Chapter 9 PLC and System Maintenance PLC System Maintenance • PLC Hardware Maintenance • Visual Inspections • Energized PLC Maintenance • Battery Maintenance • PLC Software Maintenance • Equipment and Documentation Verification • Software and Program Verification

The maintenance of a PLC must be carefully documented for future reference. A problem found during maintenance must be corrected before the problem leads to PLC failure and equipment downtime or hampers future troubleshooting efforts. System maintenance must be scheduled so maintenance does not interfere with the machine or process the PLC controls. As part of PLC system maintenance, power to a PLC is turned OFF. PLC system maintenance is typically performed annually and always recorded using maintenance logs. See Figure 9-1. See Appendix.

PLC maintenance is performed after the application is stopped and a lockout/tagout is applied. Technicians performing PLC system maintenance must understand how to safely stop the machine or process, turn the PLC OFF, and lock and tag out all energy sources. See Figure 9-2.

PLC hardware maintenance is work performed by a technician that consists of a visual inspection of a PLC, energized maintenance procedures, and battery maintenance. Hardware maintenance is maintenance that consists of a visual inspection of the PLC, energized maintenance procedures, and battery maintenance. To perform the PLC hardware maintenance, a technician must have the PLC manuals and the PLC wiring prints. Before any visual inspection is performed, power to the PLC must be turned OFF. Any foreign control voltages present in the PLC enclosure must also be turned OFF. A technician performing PLC hardware maintenance may require the assistance of a second technician or an operator because some PLC applications must be stopped in a specific sequence to prevent equipment damage. After all power is turned OFF, a lockout/tagout must be applied to the disconnect. See Figure 9-3.

A visual inspection of a PLC and PLC-related components is part of PLC hardware maintenance. A thorough visual inspection of the PLC, PLC-related components, and input devices and output components must be made periodically. See Figure 9-4. Before performing a visual inspection, use a DMM to verify that no voltage is present in a PLC enclosure.

Proper personal protective equipment (PPE) must be used when performing energized maintenance on a PLC. Energized PLC maintenance is performed after the visual inspection is successfully completed. Energized maintenance is maintenance that verifies that the master control relay (MCR) is functioning correctly, voltages are within specified range, and no abnormal conditions exist. See Figure 9-5.

Backup batteries for the CPU of a PLC can be behind the hinged door or cover of the CPU or mounted on the processor card. The processor of a PLC monitors battery voltage level. When battery voltage falls below a threshold, the “LOW” LED is turned ON and the battery must be replaced. There are two different procedures for replacing PLC batteries, depending on the location of the battery. One procedure is used when the battery is mounted behind a hinged door or cover and can be easily removed. Another procedure is used when the battery is mounted internally on the card of the CPU. See Figure 9-6.

A PC, an interface cable, and sometimes an interface card are required to connect a PC to a PLC and use the PLC programming software. Regardless of the size or type of PLC installation, a PC with an interface cable is required for programming. See Figure 9-7. Laptops are very popular for programming PLCs because laptops can be taken to the location of a PLC installation. Other equipment that can be required during PLC programming are PCMCIA interface cards, CDs or floppy disks with programming software, and keys for changing modes.

A documented program contains address descriptions, instruction comments, rung comments, and page titles. Copies of the PLC program typically contain various descriptions and comments that explain how the machine or process operates and identify the specific input and output devices. Although similarities exist, program documentation varies among programming software manufacturers. PLC programming documentation includes address descriptions, instruction comments, rung comments, and page titles. See Figure 9-8.

When connecting a PC to a PLC, the technician must be aware of certain technical issues and follow a series of steps. The first step in software and program verification is connecting the PC to the PLC and verifying that the most recent PC version (copy) of the program is in the PLC. Certain issues must be considered before connecting a PC to a PLC. See Figure 9-9.

PLCs have electrically erasable programmable read-only memory (EEPROM) chips to provide a backup to RAM memory. The second step in software and program verification is EEPROM verification. Many PLCs have electrically erasable programmable read-only memory (EEPROM) chips to provide a backup to the RAM memory. A copy of the PLC program (ladder logic only, no documentation) resides on the EEPROM. A PLC can be configured to load a program from the EEPROM to the CPU upon power-up or upon detection of a memory error. EEPROM chips can be an integral part of a PLC or a separate unit that plugs into a socket. See Figure 9-10.

After the machine or process is started, the PLC program must be copied to a separate medium and stored in a safe location. The third step in software and program verification is to back up the PLC program that resides in the PC. The PLC program must be backed up because PCs can malfunction and lose information. Program backup is also the final step in PLC maintenance. See Figure 9-11.