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:
- Where is the I/O located on the machine?
- How is the I/O collected from the machine?
Where is the I/O located?
A majority of the I/O is located in the cabinet with the PLC and is situated on one continuous backplane.
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.
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?
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.
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.
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.
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.