| The programmable logic controller, more | | | | indicator were not lit and the technician verified |
| commonly known as the PLC, plays a role in | | | | that the correct voltage is applied to the power |
| almost every automated manufacturing process. | | | | terminals. Some PLC's are equipped with an |
| The PLC is an electronically programmable device | | | | internal fuse which can also fail. These problems |
| capable of almost infinite combinations of relays, | | | | are not quite as common as a failure of an input |
| contacts, and timing circuits used to control | | | | or output terminal. The IO modules are more |
| industrial machinery of all types and sizes. It is | | | | likely to fail due to loading problems. |
| essentially a computer designed to withstand a | | | | Inputs and outputs are available in multiple |
| harsh manufacturing environment. It was designed | | | | configurations and voltages. Both AC and DC are |
| to replace complex system of relays, cam | | | | available separately and sometimes in the same |
| switches, and other electromechanical circuits. The | | | | unit. These input/outputs can be both digital and |
| first PLC's were introduced in the late 1970's and | | | | analog and can be configured as a transistor |
| they continued to grow in popularity since the day | | | | output or as a dry relay contact. On both the |
| of their release. | | | | input and output side it is critical to determine the |
| PLC's are produced today by numerous major | | | | specifications of the PLC first. Having the |
| manufacturers including Siemens, Allen Bradley, | | | | appropriate PLC software available at the time of |
| Koyo, Fuji, Telemecanique, and Mitsubishi as well | | | | troubleshooting is also necessary. The first |
| as a host of others. The programming of the PLC | | | | recommendation would be to compare the |
| is performed through proprietary software | | | | illuminated inputs and outputs against the software |
| produced by the specific manufacturer. Software | | | | in real time. An illuminated input not displaying |
| among PLC's is not compatible and the logic | | | | properly as an energized input in the software is |
| programming, although it is very similar, has many | | | | an indication of a failed or open input terminal. On |
| subtle differences across the different brands. | | | | the output side, an illuminated terminal not |
| The program stored in the PLC is called the ladder | | | | conducting the proper output voltage can mean |
| logic program. Problems in the ladder program can | | | | one of two things, the output has failed in an |
| become quite involved and that is an entirely | | | | open position, or an improper voltage is being |
| different subject altogether. Hardware problems | | | | applied to the common output terminal. The only |
| however, are fairly similar and can be easy to | | | | way to verify this would be through the use of a |
| diagnose if you know a little about the particular | | | | multimeter set on the proper voltage scale. |
| model and it's input and output configuration. | | | | Overall, PLC hardware troubleshooting is not |
| Hardware failure of the brain, or main processor, | | | | complex, yet it needs to be approached in an |
| is rare. It can be easily diagnosed visually most | | | | orderly fashion because both hardware and |
| often by an illuminated fault indicator on the face | | | | software are required to work in harmony for a |
| of the PLC. An internal power supply failure would | | | | PLC to perform its intended tasks. |
| be a possibility if the PLC screen or power | | | | |