The significant differences between PLCs and PACs are the way they handle input/output functions. PLCs constantly scan all the I/O in their systems continuously at a very high scan rate. While this enables very fast I/O response, it also limits the number of I/O points a PLC system can handle. PACs use a logical address system and a single tag name database very similar to traditional DCS and supervisory control and data acquisition systems. Thus, a PAC can identify and map I/O points as needed.
The fact that PACs handle I/O and data in the same manner as traditional supervisory control and data acquisition and DCS systems means they can more easily be interfaced directly with those systems. PACs are often used as substitutes for supervisory control and data acquisition RTUs (remote terminal units) and DCS field controllers, just because they work in a logically similar manner with data.
The IEC has created programming software standards, IEC 61131-3 and others to extend the capabilities of field automation controllers. These standards take the programming ability in about twenty ladder-logic commands and replace it with a full featured programming capability. There is the concept of function blocks. A late addition to the world of PLCs, function blocks come directly from the DCS (Distributed Control System) world. PACs are designed to utilize function blocks and function block programming rather than the more limited ladder-logic programming of the PLC.
They are capable of being programmed in a variety of higher order programming languages because PACs are in essence PCs. Suppliers of PACs have produced complete suites of programming software tools to enable very sophisticated operations to be controlled by PACs. For instance, Advantech produces software for HMI creation, PAC programming, supervisory control and data acquisition, and distributed control architectures, and an OPC server for connection to other control systems and even to MES and enterprise integration systems.