Servicing PLC 120V I/O modules

Nov 1, 1995 12:00 PM

Repairing all types of I/O modules, from fairly simple to quite complex, can be done with common tools and procedures.

Input/output (I/O ) modules used in PLCs are very reliable; however, they occasionally fail. When they do, we usually discard the defective units or send them back to the PLC manufacturer for service. There's another recourse, one that can substantially reduce the costs associated with module failures--doing the troubleshooting and repairing yourself. It's not that difficult.

Understanding I/Os

Before attempting to troubleshoot and repair a defective I/O module, you need to know how to trace out the circuits on the PLC's 120VACI/O printed circuit board. Simplified circuits are shown in Fig. 1.

[FIGURE 1 ILLUSTRATION OMITTED]

Circuit configurations vary with the different manufacturers of the equipment and the model of I/O you're working on. Generally, the input circuit reduces and rectifies the input voltage before it's fed into an opto-isolator integrated circuit. The purpose of the opto-isolator is to isolate the incoming voltage from the rest of the circuitry. The output of the opto-isolator drives some type of integrated circuit (designated "U1" in the diagram), which in turn drives an indicating LED and sends a signal to the central processing unit (CPU).

In the basic output circuit, the signal coming from the CPU goes into another opto-isolator. From here, the signal goes through a resistance network before it's used to drive a triac, which controls the line current going to the load.

Identifying the circuits

While one manufacturer's I/O module differs from another, they all have certain similarities. The components on their printed circuit boards can be cross referenced and are readily available, and you can identify the various active components and find pin and lead nomenclature.

Once the circuits are identified, you should prepare a sketch, label the edge contacts, and keep the sketch for future reference. An example of a circuit sketch is shown in Fig. 2. The keyway on the board gives you a reference point.

[FIGURE 2 ILLUSTRATION OMITTED]

The individual circuits can now be tested and repaired. Usually, only one section per module will prove to be defective. By energizing the various circuits and comparing voltage readings on the same board between a good section and a defective one, you should be able to find the problem.

Using an I/0 tester

Having a test setup for checking 120VAC printed circuit boards greatly speeds up troubleshooting. An I/O tester is shown in Fig. 3. It can only be used for a particular type of module. Since there are various types of I/O modules, you'll have to assemble a tester for each I/O being tested. Don't be alarmed; this task is fairly simple, once the card edge contacts are identified.

[FIGURE 3 ILLUSTRATION OMITTED]

Since most industrial plants standardize on one or two makes of PLCs to be used, an I/O servicing procedure to identify the circuits can be set up with little effort. The most important of the initial steps is documentation. Schematic diagrams of the circuit boards are generally available from the manufacturer. Even without this information, a lot of service work still can be performed.

Using a digital V-O-M set on the low ohms scale, it's possible to follow the various foil traces on the card and identify the edge contacts. Since most circuit boards have foil traces on both sides, tracing can be aided by holding the circuit board over a flashlight or some other source of light. A back-illuminated drawing board is excellent for circuit tracing.

More sophisticated PLCs have much more complex input and output circuit boards, and building a tester without a schematic diagram from the manufacturer in these cases could be difficult. Nevertheless, these complex boards have certain similarities to the simpler ones.

Testing procedure

First, plug in the defective board into the appropriate socket of the I/O tester (either input or output), and test each section of the module circuit board individually by moving the appropriate toggle switch (marked "Input A," "Input B," etc. in Fig. 3) to the ON position. The LEDs for circuits A, B, C, and D on the circuit board and tester should light. If an LED fails to light, this pinpoints the defective section.

Second, with the circuit board still in the I/O tester, take voltage readings with a voltmeter at the various points along the defective circuit and compare them with those taken at the same points on a circuit that's been tested and is OK.

For example, suppose we're testing an input card. We first verify that 120V is available at various parts of the input circuit. With one probe on the "AC common" terminal, place the other probe on the " AC input " card edge terminal. A 120V reading should be seen. Depending on the actual circuit configuration, moving the probe progressively toward the rectifier ahead of Terminal 1 of the opto-isolator will produce specific voltage readings that can be compared to those in correctly operating circuits. If a break exists somewhere in this path, a reading of 0V will be measured beyond that point.

If the readings taken on the AC circuit are good, then the voltage in the DC circuit should be checked. With the V-O-M set on a low DC scale and its negative probe set on the "[-V.sub.cc]" terminal, the plus probe can trace the voltage from the pins of the opto-isolator to the pins of the integrated circuit. Comparisons of readings then can be made between the circuit being tested and those obtained at similar points in a section of the card that's working properly. The DC voltage across resistors and capacitors between the opto-isolator and the integrated circuit also can be checked and compared.

Let's take an example. Suppose that on the input card of the PLC for which the tester shown in Fig. 3 is used, a voltage of +0.9VDC is measured between Pins 1 and 2 of an opto-isolator of a good section. By applying a variable DC power supply to inject this signal into the opto-isolator of a failed section, we see that the LED on the module and the tester turn ON. Thus, the problem is located somewhere ahead of Pins 1 and 2.

Breaks in printed circuits

Problems may be associated with circuit discontinuities, such as a foil break. Locating such a break is fairly easy. With the circuit removed from our tester, we can make a close inspection with a magnifying glass or use a continuity tester to reveal a break between points.

We can repair this break by soldering a small strand of wire over it. This will make the section operational again. The circuit can then be placed back into the I/O rack of the PLC. Troubleshooting and repair time is kept to a minimum.