PLC I/O Devices: More Options for Better Machine Control

PLC I/O devices have gone a long way from being wiring devices on a backplane. Once thought to be dinosaurs replaced by smart sensors and device networks, I/O devices are showing new life by offering users new ways to improve control of machines in a cost-effective manner. Several trends in the market are occurring simultaneously, but all have one direction—increased opportunities f...

By Gary A. Mintchell, CONTROL ENGINEERING March 1, 1998


Machine Control

Industrial Networking

I/O Systems and Devices

Programmable Logic Controllers (PLCs)

Sidebars: Do Intelligent Sensors Signal End of I/O Modules?

PLC I/O devices have gone a long way from being wiring devices on a backplane. Once thought to be dinosaurs replaced by smart sensors and device networks, I/O devices are showing new life by offering users new ways to improve control of machines in a cost-effective manner. Several trends in the market are occurring simultaneously, but all have one direction—increased opportunities for the user to select the control solution that best fits the needs of each unique situation.

I/O is no longer the domain of proprietary architectures. Reed Ashmore, I/O product manager at GE Fanuc Automation (Charlottesville, Va.), points out that customers no longer limit themselves to a single vendor for the various components of a control system. Customers like the modularity available now so that they are not limited by the number of open slots in a PLC rack. In addition, Mr. Ashmore sees requests for specialized modules like thermocouple, strain gage, and motion control increasing.

“One of the key trends in I/O subsystems is a wider availability of unbundled products, like distributed ‘rack, ”block,’ and ‘direct’ (sensor/actuator) I/O devices that are mated to alternative ‘brains,’ such as a personal computer,” notes Trayton Jay of Mitsubishi Electric Automation Inc. (Vernon Hills, Ill.). ” ‘Open’ standards promote inter-vendor product compatibility. This allows customers to keep the advantages of compact, rugged PLC I/O modules where appropriate and to access [what some perceive as] more open, flexible, or advanced programming and integration environments.”

Higher density means wiring woes

Mr. Ashmore calls the “biggest contradiction in customer requests” the fact that customers want more I/O points in a smaller card with the ability to switch higher energy.

Joe Pickell, product manager for PCD Inc. (Peabody, Mass.) a manufacturer of termination systems, feels that increasing density has been overlooked in most analyses of I/O modules. No longer satisfied with 8-point or even 16-point I/O cards, users now want to save space with 32-point and 64-point cards.

Since these small footprints and compact terminations are harder to wire using conventional methods, manufacturers are now using prefabricated cable assemblies rather than discrete point-to-point wiring. This has the further benefit of reducing the potential for cross wiring and other wiring problems.

Other problems that Mr. Pickell discusses include the susceptibility of high density cards to damage. Damage on one channel can potentially wipe out a large portion of a system as the high I/O count card is being replaced. This means an increased need to take protective measures in the termination area.

He adds that with systems becoming more complex and the need for automated testing growing, test ports must be provided in the connection scheme. PCs, now more prevalent in the industrial world, typically have some type of multipin connector for I/O devices creating a demand for interface modules.

“Control systems are becoming more critical to all plant operations,” concludes Mr. Pickell. “Downtimes must be minimized. This is leading to increased need for termination systems that can be quickly unplugged in the event of a failure.”

One company that has capitalized on two trends in I/O devices—increased density of modules and willingness of engineers to use third-party solutions—is Weidmuller Inc. (Richmond, Va.). It has announced new versions of its PLC interface products for Modicon’s Quantum PLCs and GE Fanuc Series 90-70 PLCs. The line of products for the Allen-Bradley PLC-5, and SLC-500 has also been expanded. These join versions that support Siemens, Mitsubishi and Omron as well as other Allen-Bradley, GE Fanuc, and Modicon products.

The Weidmuller PLC Interface system consists of either two or three components which replace the I/O card terminal strip and the rail of terminal blocks. This reduces the large number of wires that has to be individually connected by an electrician with the resulting chance for errors.

Weidmuller also contributes to the high-density trend. It has just released a family of compact, high-density interface relay modules. The PlugPak eight channel relay module is only 22.5 mm wide featuring 24 Vdc operation, LED coil indication and pluggable inputs and outputs. The UPAC relay module has 4 contacts in a 17.7 mm wide housing. The DK5R-SPNO (single-pole-normally-open) and DK5R-SPDT (single-pole-double-throw) are only 6 mm offering 4000 V surge isolation, LED status and diode protection.

Spectrum Controls (Bellevue, Wa.) is releasing an eight-channel thermocouple/millivolt analog input module specifically for the Allen-Bradley 1746 I/O structure. The 1746sc-NT8 is individually configurable for each channel for either thermocouple type or millivolt input range. Designers can save rack space and extra modules reducing costs.

Another new high-density offering is from Omron Electronics (Schaumburg, Ill.). These new eight-point analog modules offer conversion rates of 1 microsecond per point, incorporate independent offset and gain adjustments for each input or output, and continuous monitoring for real-time error detection.

