Sometime VB-OPC client cannot write to siemens s7400 plc


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We found that there is a writing problem from OPC to plc. Some time OPC client not able to write data into PLC db block. but OPC client don't get any error that data is not written etc. if write again to same area then it write successfully. OPC client is putting data into db2 & 3 block and the movement OPC put data PLC program start executing and clear the data from the area in a scan of 30 sec then opc again go and check whether the area is free. if OPC found db block free it will write again to that area if any instruction needed to be written to that block. Even we delay the written process by 2 sec to same area still we are getting this problem. It mostly happen if OPC needed to write to same block twice or more than once even if the interval is more than 5 seconds.

Please let me know if you need any more clarification and email me your contact no, better we can talk and clearify our doubts as i need to solve this problem fast due to time constraint from our customer. The environment in which the communication must work:

--W2k professional service pack 4
--S7 protcol
--attached is the citamis.str from our PC
--reading & writing very frequentlyDB2 & DB3 blocks & other blocks --Ethernet 3com card --Visual basic app as OPC client

we want to get data from PLC to safe it in database less than 1 sec Setting of Simatic net opc server setting cycle rate now set to 50ms

help is appriciated.