Remote I/O or CClink

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Remote IO can be any data collection far from the PLC. For example, you may have five thermocouples on a machine 300 feet from the PLC. One way to read them is by running five thermocouple extension wires from a thermocouple card in your PLC. But this is a lot of expensive TC wire, and subject to a lot of noise pick up. Instead, you can have one twisted pair RS485 serial line running to a remote thermocouple IO module. The protocol running over the serial line could be a common one like Modbus RTU. An example of a remote IO module like this is http://www.icpdas-usa.com/products.php?PID=480 which has 8 input channels (three extra).

As a concept, "remote I/O" is just what it says, any form of I/O equipment that is not directly attached to whatever your process controller is, a PLC, DCS, PC etc. etc. So when used as a generic term, it can mean anything and the way the processor communicates to it can be anything.

CC-Link is a communications protocol, one that would be USED by remote I/O to talk to a processor somewhere. it's a specific form of what is generically called a "fieldbus". Fieldbus systems can be EtherNet, DeviceNet, ControlNet, Hart Interface, Modbus, Modbus+, Profibus, Sercos, all kinds of different protocols exist, mostly promoted as "open" standards, but generally pushed by one or two vendors over the others so as to try to lock end users into their platform. For example DeviceNet was pushed initially by Allen Bradley, Profibus by Siemens, Modbus by Schneider (ex Modicon), Hart by Rosemount, and CC-Link is now being pushed mainly by Japanese companies like Mitsubishi and Idec.

However, "Remote I/O" (big R) was at one time a SPECIFIC form of internal communications protocol for Allen Bradley PLC-5 Series PLCs. It was how a CPU communicated to any I/O rack that was not physically attached to it. RIO was sometimes referred to a "Blue Hose" because the cable used by A-B was always blue. Because of the ubiquitous nature of A-B PLCs in North America, a lot of 3rd party vendors (of things that A-B did NOT make themselves) came up with Remote I/O interfaces for their products, and instrumentation was something that A-B did not get into, so there are a lot of instrumentation vendors that still offer "Remote I/O" interfaces for their products. A good rule-of-thumb is that if the vendor capitalizes it, as in "Remote I/O" instead of "remote I/O", then they are likely referring to the specific A-B interface protocol, not the generic use as external I/O for anything.

But even A-B hardly ever uses Remote I/O any longer, they switched to DeviceNet, ControlNet and most recently EtherNet for remote (small r) I/O communications. If you are looking at deciding on "big R" products, I wouldn't.