The days when every device in an automated system had to be installed using multiple wires are over. While proper system design can still be challenging, technology advances in I/O have greatly reduced engineering and installation time and the wiring errors that once caused frustrating delays in the commissioning of new automated equipment.
1. Motion needs deterministic I/O. Try to have deterministic data transfer. If motion is I/O sensitive, then you need deterministic I/O to synchronize to the motion task, so that events happen in the same time interval.
2. Quick connections. When selecting a terminal block for your control cabinet, look for one that has pre-built jumpers that can be installed quickly. This will save time later.
3. Match specifications. It's important to know all the information about any components you are interfacing to when you develop specifications for a project. If others are supplying parts, find out whether they are NPN or PNP, if analog signals are voltage or current, any handshaking requirements and dry contact digital or higher-level communication. It seems simple, but one key component which doesn't match up with the I/O you select can cause big headaches.
4. Field electronics. Ensure that I/O electronic interfaces can be implemented directly in the field, allowing you to replace the junction box multi-conductor back to the equipment, rack or MCC room. Field electronics need to be G3 rated per ISA standard (S71.04).
5. Shorten commissioning. I/O allocation plays a key role in reducing the commissioning duration of an automation project, followed by appropriate loop testing prior to start of commissioning activities.
6. Delay layouts. The design team working on the project should not begin the planning and layout drawings for distributed I/O in the facility until all equipment OEMs have provided general arrangements. Provide the team with an overall scope on what these cabinets are called by name, and what area of the facility each will handle. Timing of this design phase with the up-front engineering will save time and money.
7. Leave room for expansions. Make sure your new control cabinets have plenty of room for all I/O points needed in that cabinet. Also allow room for expansions that will be required for unanticipated changes and upgrades In the future. For I/O wiring, plan to label the parts of the circuit wiring that connect to the I/O points from that side of the devices with the program addresses. This will lessen the time needed for your techs to troubleshoot issues with those circuits later on. The rest of the circuit labeling should be based on the print line numbers.
8. Diagnostics. Make use of the built-in diagnostic information provided by most I/O systems. Good information for the operator can save you a lot of production time.
Remote I/O pitfalls.
There are many design pitfalls to avoid when developing I/O systems. The options now available to the designer are limited mostly by hardware price point, once you understand the purpose, location and lifecycle issues. Just because a thing can be done, however, does not mean that it should. Remote I/O options, for example, are growing, which can reduce wiring and terminal block needs in some cases by over 50-80 percent compared to what was available just 10 years ago. But serious thought has to be given to proper documentation and training of plant personnel to repair and maintain the system and prevent downtime. The documentation should include troubleshooting methodology in addition to part number, manufacturer, MAC address, etc. The control software should also be integrated with the remote I/O layout to allow for either remote troubleshooting or status information. If these simple things are not properly accounted for and implemented, the actual usage of the remote I/O may not be worth the savings.
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