SCADA doubts reg. MODBUS OPC dialup server


Thread Starter


Hi All

I Need some clarification in MODBUS Master OPC server.

In MODBUS Slave device I am creating an event record buffer of size for 100 records, where I am storing status of an event in 2 bytes (Input Register 3X) & Time stamp data in 8 bytes for each record in addition to.

I am planning to use one of MODBUS Master OPC server Gateways available from some companies.

My requirement is I have to use modem dialup (Telephone line) facility in MODBUS OPC Server & I am connecting to modem slave device once in 30 minutes & it will remain online for next 5 minutes (since I am using telephone line modem dailup, I cannot keep master & slave devices online continuously).

But I find that, All OPC servers can be configured in terms of tags for each events & I can get the current status of each tag only when I am connecting to modbus slave device.

Can somebody suggest MODBUS (SERIAL) OPC server that has feature such that I can retrive that event buffer data? Also I found that many MODBUS opc servers does not support event logging options (some servers have this option but only Events that occur within the server can be stored to disk) where I can log the event data (INPUT register 3X) which I receive from remote MODBUS Slave device.


Lynn at Alist

Not sure about an OPC server, but has rich "dial-out" support such that I'm sure they could poll your remotes for you, then you can use any OPC to poll the FS device. OPC wouldn't know that data only changes once every 30 minutes. May not be ideal, but should work.

- LynnL
One of the unsolved issues in this area is the time stamp format. There no convention I am aware of.
OPC-DA does not do exactly what I think you want. You cannot read a buffer as a chunk. You will need to read the 100 records as 100 individual parameters. You need to have an internal mechanism that ensures that the OPC server reads only the valid values (not empty or stale records from the previous read) and to ensure the slave start logging from the beginning of the buffer etc. It may be a complex mechanism. It can be done. We (SMAR) have a flow computer that logs many days of data on board. We created a utility for it that uploads the data and sump it into a database automatically, transparently to the user.

The OPC-CDA (complex data access) will be finalized any day now and it will support record sets so I think this kind of mechanism can be easier in the future. Until then I guess you need to roll your own solution.

Jonas Berge SMAR
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