Opto 22 (Temecula, Calif.) has also capitalized on two trends developing in I/O configuration. The company believes that the biggest impact on the current market is in device-level networks combined with the opening up of I/O systems to many different vendors. The Opto 22 Snap I/O is a small rack which can be panel mounted or DIN rail mounted. It is then populated with a variety of small, high-density modules and a “brain,” which communicates via various network protocols to a controller. Supported networks to date are Optomux, Pamux, Mistic, DeviceNet and SDS. Soon to come are Modbus and Profibus, the company says.

More networking choices shrink cost

The talk about one industry standard device-level network may still be continuing, but manufacturers are not waiting for the dust to settle before implementing solutions. There are several networks on the market today. Each offers users the opportunity to hang I/O devices out on the machine away from the main controller. This means that electricians only run one cable back to the panel. Savings result from less wiring and from fewer processors—sometimes with microcontrollers acting as both local logic and I/O module.

David Pokraka, I/O systems specialist of Siemens Energy & Automation Inc. (Alpharetta, Ga.) sees major benefits coming from this emphasis on networking. By standardizing on the Profibus device network, Siemens has focused on developing special I/O modules such as a waterproof block and modular high-density blocks. The user will benefit from the flexibility of tailoring a solution to the need at the machine level. Just because one part of the plant uses high-density modules, the application on the other side of the plant needing only a few points is not forced to use a larger device than it needs.

Siemens has introduced a stretch version of its “Logo!” universal logic module. The Logo!-L, while only two inches longer, has double the I/O to 12 inputs and 8 outputs. It fills the performance range just below the microPLC yet has an optional AS-interface bus module to integrate into a larger control system.

“Bus systems are differentiated by their performance, throughput, diagnostics capabilities, and the number of I/O which can be connected,” says Bjeorn Falke, application engineer with Phoenix Contact Inc. (Harrisburg, Pa.). “Interbus is designed for the real-time transport of digital and analog I/O data in addition it also supports more complex devices such as drives, robots, or ID systems. Bus systems will incorporate more PC based control and a higher degree of distributed I/O, reducing parallel wiring to the minimum in the future.”

Phoenix Contact has specialized on the Interbus network. Controller modules for the network are provided for many PLC platforms. I/O modules and blocks in many varieties are available.

Some companies have chosen Ethernet as the I/O bus of the future. PLCDirect (Cumming, Ga.) has expanded its commitment in this direction by introducing two new Ethernet communications modules. The H4-EBC Ethernet Base Controller module provides PC-based, deterministic, remote I/O control functionality for the DL405 family of controllers. In addition to the new H4-EBC module, the new H2 and H4-ECOM Ethernet Communication modules enable high-speed Ethernet communications among PLC racks and provide PLC to PC communications using PLCDirect’s DirectSoft PC-DDE server software.

Rockwell Automation/Allen-Bradley (Milwaukee, Wis.) has introduced the ArmorBlock Low Profile (LP), a smaller member of its ArmorBlock I/O family. ArmorBlock I/O combines an adapter, power supply, rack and I/O into one industrially sealed package. This family of modules distributes I/O points closer to the application which helps reduce the cost of wiring and increases flexibility for the designer. The ArmorBlock family supports the DeviceNet network.

Action Instruments (San Diego, Calif.) believes in giving I/O function that is not network specific. Action’s “buss-ticket” allows the selection of the device network or fieldbus of the user’s choice. The “I/O with I/Q” includes a rail which has connections for power and network for each module. Power, network, I/O and buss-ticket modules all plug in to the rail. Action Instruments provides a variety of special modules—signal conditioners, programmable two-wire transmitters, limit alarms and loop isolators.

Entrelec’s (Irving, Tex.) new “RIO” I/O module features a building block concept for remote bus interface and distributed intelligence. The main terminal block with plug in electronic modules is compatible with Profibus 1.5 MB, Profibus 12 MB, Interbus, Interbus “E” and WorldFip. The main bus node terminal block module includes eight digital inputs and eight digital outputs. Additional I/Os can be added with an expansion terminal block. The “RIO” line is compatible with Entrelec’s “Systron” modular I/O system. Entrelec’s product specialist, Bryan Moore, states, “The new RIO line of remote I/O modules is a flexible, cost-effective design because it offers the user options for selecting bus systems, expansion capability, two or three wire I/Os and mounting configuration.”

Wieland’s “ricos” products combine a fieldbus interface module with both digital and analog I/O units. Ricos supports Profibus, Interbus, Modbus Plus, DeviceNet, and SDS.

New products, more choices

Schneider Automation’s (North Andover, Mass.) Modicon TSX Momentum product designed as a new modular system with controller, I/O system, and networking components. Its open architecture concept gives users the flexibility to solve particular problems rather than forcing packaged solutions to fit.

