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What is a PAC (programmable automation controller) and how does it differ from a PLC (programmable logic controller)?

PACs and PLCs have many similarities, but they have significant differences as well.

• Open standards—While PLCs are based on proprietary networks and protocols, PACs are based on common, commercial networking and protocol standards like Ethernet and TCP/IP.
• Common tag database—PLCs use ladder-logic programming and require that data tags be duplicated for interoperability. PACs, in contrast, use an integrated development environment with a common tag database. Once data tags are created for control, they are automatically available for an HMI and exchange with databases or other systems based on open standards, such as OPC or Modbus/TCP.
• Exception-based scanning—PLCs constantly scan all I/O; speed is vitally important, and adding I/O points or analog functions can significantly slow down the system. PACs scan I/O only when logic requires it, and I/O processing is distributed to local processors; you can add I/O points or process control functions without straining the controller or the network.

ARC Advisory Group is generally credited with coining the term PAC and describing PAC characteristics: multifunction, multidomain, multitasking, with a modular architecture and integrated development software, based on open standards and thus able to exchange data with other manufacturers' products and systems.

Opto 22 has been manufacturing controllers with the main features that define a PAC since 1990, more than 10 years before the term was coined. Our latest SNAP PACs are easy to use and program and offer Wired+Wireless™ versions for even more communication options.