off Condition (Modbus Slave does not exist) with the help of Modbus Programming Blocks??? is there any limitation to use FB7 and FB8 blocks? How many times can I use FB7 and FB8 for one CP? Please Clarify With Regards, Srinath.
Srinath40 Posts: 31 Rating: (0)
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12/31/2012 5:17 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, you should call FB7 and FB8 only once per CP. When a Modbus request is finished with error you should delete the receive DBby the user program if you don't want the old values being kept. The CP/FB does not delete the receive DB in case of error. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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1/3/2013 5:34 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Sir, Thanks for your information.I tried the same thing before.I am not able to achieve it. 4 Queries per Slave.I briefly explained my problem below. Actual 1st and 2nd ,5th Slaves are Power OFF (doesnt exist).3rd,4th,6th,7th, 8th Slaves are in ON. 1st Slave 1st Query - Error (Send FB) 2nd Query - Error (Send FB) 3rd Query - Error (Send FB) 4th Query - Error (Send FB) 2nd Slave 1st Query - Error (Send FB) 2nd Query - Error (Send FB) 3rd Query - Error (Send FB) 4th Query - Error (Send FB) 3rd Slave 1st Query - Error (Send FB) 2nd Query - Done (Send FB) 3rd Query - Done (Send FB) 4th Query - Done (Send FB) 4th Slave 1st Query - Done (Send FB) 2nd Query - Done (Send FB) 3rd Query - Done (Send FB) 4th Query - Done (Send FB) 5th Slave 1st Query - Done (Send FB) 2nd Query - Error (Send FB) 3rd Query - Error (Send FB) 4th Query - Error (Send FB) If I test with only one slave ,all are OK (If Slave is Off,all 4 queries received Error bit.If Slave is ON,all 4 queries received Done bit) I am facing problem during multiple slaves only. As per program,PLC will send 1st query (slave 1) and check done or error. After confirmation only PLC will send 2nd Query (Slave 1).It is cyclic one.(Slave 1 to 8 then again starts from Slave 1.) I adjusted the Reply monitoring time ,cyclic time,baud rate. Even though Am not able to solve it. Please Please help me. Warm Regards, Srinath.
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1/4/2013 12:17 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, I think it is a logical sequence error in your program. Can you insert the source code of the block where the input parameters for FB7/8 are set? Regards, Kaulquappe
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1/4/2013 1:57 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear sir, please find here with attachment of source code. Warm Regards, Srinath Attachment MODBUS_SEND_RCV.zip (702 Downloads)
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1/4/2013 7:44 PM	Rate (1)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, Break on the receiving line is reported in the first cycle when calling P_SND_RK. So you won't get a positive edge to continue. I'd prefer a solution which has not got that amount of Start and Send bits. I copied thebitsof DB40 from the standard Modbus sample. Of course you can use flags instead. The suggestion is no complete program. It is just an idea how you could realise it. Some things are only comments, no code. Regards, Kaulquappe Attachment D:\temp\suggestion.pdf (682 Downloads)
Kaulquappe Posts: 3093 Rating: (324)	Last edited by: Kaulquappe at: 1/4/2013 7:49 PM
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This contribution was helpful to 1 thankful Users dchartier

1/5/2013 6:12 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Sir, Actually I am not clear about your suggestion. Today I checked the sequence with manually. I started from 2nd Slave 4th Query. 2nd Slave 4th Query - Error (Send FB) 3rd Slave 1st Query - Error (Send FB) 2nd Query - Done (Send FB) Same Problem Persists. Even I tried without Positive edge. PLC System will send the query then wait for Done or Error. After Done or Error bit Confirmation Reset the Send FB.After that It will send Next Query. Same Problem Persists. I am not able to understand where is exact problem. Please help me sir. Warm Regards, Srinath
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1/6/2013 7:29 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, what do you mean with " reset the FB" ? Do you set the reset input? I do not recommend to do that, it is not necessary. Do I understand you correctly, that thecyclic operation of FB7/8 stops when a not existing slave is addressed? Does the communication continue when you set REQ of P_SND_RK manually (in the VAT)? Regards, Kaulquappe
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1/8/2013 5:08 PM	Rate (0)
Srinath40 Posts: 31 Rating: (0)	Dear Sir, Yes correct I told about Reset input in that Send FB. No the cyclic operation never stops .It is working perfectly. PLC receives all data (Whichever slave exists) without any interruption. When Slave is in OFF Condition ,the PLC shows last value of that particular slave. I need to make receive data Zero (When Slave doesnt exist i.e slave is in OFF). With the help of error bit, we can make it zero as you told. I understood that. But problem for me is sometimes PLC receives error bit at SEND FB for queries of existing slaves also. (Slave which are ON Properly communicating) Have you got my point? Warm Regards, Srinath
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1/8/2013 5:36 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Srinath, yes, now I got it. Which STATUS or entry in the diagnostic buffer do you get when the slave is ON but ERROR is reported? Even when the slave is switched on transmission errors may occur e.g. due to electrical noise. In that case you can repeat the request immediatelly or wait for the next cycle. Regards, Kaulquappe
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