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A good backgrounder for modular I/O technology is "Ins and Outs of Modular I/O" in the Q4 2008 issue of Industrial Networking.
One of the other reasons to use modular I/O is that it maximizes efficient use of spares. Most initial project designs, especially for a conventional I/O system such as for a DCS or PLC, include a set of spare I/O points for later development or modifications. The result is that customers pay for more equipment than they need so they can add a few sensors to the design or add a single channel of analog to the system. Separating the controller and I/O permits you more-efficient cabinet designs while remaining confident that even with late scope changes you can add I/O anywhere. Another reason for modular I/O is that spare connections can be a flexible placeholder for any type of I/O until the need arises. Modular I/O also limits the exposure of the industrial network cabling that runs through the machine. A distributed, modular I/O solution keeps the network cabling out of harm's way and only the individual I/O device cables are run into aggressive environments.
Different types of I/O have different demands on the data capacity of the backplane and associated processors in the communications interface and the flexibility that modular I/O offers, so use the manufacturer's software when planning and commissioning systems to manage the physical and data processing requirements/limits of the system.
Within a modular I/O system, a network I/O node (or slave) consists of two basic components: an interface to the network and an interface to the physical I/O. Modular systems typically allow more than one I/O block to be used with a single gateway. The gateway then acts as a master for the I/O subsystem, and obtains I/O information from all connected I/O blocks and reports it as a single message to the network.
Most network interfaces have built-in functions such as communications-loss watchdogs that can trigger reversion to fail-safe mode, though this requires specification of fail-safe and power-up conditions. Many interfaces also include the ability to do additional integrity checks. Communications redundancy, I/O redundancy and hot-swap module replacement are also offered in several distributed modular I/O product families or industry bus types.
So what industrial network or bus should be used to connect the remote modular I/O to the control system? The most basic question is "What industrial networks does this solution support?" The solution needs to include both ends of the network cable, the modular I/O and the protocols directly supported by the control system without the use of gateways. And third-party interfaces add complexity to any project.
The corollary question is whether the selected I/O solution is supported by multiple vendors on an open standard. The answer is likely to be yes. If only one vendor supports the solution or if it is proprietary, it will be difficult to switch vendors or get a variety of options in the solution.
Modular I/O will continue to grow in importance for the machine industry in particular and could eventually replace conventional rack I/O in most machine control designs. This is certainly being borne out by a significant number of key industry suppliers of varying sizes who offer modular I/O systems. The key will continue to be the ease with which these devices can be integrated into systems and that will be based on the network and protocol used to make the connections.