Convergence of PACs, PLCs, IPCs

There are three common types of machine or process controllers: PLCs, PACs, and IPCs. Here’s how to select the best one for your application.


Figure 1: This AutomationDirect P2000 is an example of a PLC-based PAC as it combines the power of a PAC with the form factor and reliability of a PLC. Courtesy: AutomationDirect"Divergent" is defined as tending to be different or developing in different directions; it's common for companies to strive for differentiation from competition, often in trivial ways. For industrial controllers, "convergent" is the word best describing the state of the market and available features.

Convergent is defined as coming closer together, especially in characteristics or ideas. This more aptly defines the relationship among programmable logic controllers (PLCs), programmable automation controllers (PACs), and industrial PCs (IPCs) in today's industrial automation market. Let's take a closer look at how this convergent trend is expressed among types of industrial controllers. 

Controller background, history

First, there was the PLC invented by Dick Morley. The PLC's invention and the creation of Modicon demonstrate true entrepreneurial genius and forever changed the way machines are controlled. This relay replacer has taken on many faces over the past 45 years, growing from a simple logic solver to a true multitasking central core for many automated systems.

The PAC, a term coined by ARC Advisory Group in 2001, provided some distinction in the market from the PLC. On one end of the market, there were still many legacy controllers that were more similar to the units Morley speaks of in his memoirs. However, on the other end of the industrial control spectrum, there was a wave of new technology being introduced into the controls realm.

Much of the innovation accompanying the PAC stemmed from advancements in PCs and other commercial technologies such as cellular phones in the mid-1990s. These areas were advancing rapidly during this time, and consumer-driven markets were suddenly playing a part in the production of smaller, cheaper, and faster components that also were suitable for industrial controls. Initially, many components were not considered to be industrial quality, but this changed quickly.

The industrial personal computer (IPC) has actually been around longer than the PAC, at least since the mid-1980s. The first IPCs were quite large and expensive, as with IBM's version which was the size of a 50 hp motor and cost about $10,000. But as time progressed, the form factor and price of an IPC approached or even improved upon that of a PLC or PAC.

An issue with IPCs was the operating system, as desktop versions of Microsoft Windows weren't robust or deterministic enough for real-time control. The answer was the creation of a host of real-time operating systems, which turned the IPC hardware into a high-performance control engine suitable for industrial applications.

There are some unique characteristics among these platforms, but as we've seen in the past few years, the lines that divide them are increasingly blurring boundaries.

Recent technology has made the PLC a much more capable controller than the original relay replacers designed more than 45 years ago. Table 1 shows some generalized data that does not necessarily represent the features of each controller within its respective category, but does offer a fair assessment of the respective categories in general. 

Table 1: General PLC-PAC-IPC comparison

Table 1: General PLC-PAC-IPC comparison. Courtesy: AutomationDirect

PLC baseline

In its most basic form, the PLC is the workhorse of many industrial control applications, commonly operating for 10 or more years with little maintenance. Often built with purpose-designed gate array processors and with proprietary communication, these controllers continue to be a popular choice for machine control and for simple process control applications such as water/wastewater pump stations. Their ladder logic programming is well suited for controlling simple and complex automated sequences, but is lacking when it comes to control of analog variables and data handling.

PLCs work well in applications where space is at a premium due to their small fixed or modular form factor. While there are limited expandability options, their low cost hardware and programming software make them a popular choice for original equipment manufacturers (OEM) and other applications.

Even though the program memory is typically in the low to mid-range, PLCs can be integrated with motion and vision systems to provide more sophisticated control. 

Convergence to PLC-based PACs

Although PLCs have been around since the 1960s, today's electronics advancements provide an atmosphere favorable for the development of a new class of controllers (see Figure 1). Components are smaller, faster, cheaper, and more reliable than ever before. This provides the ability to increase the capabilities of the PLC without driving the cost too high and to lower the cost of a typical high-end control system without sacrificing performance, thus creating the PAC from the bones of the PLC.

Figure 2: This PAC controller provides a wide range of capabilities and is thus suitable for simple to complex machine and process control applications. Courtesy: AutomationDirectBecause the PAC grew from the PLC and possesses many of its best characteristics such as reliability and a compact form factor, it makes sense to refer to it as a PLC-based PAC. This is the perfect example of convergence of technology in the industrial realm. These controllers can satisfy a broad range of applications ranging from simple, low-cost machine control to complex control of large systems with many input/output (I/O) points.

