Overview

ISA SP95 was chartered in 1995 to define the interface between business and manufacturing systems. The original idea was to define a firewall that would prevent business systems from sending arbitrary commands to manufacturing equipment. Some vendors were pushing the idea of integrated control from the boardroom to the factory floor. The people in the boardroom have high control needs, so they thought the idea was excellent.

A firewall isn't popular with people who see the enterprise as a single information problem to be solved by computers all the way down. Manufacturing provides mission fulfillment for the enterprise by making the products that generate cash flow, so it ought to be tightly integrated into an enterprise-wide information system, right? Well, no, because manufacturing systems are actually fragile.

There are things that business systems should not be able to do to manufacturing systems. Control networks tend to be bandwidth-limited. Using each piece of computing and communications equipment to its maximum capacity saves money on the control project. The result is that too many unsolicited queries from business systems can shut down real-time control.

Every process is run to its fullest capability, which means pushing the equipment to its limits. There is no room for an error made by someone who was not trained in the consequences of their actions. In most processes, an error can destroy expensive machinery that cannot be readily replaced, or kill people by releasing very hot or poisonous material, or destroy expensive products. Operators require extensive...