The first step in PLC troubleshooting is to decide if the problem is internal to the processor or in the I/O system. It's also assumed that the PLC is programmed using some form of ladder-logic and not a higher-level language, and the discussion is limited to the most common I/O module types, namely those that support digital and analog inputs and outputs. First, it's assumed that the PLC system under analysis was operating correctly at some time in the recent past, so the problems of program debugging and wiring errors that are more typical of a startup situation will not be addressed. The intent of this article is to cover only the basics of PLC troubleshooting, so there are some limitations on what will be discussed. The internal operation of a PLC can be monitored via a handheld programmer, terminal, or personal computer, and many indicator lights are provided for I/O troubleshooting. Those unfamiliar with PLCs often fear troubleshooting a device that appears to be a mysterious "black box" (see side bar on page 22), but in fact today's PLCs actually are very open systems that lend themselves to relatively easy diagnosis.
One reason for the popularity of PLCs is their high reliability in harsh industrial environments occasionally, however, things do go wrong and troubleshooting becomes necessary. The PLC reads inputs, processes them through a program, and generates outputs. I/O modules connected to the PLC provide analog or digital electronic interfaces to the external world. These functions usually are used to monitor and control individual machines or complex processes via inputs and outputs (I/Os). Those instructions enable the PLC to perform various useful control functions like relay logic, counting, timing, sequencing, and arithmetic computation. What is a PLC? In brief terms, a PLC is a digital electronic device that contains a programmable (changeable) memory in which a sequence of instructions is stored. Because they have constantly increased in capability while decreasing in cost, PLCs have solidified their position as the device of choice for a wide variety of control tasks. Programmable logic controllers (PLCs) have become important building blocks for automated systems.
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