Today's I/O systems can add intelligence, broader communications, and flexibility across multiple logic platforms, central or distributed. I/O modules, installation, retrofits, and associated labor can account for half of an automation project's total cost. "Designers can now mix-and-match some I/O modules across PLC, DCS, and PC control vendor product lines," explains Connie Chick, business ma...
Today's I/O systems can add intelligence, broader communications, and flexibility across multiple logic platforms, central or distributed. I/O modules, installation, retrofits, and associated labor can account for half of an automation project's total cost.
'Designers can now mix-and-match some I/O modules across PLC, DCS, and PC control vendor product lines,' explains Connie Chick, business manager for controllers at GE Fanuc. 'This freedom may complicate the selection process, but it ultimately allows engineers to create better systems—if they understand the ins and outs of I/O.' New I/O modules can ease the selection process, she suggested, by spanning control architectures.
To design the best I/O system, Chick says:
Consider I/O and control system structures before selecting products to meet application needs.
Evaluate system requirements, then define overall control attributes, such as response times, network communications, diagnostics, redundancy, and scalability.
Ensure network compatibility; designers can inadvertently increase number of required local area networks (LANs) by selecting incompatible devices.
Identify the physical structure or layout of the application (does the machine cover a small or large area of the factory) to help determine whether I/O modules should be local, remote, distributed, or a combination.
Figure the blend of rack-mounted and distributed. In a mixed application with rack-mounted and distributed I/O systems, rack-mounted I/O is used for those points requiring fast response and for local devices where wiring is minimal. Distributed I/O modules connect to some remote devices.
Prioritize such features as auto-configuration, addressing, and hot insertion.
Determine processor configuration. In some systems, a single processor (PC or PLC) provides all control capability, and I/O system can be either local or remote. Other systems split control across several processors with each acting nearly autonomously, often with another processor coordinating the system. Specialized tasks (motion control) or high-speed operations may require multiple processors. Each processor often controls its own I/O to maximize system speed, and provide a unique input or specialized output.
Consider response time, which is related to bus communications, diagnostics, redundancy, scalability, and modularity. It includes input signal filtering time, I/O scan time to get the signal to the CPU, time for the processor to act on the input, time to send the signal to the output device, and time for the output device to generate the output signal.
Report system health and device failure with diagnostics.
Build redundancy, even in I/O systems, for the most-critical applications.
Ensure scalability and modularity.
Permit auto-configuration and automatic addressing, which can cut setup time up to 50%.
Mark T. Hoske, Control Engineering, MHoske@cfemedia.com
National Instruments discusses I/O topics, including analog input/output, counter/timer, digital input/output, advanced programming techniques, at this area of the NI site. Click here