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Troubleshooting Tutorial: Common SLC 500 Faults – Part 1

I. Introduction
Programmable Logic Controllers (PLC) such as Allen-Bradley’s SLC 500 Modular or Fixed style hardware are designed with capability of self-diagnostics. With Self-diagnostics, the controller is able to detect the operational status of every member module and report this externally through varying behavior of built-in LED indicators and internally via the diagnostic buffer.

II. SLC 500 LED indicators
A SLC 500 controller is provided with LED indicators such as:

                –              RUN                       (All controllers)
                –              CPU FAULT          (SLC 5/01 and SLC 5/02)
                –              FLT                         ( SLC 5/03; SLC 5/04 and SLC 5/05)
                –              BATTERY LOW   (SLC 5/01 and SLC 5/02)
                –              BATT                     ( SLC 5/03; SLC 5/04 and SLC 5/05)
                –              FORCED I/O        (SLC 5/01 and SLC 5/02)
                –              FORCE                   ( SLC 5/03; SLC 5/04 and SLC 5/05)
                –              RS232                    ( SLC 5/03; SLC 5/04 and SLC 5/05)
                –              DH485                   (SLC 5/03)
                –              DH+                       (SLC 5/04)
                –              ETHERNET           (SLC 5/05)

Generally, faults are made known when the CPU FAULT or FLT LED is energized RED. Similarly, other controller states such as Battery status, I/O status, communication and controller modes are indicated by the energization states of each individual LEDs.

When the FAULT LED is activated, this prompts the user to take action. It is of top priority that the FAULT message be identified before trying to remove the error or else, it will simply return. There are several ways to remove the FAULT from the controller.  

III. FAULT Handling
There are ways to remove error from a faulted controller. The most common are:

                A. Hardware corrections:
                –              Switching controller mode from RUN to STOP then back to RUN mode.
                –              Power cycling the controller or the entire SLC 500 rack.

B. Software corrections
                –              Downloading entire program
                –              Clearing Fault
                –              Identifying fault via error codes listed in the diagnostic buffer and applying                                                        appropriate correction.

Out of the listed methods, the most efficient way of Fault handling is by visiting the diagnostic buffer, identifying the error codes that correspond to the actual Fault. This way, the user may apply the necessary correction to prevent the fault from returning. Other mentioned methods may remove fault temporarily however, without knowing the source of the problem, it is most likely to return.

IV. Accessing the Diagnostic buffer

The diagnostic buffer is actually the Data File S2. This data file is accessible by:

                A. Going ONLINE with the controller.
                It is imperative that the Data File S2 is accessed ONLINE otherwise, it will appear that the processor has no fault or information displayed are outdated when viewed OFFLINE.

To access the data file of a FAULTED controller, simply navigate through the controller panel located on the upper left of the software. Click on the drop-down arrow and select “Goto Error” option. This will directly send the user to the “Errors” tab.

B. Directly through the Data Files
                Another way of accessing the diagnostic buffer is by opening the Data File S2 directly.

Generally, when an error is present, a phase or paragraph describing the error appears inside      the error description box. Error codes are also displayed in HEX code. Both these information provide reference to the appropriate action needed to perform to correct the error.

V. Tools and skills needed to efficiently troubleshoot an SLC500 fault.
Systematic approach is required to correct the fault. This is needed to determine the root cause of the error to be able to perform the necessary action and prevent the controller from entering Faulted state again. To systematically troubleshoot the error, one may need the following:

  •  Programming workstation with RSLogix 500 software installed.
  •  Communication cable for getting online with the controller.
  • SLC 500 Instruction Set Reference Manual. This manual has a dedicated section that discusses fault codes and resolutions.

Additionally, the user must be familiar with downloading the user program as this capability may be used to temporarily correct the error or an integrated step taken to permanently resolve of the error. Program download procedure can be found on the appendix of this user guide. 

VI. Common SLC 500 Faults
Now that we have understand how to properly identify the error, it is also useful to know the common faults and corrective actions that may be experienced and applied to an SLC 500 Processor. Having this knowledge will help increase the availability of the controller, preventing unwanted or unplanned shutdown.

In the next sections, we will discuss common SLC 500 Processor Faults and I/O Hardware faults.

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This entry was posted on March 12th, 2021 and is filed under Allen-Bradley, Automation, PLC, Technology, Troubleshooting. Both comments and pings are currently closed.

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