There have been countless articles comparing Programmable Logic Controllers ( PLCs ) and Programmable Automation Controllers (PACs). Yes, there are differences between these two terms, but are these differences really important? In some cases, they may not be, because with the continuous development and improvement of PLCs, they have already achieved functions and performance comparable to PACs.
PLC -based PAC (a term I coined). In terms of characteristics, functions, and suitable applications, PLC-based PACs can effectively replace PACs in most situations due to significant overlap. Although there are many similarities between PLC-based PACs and PACs, some key differences need to be discussed. Let's first look at the evolution of PLCs to see how PLC-based PACs have gradually developed and grown.
Origin of the name
In the era of first-generation PLCs, PLCs were mainly used to replace hard-wired relays and pneumatic timers. At that time, PLCs were actually called programmable controllers, or PCs for short. However, in the early 1980s, the name PC was quickly taken over by personal computers, and the term PLC became more commonly used. The name PAC appeared about 15 years ago, perhaps to distinguish the most powerful PLC from its less powerful counterparts.
Compared to PAC, PLC-based PAC might be a better name, as these controllers incorporate the advancements made in PLCs and other technologies over the past few decades. Manufacturers have adopted proven, more powerful PLC hardware designs and incorporated the latest, low-cost technologies from the PC and mobile device sectors. They have integrated these technological advancements to meet evolving user needs, providing PLC-based PAC systems.
Common functions
Currently, finding an industrial controller that lacks many of the features of a PLC or PAC is quite difficult. However, the definition of a PAC varies considerably. Many manufacturers struggle to differentiate between PACs and PLCs because they share many important but common characteristics. These common characteristics and functions include:
1. High-speed CPU, providing fast scanning;
2. Functionality based on tag name;
3. Large-capacity onboard storage;
4. The document is stored in the controller;
5. Task management procedure organization;
6. Multiple built-in network protocols;
7. Data collection.
Both share many common characteristics, representing more new technologies than branches of a particular category. For example, faster scan times. The latest PLC and PAC processor chips have significantly higher processing frequencies compared to most PCs from the early 2000s. This technological advancement applies to controllers at all levels. In terms of CPU performance and cost, this is more of a manufacturer's preference. These high-speed CPUs are essential in many machine control applications and other situations requiring very fast execution speeds.
Other common functions are part of the natural progress or evolution of PLCs. Tag-based controllers are one example. As PLCs are increasingly becoming part of integrated systems—compared to standalone controllers—it makes sense to shift from fixed-address designs to tag-based systems. This allows multiple platforms within the same control system to share a common tag name database, which generally significantly reduces front-end development work.
Lower-cost memory makes tag-based systems possible. Compared to typical fixed-address PLC systems, tag names consume more memory, thus requiring more total memory to achieve the same application functionality. More memory also allows vendors to store program files on the controller. This is a significant convenience for field troubleshooting and solves a common problem: the potential loss of tag name interpreter files when these files are not stored on the controller.
For some PLCs and PACs, the task manager and the method of managing programs are quite similar. For large projects that cover multiple devices/processes, program organization functions are an ideal tool.
Common communication
Offering integrated or optional communication protocols is often a matter of supplier preference rather than technical limitation. While some high-end controllers still have only a single communication port, many mid-range and low-end PLCs (even those from the late 1990s to the early 2000s) have multiple communication ports built in. There are also many other options for implementing additional ports and communication protocols.
PLCs and PACs commonly use Ethernet protocols, including EtherNet/IP and Modbus TCP/IP. These communication protocols provide a convenient way to connect to various devices and systems, including ERP and business systems. Many PLCs and PACs also provide serial Modbus and ASCII communication. These communication methods are popular in barcode scanners, information displays, electronic scales, frequency converters, temperature controllers, timers/counters, and other devices.
Key differences
Despite the many similarities between PLCs and PACs, there are still some key differences. These differences are mostly related to high-end functionality. In some very large and complex applications, due to the number of instruments, remote devices, extensive process control and monitoring, and other requirements, a PAC system may be necessary. These differences generally relate to hardware configuration, extended functionality, and cost (see Table 1).
The overall scale of the application is often a key differentiating characteristic. Many small PLCs do indeed have the capability to expand the initial boundaries of the control system by adding a bus controller master module. Whether utilizing multiple racks or dedicated remote I/O, the number of I/O points can be expanded to 100,000 or more. This can significantly reduce the manual time required for system configuration and development. Newer PLC-based PACs are generally smaller in size, and therefore, in many cases much smaller than a PAC, allowing for the addition of even more external I/O.
There are also some cross-domain characteristics, which were originally generally classified as special controllers, such as redundancy, multi-language programming, and specific hardware specifications.
Although PLC-based PACs may only have ladder logic and certain specific function blocks to simplify motion control, most PACs support the five programming languages listed in IEC 61131-3:
1. Trapezoid diagram;
2. Function block diagram;
3. Instruction list;
4. Structured text;
5. Sequence Function Chart.
PLC-based PAC in Action
Currently, PLC-based PACs can meet the needs of many applications, from simple machine control to more advanced PAC applications (see Figure 2). To achieve this, many new technologies have been applied to create controllers that are lower cost and better than traditional control systems.
Technological advancements have enabled suppliers to offer faster, more feature-rich controllers in smaller physical sizes. This allows PLC-based PACs to be used in a wider range of machine control applications. Many of these machines require fast scan times, giving their manufacturers a competitive edge and enabling them to meet design specifications.
Whether it's a PLC-based PAC or a PAC, both can effectively control this large-scale filling production line. The final choice depends on specific functional and performance requirements.
In the past, machine manufacturers faced a dilemma: using smaller PLCs could meet I/O point requirements and physical space constraints, but performance would be less than ideal. An alternative was to choose a large PLC or PAC system. In most cases, for simple machine control needs, a large controller is overkill, but it's indispensable to meet specific application requirements.
PLC-based PAC systems are well-suited for small, cost-sensitive applications, and can also handle applications requiring the processing of hundreds or thousands of analog channels. Many of these controllers can record data points to files stored in an integrated storage port, and then retrieve these files via a standard web browser using a built-in web server.
The large storage capacity of a typical PLC-based PAC system makes it ideal for creating 1-D or 2-D matrices to track product, quality characteristics, shipping data, and customer information. PLC-based PACs also feature tag-based functionality, meaning they can interface easily with HMI/SCADA systems, OPC servers, and databases.
PAC meets high-end challenges
By leveraging more domain-specific integrations, PAC environments can offer unique benefits. Advanced motion control and vision applications, which frequently require the powerful capabilities of PACs, are two excellent examples. Utilizing a single PAC platform to integrate multi-axis coordinated motion, trigger vision systems, and collect inspection results offers significant advantages in these areas.
On larger, but also more expensive, PAC platforms, manufacturers typically offer options for programmable security and other specific functions. These highly specialized requirements represent a small market share but are crucial for certain users.
Choose a suitable solution
When designing a control system, one should not choose based on the definitions of PAC, PLC, or PLC-based PAC. Instead, users should determine their controller requirements based on their specific application and select the most suitable product.
Regardless of whether the application is machine control, motion control, process control, surveying and data acquisition, distributed control and enterprise interconnection, or a combination of the above characteristics, the definition of a controller should be based on application requirements and the controller's functions.
