Remote and Distributed Input / Output Devices
Traditionally, Input/Output (I/O) cards and devices reside on the PLC rack (or chassis). However, another form that I/O can take is that I/O can take is that of distributed or networked IO. There are many different terms used to describe distributed IO – remote I/O, I/O bus, I/O field bus, I/O network, etc. What these all have in common is the ability for the I/O devices to be placed somewhere other than in the PLC rack.
Remote I/O Rack
The simplest remote IO solution to explain is the remote rack. With a remote rack, the main PLC rack is expanded by connecting a second rack via a connector cable. Usually, the second or remote rack is nearly identical to the main rack. The remote rack is essentially an extension of the PLC chassis. Many times the second rack will be installed in the same housing as the main rack, but it may be possible (depending on the rack, manufacturer, etc.) to place the second rack quite a distance from the main rack. Most PLC platforms allow multiple racks to be connected to the main rack in a daisy-chain fashion – there are usually limits to the total number of remote racks and total distance from the main rack.