The great PLC-based PAC enabler is today's new technologies, which allow suppliers to make a better controller at a lower cost as compared to legacy control systems. The technology inside these controllers is also becoming more compact, making it a perfect fit for many machine control and other OEM applications.

Many of these machines require moderately fast scan times to meet design specifications set forth by OEMs in highly competitive markets. In the past, machine builders were often faced with using a PLC that met requirements for I/O requirements and form factor, constraints, and compromise on performance.

The alternative was to make the switch to a high-end PAC or IPC, often necessary to accomplish the goals set forth by the design engineers. In many cases, these larger controllers were overkill and drove up the cost of the overall design by requiring a larger enclosure, along with higher hardware and software costs. But an answer emerged in the form of the PLC-based PAC, which provided the required performance in a compact form factor and at competitive cost.

Stepping up control with the PAC

In addition to machine control, today's PLC-based PAC is also often a good fit for quite complex applications. For examples, a modern PAC could scan hundreds of analog channels and log data points to a file on an integrated memory port and then allow access to those files via a standard web browser through its built-in Web server (see Figure 2).

These more capable PAC controllers typically include a general purpose processor. They also are offered with a wide-range of standard communication protocols, enabling connection to a variety of devices. And with access to multiple programming languages as defined in the IEC 61131-3 standard, they have extensive control capabilities. These medium-to-large form factor, rack-based systems also provide medium to high expandability for I/O count and memory.

These capabilities do make the hardware and programming software more expensive than a PLC, but the PLC-based PAC delivers the higher level of performance required in certain applications.

For example, its large memory capacity makes it ideal for creating 1-D or 2-D arrays to track products through an exhaustive application requiring the tracking of quality attributes, shipping data, customer information, etc. The tag-name capability of the PLC-based PAC also eases interface to human-machine interface/supervisory control and data acquisition (HMI/SCADA), OPC servers, database ERP software, and motion and vision applications. 

Highly capable IPC

Figure 3: Only the largest and most complicated applications required the capabilities of the IPC, the most capable but also the most expensive of the controllers covered in this article. Courtesy: AutomationDirectFor the more specialized areas of automation and integration, an IPC environment can provide unique benefits (see Figure 3). Advanced motion control and vision are two good examples, as one can benefit from an IPC's built-in motion control capability to accomplish multiple coordinated axis control. If there is a need to design a vision application with one programming environment, the advantages can be seen.

IPC processors are typically PC-based, multi-core processors, and the communication options are open in nature and only limited by protocols requiring specialized hardware. Ethernet is by far the most popular connectivity option, with many protocols supported, such as EtherNet/IP, Modbus TCP, and Profinet (supported by ODVA, The Modbus Organization, and PI North America, respectively).

IPC programming capabilities are the most extensive of any controller. Not only are the IEC 61131-3 languages available, so are PC-based software languages, such as C++ and the Microsoft .NET framework. And because of its PC roots, a PC-based HMI can reside on the same chassis and run on its dedicated processor in a multicore system.

An IPC is usually connected to I/O points through a bus coupler, creating a distributed control system. Their design also makes them the most expandable option on the market, usually only limited by the bus specification.

These extensive control capabilities and high performance come with a correspondingly high cost for both the hardware and programming software. In return, the IPC provides a great deal of program memory, which can be expanded as needed, and provides best-in-class integrated motion and vision capabilities.

There are many applications crossing the lines of these classifications, but some specialized requirements often exist that tend to favor one control platform over another. The PLC and PAC have much in common, and new hardware technology just strengthens this overlap as it made the PLC-based PAC a reality, along with the IPC.

- Jeff Payne is the automation controls group product manager at AutomationDirect. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering,

Key concepts

  • Much of the innovation accompanying the PAC stemmed from advancements in PCs and other commercial technologies such as cellular phones in the mid-1990s.
  • The industrial personal computer (IPC) has actually been around longer than the PAC, at least since the mid-1980s.

Consider this

The PLC and PAC have much in common, and new hardware technology just strengthens this overlap as it made the PLC-based PAC a reality, along with the IPC.

ONLINE extra 

Read about controller history at

- See related articled about PLCs and PACs linked below.

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