Programmable Logic Controllers

When buying a programmable logic controller (PLC), it's best to use TLC—technology and language considerations. Because a PLC is a type of computer, one factor in any budget should be the cost of developing an application on the chosen PLC—an expenditure that typically arises after the hardware is purchased.


PLC checklist

When buying a programmable logic controller (PLC), it's best to use TLC—technology and language considerations. Because a PLC is a type of computer, one factor in any budget should be the cost of developing an application on the chosen PLC—an expenditure that typically arises after the hardware is purchased. Because software development can consume time and money, cost of programming tools, talent, and time have to be considered in the buying decision, along with hardware capabilities, such as processor speed and I/O count.

I/O count in particular is responsible for much of the price differences within a given PLC family, with analog I/O points costing much more than the discrete variety. A four-point analog input, for example, may list for over $600, while an eight point discrete card may list for only $125 or so. Processors run from $100 to as much as $10,000, with the extra money buying speed, memory, and such features as hot swapping cards or seamlessly transferring to a backup system. It's the combination of processor, I/O count, and features that determine a system's cost; thus the final PLC price depends upon your particular application.


Five standard and open IEC 61131-3 languages are generally used to program PLCs, regardless of vendor origin. There are also some proprietary software language offerings, some of which are widely used. While being able to handle this entire array of open and proprietary programming languages is desirable and perhaps necessary, the most important language for a given application is also likely to be the one with which your operations personnel are most familiar.

"If you are more comfortable programming brand X, you will be more productive and proficient in your project development," said Jeff Payne, product manager for PLC, I/O, and PC controls at AutomationDirect.

However, since technology and business aren't static, if a one language is used today, that may not be the case tomorrow. As a result, it's important for a PLC to be able to handle a programming Tower of Babel. One way to do that is to have a single configuration tool that supports multiple languages—at least all five under the IEC umbrella. This support ideally involves a single configuration tool and a common runtime environment. In that case, solutions can be reused and data exchanged among languages, cutting development time and costs. The difference in development environments can be an important differentiator among PLC providers, with some offering an easier setup experience than others.

Despite this happy picture of being able to effortlessly port code from one language to another, be aware that vendors often implement tweaks, changes, and extensions to their PLC programming tools, which they claim allows their hardware to provide a competitive edge. In the end, this means that porting information between systems from different vendors may not be as easy as intended by the original IEC 61131 standard developers.

Hard time with hardware

On the hardware side, it's important to remember that PLCs are on a technology curve. It's not as steep as the one that PCs are on, largely because PLC technology tends to be more conservative, with fewer generational upgrades than office and consumer markets.

However, PLC technology is still changing at a fairly rapid pace. Therefore, it's important to consider scalability of a PLC—which could involve putting multiple processors in a given rack at some later date. Another aspect of technological change that must be taken into account is the vendor's capability to upgrade a system or provide an upgrade path. If the software language adheres to an open standard (or widely used environment), an upgrade might only involve a hardware change and a minimal rewriting of software. Be sure to find out.

One thing that money buys is processing speed. A system's scan time—the time it takes to check on connected nodes—can run in the milliseconds. The more nodes there are, the longer that time can be. For some applications, such as running machining equipment or textile equipment or plastic production, execution speed is vitally important. In such cases, it may be worth the money to get the extra processing speed. As for how fast a PLC can go, that depends on the vendor.

"Our claim to fame is that we hold the world's fastest PLC, with scan times of 20 microseconds," said Craig LaRose, network solutions marketing manager at PLC-vendor Yokogawa. That speed costs money, with the bottom of the company's product line several hundred dollars more than the least expensive processors available from others. But this can be money well spent if the faster execution speed is needed.

Making connections

PLCs make two types of network connections. One is to the factory floor. The other is to the company's business network. For the latter, standard communication path is Ethernet. Therefore, this capability should be built-in to the PLC, if possible. Another component of this connection is ability to display human-machine interface (HMI) screens. Like high-level programming languages, HMIs show up in mid-range PLC systems, particularly in those with processors that run a few hundred dollars and up. Another type of human interface, access through a Web browser, allows PLCs to be reached from virtually any connected desktop or mobile device.

As for connections to the factory floor or other places where controls and sensors reside, an important factor to consider is number of PLC connections. Clearly, a PLC must support the minimum I/O count needed for an application. Some value beyond this should often be considered to handle expansion. However, adding I/O count also adds cost. At some point the extra expense may not be justifiable.

Some applications require special approvals, such as marine environments or hazardous locations such as refineries.

PLC checklist

I/O count and connection types: spring, screw, both


Languages (all five IEC 61131-3)

Development tools

Open platform






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