Choosing Between a PLC and a PAC

The term “programmable automation controller” (PAC) has been around since the mid 1980s. Over the past five years or so, PACs have been offered by an increasing number of control vendors and are now commonly chosen for machine control systems. Traditional programmable logic controllers (PLCs) are typically rack-mounted systems with local I/O or distributed I/O, and, to some extent, as soft-logic PLCs (PLC programming in a PC-based platform).

PAC vendors often promote their products’ open architecture features, which provide engineers with a variety of programming tools beyond Ladder Diagram programming, such as Structured Text and the other languages of IEC 61131-3, PLCopen motion, Visual Basic, XML, C++, etc.). Standard communication methods (Ethernet TCP/IP, OPC, EtherNet/IP, Profinet, SERCOS III, etc.) are used for I/O, multiple device integration, and motion control, as well as diagnostic-data transfer and remote setup.

Many PACs provide tight integration of logic, motion, integrated safety functions, machine interface and communication programming languages, allowing for increased performance and improved integration into the manufacturing execution infrastructure.

Also, PACs are not bound to the rack-based hardware form-factor. Today, a PAC can be included as a soft-controller in a PC with Microsoft Windows XP or Microsoft Windows CE, or built as a rack or terminal based system. In the past couple of years PAC functionality is even provided in servo-drive amplifiers.

In the end, control engineers must not get hung up on the terms PLC and PAC as they are mainly vendor marketing terms. Rather, one should compare the offered controller functions, features, and benefits, and assess how they contribute to the success of the machine integration need at hand.

- by Karl Rapp of Bosch Rexroth Corp