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