Troubleshooting Tutorial: Common SLC 500 Faults – Part 2 : Processor Faults

Programmable Logic Controllers (PLCs) such as Allen-Bradley’s SLC 500 Modular or Fixed style hardware are designed with the capability of self-diagnostics. With self-diagnostics, the controller can detect the operational status of every module and report this externally through varying behavior of built-in LED indicators and internally via the diagnostic buffer. This is part 2 of our tutorial for troubleshooting common SLC 500 faults.

In this section, we will discuss common SLC 500 processor faults.

NVRAM Error

• Hardware Fault Type: Processor

• Error Type: Powerup

• Error Code: 0001 (HEX)

• Error Description: NVRAM Error

• Cause: Mostly due to noise, power surge, grounding, and loss of capacitor or battery backup power.

• Resolution:

  • a. Program Download and Run

  • b. Automatic loading of program

• Detailed Steps:

  • To download program and Run, refer to the appendix.

  • To configure automatic program loading:

    • Access the Custom Data Monitor. Type in S:1/10 and put 1 in the Value tab. This forces automatic loading of the program every time a power cycle is executed.

Watchdog Timeout

• Hardware Fault Type: Processor

• Error Type: Powerup / User Program Instructions

• Error Code: 0002 (HEX) / 0022 (HEX)

• Error Description: Unexpected hardware watchdog timeout; The user watchdog scan time has been exceeded.

• Cause: Noise, power surge, grounding issues, loss of backup power, improper use of JMP, LBL, or complex nested routines and loops.

• Resolution:

  • a. Program Download and Run

  • b. Automatic loading of program

  • c. Restructure and simplify program

  • d. Adjust watchdog time duration

• Detailed Steps:

  • To download program and run, refer to appendix.

  • To adjust watchdog time duration:

    • Open Data File S2

    • Locate the SCAN TIMES tab

    • Adjust default value (typically 10 = 100 ms) up to a max of 250 (2.5 seconds) cautiously.

Math Overflow

• Hardware Fault Type: Processor

• Error Type: User Program Instructions

• Error Code:

  • Major Halt S:1/3 set to 1

  • Math Overflow Trap S:5/0 set to 1

  • Major Error S:6 shows error code 20h

• Error Description: A minor error bit is set at the end of the scan.

• Cause: Division by zero; register overflow; data out of range.

• Resolution:

  • a. Program Download and Run

  • b. Automatic program loading

  • c. Reset Overflow Trap bit S:5/0

  • d. Reassign a register with appropriate length (e.g., double integer or float)

• Detailed Steps:

  • Insert OTU instruction to a new rung; assign address to S:5/0.

  • Reassign N9 registers to F8 or other higher capacity data types as needed.

Memory Module Error

• Hardware Fault Type: Processor

• Error Type: Powerup

• Error Code: 0003 / 0007

• Error Description: Memory module memory error; can occur during REM Run mode or memory transfer.

• Cause: Memory module corruption

• Resolution:

  • a. Reprogram memory module

  • b. Replace memory module

• Detailed Steps:

  • Turn off controller power

  • Locate and remove old memory module

  • Install new memory module ensuring correct alignment

  • Reapply power and download user program

Negative Value for Timer and HSC Preset

• Hardware Fault Type: Processor

• Error Type: Program Instruction Error

• Error Code: 0034

• Error Description: Negative accumulator or preset timer value detected. For fixed processors with 24 VDC input only: Negative or zero HSC preset detected.

• Cause: Arithmetic operations resulting in zero or negative timer/HSC presets.

• Resolution: Program correction. Ensure preset values remain positive through appropriate calculations and validations.

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