Ins and Outs of Modular I/O

Increased Granularity Improves Flexibility and Can Help to Reduce Costs by Eliminating the Need to Buy Unnecessary Components

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By Paul Miller

While many device users and specifiers in our industry consider input/output (I/O) devices to be pretty much of a commodity these days, that’s not always the case. The latest generation of advanced, modular I/O provides the key enabling technology for today’s widely distributed, open multi-vendor automation solutions.

All I/O, regardless of configuration, is modular to a certain degree. So first, we should define the term and determine how modular I/O might differ from conventional rack-mounted or high-density brick I/O.

The Big Three

As most everyone who has used I/O to some degree in the industry knows, there are three main types of I/O. Conventional rack-mounted I/O modules typically plug in to the PLC or DCS backplane and require a separate power supply. Block I/O is a solid brick with a set number of inputs or outputs. Modular I/O comprises a CPU or communications head unit with stackable modules, typically mounted on a DIN rail.

Conventional rack-style I/O usually comes from the same vendors that provide the controllers. This might or might not be the case for brick or modular I/O. Further, while conventional rack-style I/O almost always needs to be mounted inside a cabinet, both block and modular I/O increasingly are available for inside or outside-the-cabinet configurations.

Unlike conventional rack-mounted I/O that only plugs into and communicates with the PLC backplane, modular I/O typically can plug into a number of different industry-standard networks via vendor-provided connectors. Networking standards often allow modular I/O from one vendor to be used with other vendors’ PLCs, DCSs or other computing or control platforms.

In addition to providing basic input and output capabilities for both discrete and analog channels, most modular I/O systems also offer options for onboard discrete control, PID control or motion-control functionality to allow intelligence to be widely distributed throughout an industrial facility.

“Modular I/O allows for fairly granular channel counts and a variety of channel types to be dealt with in a uniform manner,” explains Bruce Thomason, chief technical officer at CyberMetrix, an automation and control solution provider based in Columbus, Ind. “The different modular I/O channel types are hosted with a common interface, using a common bus, physical mounting system, termination provisions and power supply. Modular I/O also allows users to freely change or add channels. Conventional I/O in-chassis or in a larger channel-count brick would not support the changing or adding of channels.”

Thomason says the two differ primarily in flexibility and scalability, with modular I/O winning out on both counts, “unless you just happen to need exactly what the brick has to offer,” he adds. “For us, the beauties of distributed and modular I/O are that it scales up, down and sideways very effectively, with up/down meaning count and sideways meaning the variety of different signals supported. It also allows us to place the right mix of I/O in close proximity to the actuators and sensors we manage.” This, says Thomason, reduces field wiring length requirements, which not only reduces costs, but also reduces EMI-induced noise.

“We use modular I/O in projects across a variety of industries for a number of reasons,” says Matthew Youney of Youney Instrumentation and Control Systems Engineering, an engineering services provider based in West Palm Beach, Fla. “First is expandability. If you run out of slots with conventional rack I/O, you have to add a whole new PLC or at least an expansion backplane. With modular I/O, you just stack it up as you need it.

Modular I/O also offers more granularity so you don’t have to buy stuff that you don’t really need. I also like the small size of most modular I/O. With Wago 750 I/O, for example, a module with eight digital inputs or four analog inputs is just a ½-in. wide. The distributed processing capability of modular I/O via a network reduces field wiring requirements and improves fault-tolerance.”

According to Bob Gardner, product manager, network and interface division, with Turck in Minneapolis, the term “modular I/O” can be a little confusing. “This is because conventional PLC I/O racks are obviously modular, since you can add input and output cards in different combinations for your particular I/O need,” he explains. “That said, the best modular systems available today have a number of features not available with conventional I/O. For example, modular I/O systems can be attached to any number of industrial networks and include power distribution, eliminating the need for many other terminal points in the panel. Modular I/O systems provide I/O in many more counts and varieties than conventional I/O and typically have removable electronic cards, so you can replace the electronics without disturbing the wiring. Modular I/O systems do not rely on a conventional fixed chassis size of four, eight or 16 cards. Instead you can add cards individually, from one to 50 or more.”

The modular I/O concept was developed to fill the void left with conventional I/O, comments Jason Haldeman, product marketing specialist for the Automation Systems Group at Phoenix Contact. “If an application calls for a variation of digital and analog signals, it is difficult to find a vendor that sells a module that contains the exact variation you need,” says Haldeman. “With modular I/O, you create your own custom station by selecting bus interface modules and the exact variation I/O required. If later during installation you realize that you need more or less I/O, you simply add or remove the points.”

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