I just had to add my two cents to the comment “but if we had to do it over again, we’d have gone with Modbus”.  Modbus is another well known and reliable protocol, but it too has limitations.  I for one would not want to attempt a high point count, triple layer redundant link, with multiple failover conditions and condition logic using a Master/Slave based protocol that does not natively supply timestamps or quality information.

Don’t get me wrong.  Modbus is a very good messaging structure, and has gone a long way to standardizing distributed automation for PLC and RTU networks.  Just as with OPC, there are times to use Modbus, and times to use a different protocol.  As with any software interface architecture, a clear understanding of the requirements and capabilities of the products and/or protocol is essential for a smooth implementation.

More often than not you will find that Modbus and OPC are complementary protocols in a given architecture.  One of the most common OPC servers on the market, is the OPC Server for Modbus.  In fact, Modbus-IDA is one of the sponsors for the 2006 MatrikonOPC User Group.

I’ve implemented redundancy projects that use Modbus, and many more that use OPC.  Every project is different, but personally I see more options with OPC.