Over the past few decades, some innovations have indeed given industrial automation new surges of growth: The programmable logic controller (PLC) was designed to replace relay-logic; it generated growth in applications where custom logic was difficult to implement and change. The PLC was a lot more reliable than relay-contacts, and much easier to program and reprogram.
PACs are relatively new to the automation market, using the term coined by the market research firm ARC in 2001. Since then, there has been no specific agreement as to what differentiates a PAC from a PLC. Some users feel the term PAC is simply marketing jargon to describe highly advanced PLCs, while others believe there is a definite distinction between a PLC and a PAC. In any case, defining exactly what constitutes a PAC isn’t as important as having users understand the types of applications for which each is best suited.
PLC vs PAC Functional differences
The demarcation line between PLCs and PACs has become less clear, but there are still some applications that clearly favor a PAC, due to its greater range of features, functions, and capabilities (Table 1). Here are a few observations:
From a programming perspective, a PLC typically has a fixed memory map and addressing. In contrast, a PAC allows tag naming, letting users define data types as they program. This provides more flexibility, especially when expanding the system.
While many high-level PLCs have excellent execution speeds, PACs typically offer much greater I/O capacity and user memory size for larger projects and larger overall system sizes. This often makes them a better choice for large systems encompassing several areas of a plant.
While advanced PLCs have increased communication and data handling options, PACs still offer more built-in features such as USB data logging ports, a web server to view system data and data log files, and an LCD screen for enhanced user interface and diagnostics.
PACs are designed to be integrated more tightly with SQL and other databases. They often are still the choice for process control applications because they deliver other advantages such as standard 16-bit resolution analog for higher precision measurements.
1 More open architecture
2 Modular design
3 More analog I/O options
4 Better process control capability
5 Designed for tight integration with SQL and other databases
6 Tag naming that lets users define data types
7 Greater I/O capacity
8 More built-in communications ports
9 USB data logging
10 Programming via USB port
Modern PLCs and PACs share many of the same features, and either will work in many applications.