I realized that I oversimplified OPC by lumping DA and UA together and by impugning the speed of OPC. I think the oversimplification was valuable in noting the differences between PROFINET and OPC, but the engineer in me cannot avoid adding some details and clarification. Here they are:
HMI. Although I pictured OPC as just connecting PLCs to HMIs and although that may have been true to some extent with OPC DA, it’s different today. Now OPC is often used to move information to “the enterprise.” By that I do not mean moving information to a starship labeled NCC-1701, but to MES and ERP systems. Plant information needs to be displayed (in HMIs), but also needs to be sent to and received from higher level systems that coordinate production, manage recipes, etc.
Data versus information. I subtly described this as a difference between what PROFINET and OPC move. What is the difference? An example of data is the output in channel 3, slot 5, IO rack 17, is it on or off. Additional data might be an alarm that that channel has experienced a wire break. Information might be the set of temperature controller parameters used that time-proportion that channel’s output.
Speed. I always get in trouble assuming OPC is slow. In fact, I remember a conversation back in the 1990’s with Al Chisholm, one of the OPC founders, at the Johannesburg airport. Al was with Intellution and I was with Wonderware so this was like the Hatfields and McCoys in friendly conversation. But it was a friendly conversation in which Al reported on some test results on OPC speeds. They were fast. Today I checked with my contemporary OPC colleagues. (PI North America shares office space with OPC, so checking with them was not difficult.) Surprise. OPC is still fast. And OPC can achieve updatess in milliseconds. Of course, PROFINET can achieve updates in microseconds, so we’re still faster. <smile>