The short answer
A flashing red FLT LED on an SLC 500 means the processor found a major error in itself, the chassis or memory: it stopped the program, set the major error halted bit S:1/13 and put a hex code in S:6. Rockwell groups the codes: powerup (0001–0009), going to run (0010–0019), run and instruction errors (mostly 0020–003C) and I/O (xx50–xx94, where xx is the slot number in hex).
This covers Allen-Bradley SLC 500 modular processors – SLC 5/01 to 5/05, 1747-L511 to 1747-L553 – programmed in Rockwell Automation RSLogix 500.
Key takeaways
- Write down S:6 before anyone clears it – on a 5/03 and up, a keyswitch RUN to PROG to RUN cycle clears it along with S:1/13.
- On an SLC 5/03 or higher, clearing a fault with the keyswitch in RUN puts the processor straight into Run.
- For an xx code, convert the first two digits from hex to find the slot: 0A is slot 10.
Where the fault shows: FLT, S:1/13 and S:6
- FLT LED (SLC 5/03, 5/04, 5/05): flashing during operation is a major error in the processor, chassis or memory; steady red is a fatal error with no communication; flashing at power-up means the processor has not been configured.
- S:1/13, the major error halted bit: set by the processor on every major error. Until it is cleared, the processor won't accept a mode change.
- S:6, the major error code, in hex. To compare it in ladder logic, use the decimal value:
0016hex is 22. - S:5, the minor error bits: overflow (
S:5/0), battery low (S:5/11) and others. A minor bit in the lower byte (S:5/0–S:5/7) still set at the end of the scan becomes fault0020.
In RSLogix 500, go online and open the Processor Status file, Errors tab. S:20 holds the rung number and S:21 the file number the processor last ran before the fault or power-down – built in on a 5/03 and up; on a 5/02 only if the program was saved with Test Single Step selected.
Common SLC 500 fault codes
| Code (hex) | What it means (our words) |
|---|---|
0001 | NVRAM error: memory lost, for example from noise, power or a lost battery or capacitor backup. |
0002 | Unexpected hardware watchdog timeout. |
0010 | The processor's revision level isn't compatible with the program. |
0013 | A required memory module is missing, or its autoload bits don't match. |
0016 | Startup protection: power was lost while running and S:1/9 is set. |
0020 | A minor error bit in S:5 was still set at the end of the scan. |
0021 | An expansion chassis lost power; on a 5/02 and up it clears itself when power returns, and the processor restarts and goes straight back to Run – expect the machine to start. |
0022 | The scan took longer than the watchdog time in S:3H. |
002A | An indexed address points past the end of its data file. |
0031 | The program uses an instruction this processor doesn't support. |
0034 | A timer has a negative preset or accumulated value. |
xx52 | A module the program needs in slot xx is missing. |
xx54 / xx55 | The module in slot xx is the wrong type, or has the wrong I/O count. |
xx56 | The chassis configuration in the program doesn't match the hardware. |
xx94 | A module was inserted under power, or reset itself. |
First checks by code group
- Powerup (0001–0009): Rockwell's listed causes are noise, lightning, poor grounding, missing surge suppression on inductive outputs and a poor power source. Fix that, reload the program and run; an autoload from a memory module (
S:1/10–S:1/12) can reload it for you. - Going to run (0010–0019): the program and the processor or memory module don't match – revision level, missing program file 2, memory module or
S:2/9settings. - Run and instruction errors (mostly 0020–003C): a programming problem. The code names the instruction type; correct the logic and reload.
- I/O (xx50–xx94): go to the slot. Is the module seated, the right catalog number and in the I/O configuration? A severely damaged I/O card may make the processor report slot 1, and no module should ever be inserted under power. On a 5/02 and up, many I/O faults are recoverable only if a fault routine disables that slot; otherwise the processor faults at the end of the scan.
- Steady FLT, no communication: cycle power. If it stays, remove power, take out the memory module and power up again: if the steady FLT turns to flashing, replace the memory module. Otherwise try the processor in another chassis.
