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By Dan Hebert, Senior Technical Editor
Remember when PLC vendors used to neuter their low-end PLCs to force you up the food chain? Back in the late 1980s, all the low-end PLCs lacked one critical function that we needed for our process skid application—remote I/O. To get that feature, we would have had to buy a high-end PLC at $5000+.
However, we found a way out, moving to an industrial PC-based control system with Opto 22 I/O. This provided us with remote I/O and some other nifty features, all for less than the price of the high-end PLC.
But, the more things change the more they stay the same. And today, some PLCs of the process automation controller (PAC) vendors are still playing a similar game, albeit with different features.
In my May column, I looked at how some end users are turning to industrial PCs to avoid the upsell. Now, we'll look at how relatively inexpensive PACs and PLCs are providing the features and performance of high-end PACs, PLCs and even DCSs at low-end prices.
Opto 22 is among the price and technology leaders in wireless I/O. "Our PACs offer 802.11a/b/g wireless networking in addition to standard Ethernet networking," says Ben Orchard, application engineer at Opto 22. "Our PAC programming software is free, and our wired-plus-wireless PAC sells for $1995.
"Process control engineers can design control systems with traditional Ethernet wiring, wireless or a combination of the two with no restrictions—and we're the first to offer a wireless PAC that operates in the 802.11a, 5GHz range."
Orchard adds that, "Other vendors' wireless networking hardware often comes in configurations and form factors incompatible with their standard line. With many suppliers, standard analog input and analog output modules are not available for wireless. Our wireless system encompasses our entire line of I/O modules."
In addition to wireless I/O, another high-end feature typically reserved for expensive PLCs is the capability to handle large amounts of I/O devices and data. Phoenix Contact offers PLCs that can handle up to 4,096 I/O points at prices starting at $400.
Intrinsically safe I/O that can be mixed and matched with standard I/O is a feature offered by many DCS vendors, and is often needed when distributing control and I/O in hazardous areas. But, it's not necessary to pay DCS prices for this capability, as Wago's line of programmable fieldbus controllers (PFCs) offers this feature and more at prices starting at $350.
"We offer a full line of intrinsically safe modules that can be co-located with our standard I/O, eliminating the need to route wiring through barrier terminal blocks," says Charlie Norz, product manager for I/O Systems.
Wago's PFC controllers and intrinsically safe I/O are available with IP67 ratings, meaning that you can deploy an inexpensive off-the-shelf control system in a hazardous area with no need for explosion-proof or purged cabinets.
Similarly, sophisticated PID control and data handling are often excluded from low-price PLCs, but AutomationDirect's Productivity3000 programmable controller family "starts at $599 and includes free programming software," notes Jeff Payne, product manager for the programmable controllers group. "Our PID instructions include built-in auto tuning, typically performed by expensive third-party software. When working with curing, annealing, pressurizing, forming or other applications, the Ramp/Soak features monitor the process and maintain a structured cycle that would be complicated to accomplish otherwise," he says.
"Data Logging is a built-in feature that allows specific tags or variables to be logged to a file based on a pre-configured schedule or on events. All this data can be sent to a removable USB drive that connects to a designated port on the CPU. Files are automatically stored in a folder on the drive with a date and time stamp, and new files can be created on a user-defined schedule," adds Payne.