Industrial Network Basics: Simplifying I/O Terminology

There are many terms used for I/O technology in industrial automation: Remote I/O, Distributed I/O,  Modular I/O, Expandable I/O, Block I/O, Conventional I/O and the list can go on.  What do they all mean?  Can they be used interchangeably?  What is the difference?

Lets be honest… this is a muddled topic and many people use different things interchangeably.  I’ve done a bit of research and reading of automation magazines, forums and websites and have tried to piece it together.

Basically you have to think about it two ways:

  1. Where is the I/O located on the machine?
  2. How is the I/O collected from the machine?

Where is the I/O located?

Centralized I/O:
A majority of the I/O is located in the cabinet with the PLC and is situated on one continuous backplane.

Distributed I/O:
Small percentages of the I/O are located in many locations that are not the same as the controller.  This data is usually collected over an industrial network.  Distributed I/O solutions usually generate a total cost of ownership per point lower than other types.

Remote I/O: 
This has evolved over time but in 2011, I believe this is the new definition.   A big percentage of the I/O is in a single or few locations that are not the same as the location of the PLC.  It is a hybrid of Distributed and Centralized I/O solutions.  This data is usually collected over an extended backplane or an industrial network.

How is the I/O collected?

Conventional I/O:
Normally mounted inside a cabinet on DIN rail or rack; this utilizes a backplane providing communication and power supply to the I/O devices, which are unique to the version of the master device.  This is usually found in a centralized or remote I/O configuration.

Expandable I/O:
Found both inside and outside the cabinet, expandable I/O solutions utilize the “slice” concept and use a backplane/sub-bus to communicate between I/O devices, which are unique to the version of the master device.  These can implement large numbers of additional slices of different I/O types to be added including valve manifolds.  It is usually used in a remote I/O configuration.

Block I/O:
A set number of I/O points typically 8 or 16 discrete points.  Block I/O usually communicates over an industrial network and is rated for use on the machine, outside of a controls cabinet.  It is almost always found in a distributed I/O configuration.

Modular I/O:
Normally found outside the cabinet fitted with an industrial network communications head with the ability to combine multiple types of I/O devices, including valve manifolds.  Modular I/O is typically used in a distributed or remote I/O configuration.

Click here for a full line of Block I/O devices as well as Distributed Modular I/O devices.

Will Healy III is the Industry Marketing Director at Balluff Inc. in Florence, Kentucky and he is enthusiastic about smart manufacturing, automation and STEM education. Will graduated from Purdue University with a degree in mechanical engineering and has been sharing his passion for automation for more than 10 years in a variety of industries. He is published and quoted in various trade magazines, works as an industrial adviser for multiple universities and has widely presented on the value sensors, networking and IIoT bring to manufacturing.

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