FLT lit while the other LEDs flash every 2 seconds means a self-test or operating system download error on a 5/03 or higher; the LEDs that come on with it name it. FLT with BATT is an NVRAM error, FLT with BATT and FORCE a flash EEPROM failure, and FLT with BATT, FORCE and the network LED a missing or corrupt operating system.
How to clear an SLC 500 fault safely
- Record it: the S:6 code, the LED states and S:20 / S:21. Write them down first: the keyswitch cycle below clears both
S:1/13andS:6. - Fix the cause, or the fault comes straight back.
- Make the machine safe with the people who run it: clearing can restart the program.
- Clear it: on a 5/03, 5/04 or 5/05, turn the keyswitch from RUN to PROG and back to RUN – the processor goes straight into Run. Or, online, put the processor in REM Program, clear
S:1/13and the minor bitsS:5/0–S:5/7(andS:6, to avoid confusion), then return to REM Run. - Check the indicators: FLT off, RUN steady green.
Automatic options exist: S:1/8 clears a fault at power-up if the processor was running when it faulted, and the processor then tries to go back to REM Run on its own, so the machine can start. A fault routine named in S:29 can clear recoverable user faults – never non-user faults. Use them only if you understand why the machine faults.
Safety first: work inside a control cabinet only as a qualified person, under your site's lockout/tagout procedure, and follow Rockwell Automation's procedure for the processor. Never force outputs, jumper interlocks or bypass a safety circuit to get a machine running after a fault. The keyswitch is not a safety device; it never replaces the hardwired master control relay or E-stop.
When the fault keeps coming back: what to collect
- The S:6 code and the LED states each time, with date and time.
- The processor catalog number, series and firmware from the label on its side.
- The I/O modules and chassis in the system, and the RSLogix 500 version.
- A copy of the program: Rockwell notes a flashing FLT can mean the program in the processor is corrupt.
That is what Rockwell support or an integrator will ask for; our engineering team does PLC programming and changes. Planning ahead? Read the SLC 500 migration guide, and for batteries the PLC battery guide.
Parts that commonly fail
Processors, I/O modules, power supplies, chassis and batteries are the hardware behind many of these codes. The part numbers we have the most of in stock right now:
SLC 500 processors
SLC 500 I/O modules
SLC 500 power supplies and chassis
Stock changes every day. Every listing is photographed, so check the catalog number, series or product version and firmware on the label photo before you buy.
Not listed, or need several? Send us the part number and we'll check our shelves and our sourcing network and quote it in writing.
Sources
Every code, LED state and menu path on this page was checked against Rockwell Automation's own documents, listed below, and is described here in our own words. Manuals change between editions and firmware revisions: for your exact catalog or order number, check the current edition.
- Rockwell Automation, SLC 500 Instruction Set Reference Manual, publication 1747-RM001G-EN-P, November 2008.
- Rockwell Automation, SLC 500 Modular Hardware Style User Manual, publication 1747-UM011G-EN-P, June 2008.
Frequently asked questions
What does SLC 500 fault code 0020 mean?
A minor error bit in the lower byte of S:5 was still set at the end of the scan, so the processor faulted. Typical causes are a math overflow or divide by zero (S:5/0) or a sequencer or shift register error. Fix the logic; where an overflow is expected, Rockwell shows unlatching S:5/0 on a rung before the END.
How do I clear a fault on an SLC 5/03, 5/04 or 5/05?
Fix the cause first. Then either turn the keyswitch from RUN to PROG and back to RUN, which puts the processor straight into Run, or go online in RSLogix 500, put the processor in REM Program, clear S:1/13 and the S:5 minor error bits, and return it to REM Run.
What does xx mean in an SLC I/O fault code like xx52?
The first two digits are the slot number in hexadecimal. 0352 means the module in slot 3 is missing; 0A54 means the module in slot 10 is the wrong type. When the processor can't tell which slot, a modular system shows 1F.
Why is the FLT LED flashing with the BATT LED on?
Rockwell's table reads it as a major error with low or no battery backup: RAM was lost during power-down. Check the battery is connected, replace it (1747-BA), then reload the program or let the autoload from a memory module do it.