Quantum CPU Run Light Blinking: What Each Code Means
On many Modicon Quantum CPUs, a blinking Run light is an error code. The number of blinks tells you which group of fault you have. Two blinks is a RAM sizing error. Five or six blinks is a RAM test error. Seven or eight blinks points to the executive or kernel firmware. Count the blinks first, then check the battery, then decide whether the CPU needs replacing.
A CPU that won’t run stops the whole rack. This guide explains what the Quantum CPU Run light is telling you, what the other status lights mean, and what to do next.
What Each Quantum CPU Light Means
| Light | Color | What it means |
|---|---|---|
| Ready | Green | CPU passed its power-up diagnostics |
| Run | Green | CPU has started and is solving logic. Blinking means an error code |
| Modbus | Green | Communication is active on the Modbus port |
| Modbus+ | Green | Communication is active on the Modbus Plus port |
| Mem Prt | Amber | Memory is write-protected (memory protect switch is on) |
| Bat Low | Red | Battery needs replacing or is missing |
| Error A | Red | Communication error on the Modbus Plus network |
These are the lights on the standard CPUs with an LED panel. Some larger CPUs show state on a screen instead. See the section on LCD models below.
Run Light Blink Codes
Count the blinks between pauses. Schneider groups the error codes by blink count.
| Run light | What it means |
|---|---|
| Continuous blinking | Kernel mode was requested |
| 2 blinks | RAM error during sizing |
| 3 or 4 blinks | Bus, ASIC, communication or Modbus-related errors |
| 5 blinks | RAM address test error |
| 6 blinks | RAM data test error |
| 7 blinks | Executive not loaded, or executive checksum error |
| 8 blinks | Kernel PROM checksum error, or flash program/erase error |
Each group holds several hex crash codes. Schneider lists them in its Run LED error code FAQ. This applies to the lower-end CPUs. Check the manual for your exact model.
What to Do for Each Group
2, 5 or 6 blinks (RAM errors). The memory failed a test. Power-cycle once. If the blinks return, the fault is usually in the hardware. Plan to replace the CPU.
7 or 8 blinks (executive or kernel). The CPU’s operating firmware is missing or corrupt. That usually means a reload by someone with the right tools, or a replacement.
3 or 4 blinks (bus and communication). Reseat the CPU, check the rack and the power supply. Then look at the CPU’s ports.
Continuous blinking (kernel mode). The CPU is in kernel mode, which is used when loading firmware. If nobody did that on purpose, treat it as a fault.
Check the Battery First
A red Bat Low light is the easy fix. A dead battery can cost you the program at the next power-down. Replace it with the rack powered, following Schneider’s battery procedure, so memory isn’t lost.
What About CPUs With a Screen?
Larger Quantum CPUs, like the 140CPU65xxx series, show state on an LCD instead of blinking lights:
- RUN: the program is running
- STOP: the program isn’t running
- No conf: the CPU has no application program
- HALT: the CPU detected a state error
- BatL blinking: the battery is low
Compare Two Quantum CPUs
Before ordering a replacement, check what you’re matching. These figures come from the product pages and Schneider’s datasheet.
| Spec | 140CPU43412A | 140CPU65860 | 140CPU43412U |
|---|---|---|---|
| Software | Concept / ProWORX 32 | Unity Pro | Unity |
| Clock frequency | 66 MHz | 266 MHz | 66 MHz |
| Internal RAM | 896 kB | 11,264 kB | 1,056 kB |
| Built-in ports | 1 Modbus Plus, 2 Modbus RS232 | Ethernet TCP/IP, Modbus Plus, Modbus, USB | 1 Modbus Plus, 2 Modbus |
| Optional comm modules | 6 | 6 | 6 |
| Bus current draw | 1,250 mA | 2,760 mA | 1,800 mA |
Note the 43412A and 43412U. Same number, different suffix. One is a Concept CPU. The other is a Unity CPU. They are not the same part.
Also check the bus current. The 140CPU65860 draws more than twice what the 43412A does. Make sure your 140CPS11420 power supply or other rack supply has headroom.
When to Replace the CPU
Replace the CPU when:
- The same blink code returns after a power cycle
- RAM, executive or kernel errors persist
- The rack and power supply check out fine
Browse tested stock in the PAC/PLC category. Ask the supplier which software the CPU is built for, and whether it has been powered up and tested.
Frequently Asked Questions
Q: What does the Run light blinking mean on a Quantum CPU? A: On many Quantum CPUs it’s an error code. The number of blinks identifies the error group. Count the blinks between pauses, then match the count to the table above.
Q: What does the Bat Low light mean? A: The battery needs replacing or is missing. Replace it soon, because a dead battery can cost you the program at the next power-down.
Q: What does the amber Mem Prt light mean? A: Memory is write-protected. The memory protect switch is on. It’s often normal.
Q: Why does my 140CPU65xxx not show blinking lights? A: It shows state on an LCD screen instead: RUN, STOP, No conf or HALT. A blinking BatL indicates a low battery.
Q: Are the 140CPU43412A and 140CPU43412U the same? A: No. The 43412A is a Concept CPU with 896 kB of RAM. The 43412U is a Unity CPU with 1,056 kB of RAM.
The Quantum CPU Run light is a message, not a mystery. Count the blinks, check the battery, check the rack, and only then replace the CPU. When you do, match the software, memory and bus current to your system.
