Your PLC has stopped. The fault light is on. Someone plugs in a laptop and the program is just gone. The controller that was running your whole line yesterday is now an empty box.
This happens more often than people think, and the outcome depends entirely on one thing: whether someone has a backup of that program.
Why PLCs Lose Their Programs
The most common cause is battery failure. Older PLCs store their programs in volatile memory that needs a small lithium battery to hold the data when the power is off. That battery lasts a few years in ideal conditions, but in FNQ's heat it can fail faster. When the battery dies and the PLC loses power, even briefly, the program is wiped.
This is especially common with:
- Allen-Bradley SLC 500 series - the SLC-5/01 and 5/02 have no backup capacitor at all. Pull the battery while it's off and the program is instantly gone.
- Siemens S7-300 (older models without a memory card) - battery-dependent for program storage.
- Older Mitsubishi FX and Omron CPM series - same deal, battery-backed RAM.
Other causes include power surges (common during storm season up here), failed firmware updates, memory corruption, or someone accidentally overwriting the program.
The tricky part is that a dead battery gives you no warning while the PLC is running. The program sits in RAM and stays there as long as there's power. The battery only matters when the power goes off. So you can have a dead battery for months and never know it until the next power outage wipes everything.
What Happens Next
If you have a backup of the program file, recovery is straightforward. Someone with the right programming software connects to the PLC, downloads the program, verifies it, and you're running again. That's a few hours of work at most.
If you don't have a backup, you're looking at one of two scenarios:
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Reverse engineering - someone has to figure out what the program did by tracing the wiring, watching the process, and rebuilding the logic from scratch. This takes days to weeks depending on how complex the system is. And there's always a risk of missing something the original programmer put in that nobody documented.
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Complete rebuild - if the system is complex enough that reverse engineering isn't practical, the whole control system gets reprogrammed from the ground up. This is the most expensive outcome.
Either way, your machine is down for the duration.
What You Should Do Right Now
If your PLCs are running and you're not sure whether backups exist, find out. Ask your electrician, your maintenance crew, or whoever originally set the system up. Check if anyone has project files on a laptop or a USB drive somewhere.
If nobody has the files, get them backed up while the PLCs are still running. It's a fraction of the cost compared to what recovery looks like after a failure.
A few other things worth doing:
- Check battery status on older PLCs during routine maintenance. Most have a battery indicator light or a status bit you can read through the programming software.
- Replace batteries on a schedule - don't wait for them to die. Every 2-3 years is a good starting point, more often in hot environments.
- If you have seasonal equipment that sits idle for months (sugar mill gear, seasonal processing lines), those are the highest risk. The battery drains while it's powered off with nobody watching.
Modern PLCs Are Better, But Not Bulletproof
Newer PLCs like the Siemens S7-1200/1500 and Allen-Bradley CompactLogix use non-volatile flash memory that doesn't need a battery to hold the program. That's a big improvement. But they can still lose programs to firmware issues, accidental overwrites, or ransomware attacks on the connected network.
A backup is still essential regardless of how new your gear is.
Related Services
- HMI & PLC Backup - get your systems backed up while they're still running
- PLC Software Services - programming, backup, and configuration
- Reverse Engineering & Recovery - when the backup doesn't exist and the hardware has already failed
We back up PLCs from Delta, Fatek, Siemens, Mitsubishi, and many other brands across Cairns and Far North Queensland. Get in touch before you need us in a hurry.