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Electrical Noise Forces the Choice

Nov. 9, 2014
When Is Digital I/O Preferred to Analog I/O?

We've always used a lot of analog I/O to capture process variables. Electrical noise is becoming a bigger headache for us and our customers as the machines' locations co-exist with a lot more legacy electronics, including old drives and motors, power supplies, as well as just an increased density of devices everywhere. We already use digital I/O via EtherNet/IP (and sometimes other fieldbuses) more frequently for other types of machine signals to save on wiring, gain some device intelligence and increase system flexibility. Are we better off just moving entirely to digital I/O?


There is a migration across the industry toward intelligent I/O, for good reason. Intelligent sensors provide more valuable information on the health of the machine than standard on/off sensors.

Also Read: Open I/O Systems Arrive

Intelligent I/O encapsulates many technologies that provide more than just object detection information to the controller or PLC. This includes DeviceNet, Ethernet, AS-i, Profibus and IO-Link. A recent transition in this space has been to IO-Link-enabled sensors. These sensors can provide significant benefits that prior open standards were not able to achieve. IO-Link technology can provide diagnostic information, such as temperature, operating hours and the received signal strength of the photoeye or proximity sensor. The technology also delivers digitized information and is less susceptible to electrical noise compared to analog solutions—helping to simplify installation and maintenance.

In addition, IO-Link was designed to be forward- and backward-compatible. Users can continue using the same connectivity and sensor catalog numbers. Sensor companies are adding IO-Link functionality to their existing sensor models as opposed to introducing specific IO-Link catalog numbers. Users can apply a single product in standard IO and IO-Link applications as needed, without an increase in the number of product models they need to stock or the confusion associated with more part numbers. To enable the IO-Link functionality the user must install an IO-Link master where the additional diagnostics are requested. This also gives flexibility to upgrade or modify a machine easily in the future without re-wiring the machine or buying all new sensors.

One more consideration is speed requirements. For the most demanding high-speed applications, analog I/O may still be necessary. While more costly, analog I/O can perform at speeds higher than the speed for which I-O Link technology was designed. However, we estimate that IO-Link will meet or exceed the vast majority of application requirements.

Kevin Zomchek, product manager,
Rockwell Automation

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