Momentum I/O components work well with distributed control systems (DCS), PC-based control and traditional PLC platforms. Supporting Modbus Plus, Redundant Modbus Plus, FIP I/O, Interbus, and Profibus now, with Ethernet I/O, DeviceNet, ControlNet, ASi, and Seriplex to come, Momentum will work in almost any control environment. An engineer will not be forced to throw out existing controls that still work to add new functions.

The Momentum Processor Adapter allows the system to act as distributed intelligence in a larger system controlled by a Modicon Compact or Quantum PLC system.

For those who are installing a DeviceNet system in a machine with existing input and output devices, Cutler-Hammer/Eaton (Milwaukee, Wis.) has just released the DN50 I/O block. According to Greg Bell, PLC/Operator Interface product manager, “The features designed into this I/O block offer users the flexibility to adapt to applications easily, as well as make changes as their requirements change. With the variety of I/O configurations offered, virtually any device can be connected to a DeviceNet system.”

A device without a CAN (Controller Area Network) communications chip is wired to the DN50 and can communicate over the network. Programming and configuring is done with Cutler-Hammer’s NetSolver and NetView software packages. The user can troubleshoot, set attributes and gather diagnostic information from these devices.

Specialty modules increase options

GE Fanuc has released a high-density, standalone thermocouple module for its Series 90-30 PLCs. The TCM provides eight thermocouple inputs (J, K or L type), one RTD input, and eight PID-controlled solid state 24 Vdc outputs for controlling heaters. Each channel can operate in either closed- or open- loop mode. It features autotuning and diagnostics.

An interesting approach to distributing I/O devices is taken by Festo Corp. (Hauppauge, N.Y.). It has embedded Allen-Bradley SLC 5/02 technology into a pneumatic valve manifold. Communicating over either DeviceNet or Allen-Bradley’s DH-485 network, the smart manifold can operate as part of a larger control system or operate as a stand-alone controller.

Klöckner-Moeller has added two specialty modules to its PS4 family. The EM4-101-TX1 is a network temperature module that monitors locally and communicates via RS-485 to the controller. It has six inputs for measuring temperature plus two standard

Is software important?

Intellution Inc. (Norwood, Mass.) and Grayhill Inc. (LaGrange, Ill.) have announced an agreement to port Intellution’s Paradym-31 SoftLogic control software to Grayhill’s OpenLine controller (see related News article). Brian May, Grayhill’s president of the Electronic Systems Division, believes that software is a crucial element for I/O device manufacturers. They need software support so that the devices can be easily used by various hardware platforms and human-machine interfaces. This software integration “will result in the simplest system to configure, program and troubleshoot on the market today,” he says.

Steve Blakely, SST (Waterloo, Ontario, Canada) product manager, notes, “One trend we’ve noticed over the last few years with our network interface cards is the increased value customers place on the software available from the card vendor. We have taken a unique position in the marketplace by including all this software free-of-charge with our DeviceNet, Profibus, and ControlNet interface cards.” SST (formerly SS Technologies) has just announced the DeviceNet Pro interface card. With enhanced features over the original DN card, The 5136-DNP is available for ISA, VME, and Multibus.

Given all these new developments in I/O modules and devices, engineers have a better opportunity to find the appropriate technical and economic solution for increasingly demanding problems.

Do Intelligent Sensors Signal End of I/O Modules?

A computer chip can be implanted into almost anything today. A few years ago, the industry started putting more and more intelligence in its sensors. Remember the introduction of fuzzy logic in various kinds of sensors? Add to this the acceptance of device-level networks and many argued there may be little or no need for a controller.

At the introduction of these new intelligent sensors, manufacturers said that many, if not most, decisions could now be made directly from the sensor to the output device, completely bypassing the controller. The only purpose of the controller for this type of transaction was to monitor and override. The user saw two major benefits. First, the speed of the system could be greatly enhanced with no need to wait for the scan time of the processor. The size of the controller and its complementary I/O racks and modules could be reduced, saving money.

But these features also increase cost and complexity of the sensors. It often makes more sense to use the same sensors and other field devices that are already in stock and add value to the I/O modules and devices (the component between the sensor, contactor, etc. and the controller).

Rockwell Automation’s (Milwaukee, Wis.) Bob Lennon, vice president of the I/O business, sees two trends in the market, both placing more value on the I/O solution. “The first shows I/O products becoming more highly distributed and operating more independently from the master controller,” says Mr. Lennon. “The other trend shows I/O products becoming more tightly integrated with the controller, which supports centralized I/O solutions.” In both environments Rockwell Automation sees more and more value migrating into the I/O solution.

Customers are driving manufacturers to find new and innovative ways to find new solutions to specific manufacturing problems. This means open systems, multiple-vendor solutions, and application specific devices.

Are intelligent sensors going to replace I/O modules in the near future? The answer is resoundingly, ” No.”

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