To keep the gremlins out of a programmable controller system, avoid those areas where mistakes are most likely to occur. Here are the five most common planning and installation errors that cause poor operation. And, they even occur at companies with the best engineering staffs.

Lack of memory mapping

An electrician at a large steel mill uses documentation to identify a particular module in a programmable controller I/O rack. Good documentation makes it easy to locate modules that need wiring, adjustment, or repair.

One of the pitfalls frequently encountered is the failure to plan software programs — through memory mapping — before configuration. Memory mapping establishes the design concept and the method of arranging files. Setting up all programs in a consistent manner achieves the same look and feel, which makes it easier to design and maintain programs.

Memory mapping not only makes writing programs easier, it speeds troubleshooting. For example, a software bug in the interface program for a group of robots can be fixed more quickly if the applicable program is easy to find. It should be located under the same file name in controller No. 1 as in controller No. 10.

Users should establish standard memory- mapping conventions and apply them to all of their systems — even for controllers from different vendors — as consistently as possible.

For example, a maintenance engineer should be able to find the communications interface, message, or diagnostic programs in the same location and under the same file name in any controller, regardless of the controller function. In other words, programs that perform a certain type of function, such as enabling a robot to run or a programmable controller to communicate, should be placed in the same file and location every time, regardless of the machine manufacturer.

Improper grounding

Poor grounding of equipment in a manufacturing environment can cause random problems that are difficult to troubleshoot. For example, stations on an improperly grounded system can disappear from the screen of the network manager (person who monitors network functions) and then re-appear.

When application engineers are asked to assist a customer who can’t get a programmable controller running, they start by ensuring good grounding practices. Often, that is all it takes to get the system running. No hardware or software changes are needed.

Making sure that your programmable controller is properly grounded is generally a simple matter of following a few guidelines:

• Use good grounding practices in accordance with the manufacturer’s recommendations and the National Electrical Code (NEC).
• Provide the installer with customer-specific guidelines for installing the equipment.
• Use the correct size ground wire based on the equipment vendor’s specifications and the NEC.
• Attach the ground wire to the correct lugs on the power supply and the controller.

As simple as these guidelines may seem, they ensure that grounding is done properly and save hours of troubleshooting.

Improper network connections

Often, users or vendors fail to make proper cabling and termination connections. Sometimes, very simple things are overlooked. For example, terminating resisters on remote I/O links and communications networks provide proper impedance matching. If a link is not terminated, it can act as an antenna, picking up noise from other devices in the facility. To determine the proper terminating resistance, check the specific controller instruction manual for recommendations.

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