Program correction for machines that already run

• Machine hangs mid-cycle, or stops at the same step every few hundred cycles.
• Good parts rejected, or bad parts passed, after a sensor or a recipe change.
• A new sensor, station, interlock or test has to be added to a running machine.
• Operators have found a way round an interlock, and it has to be closed properly.
• The HMI shows wrong values, freezes, or lost communication after a replacement.
• The machine builder is gone and nobody understands the program.
• The program was lost after a battery, memory card or power failure.
• The PLC is obsolete and spares are no longer available.

1. STEP 01
   Back up first
   The PLC program, HMI project, drive parameters and recipes are copied before anything else, and kept with you.
2. STEP 02
   Read and document
   We read the program, name what is unnamed, and note how the sequence actually works — not how anyone remembers it.
3. STEP 03
   Reproduce the fault
   We find the exact condition that causes the problem, from the logic, the I/O and the machine itself.
4. STEP 04
   Change in small steps
   One change at a time, each tested on the machine, rather than a large edit loaded all at once.
5. STEP 05
   Test the edges
   Normal cycles, then the awkward ones: a sensor that fails, an emergency stop mid-cycle, a restart after a power cut.
6. STEP 06
   Hand over
   The new program, the old one, and a short note of what changed and why — so the next person does not start from zero.

A PLC that has lost its program is a stopped machine with no instructions. The fastest route is a backup, so we search for one first: the programming laptop, a memory card, the HMI, old emails. When none exists, the program can be rebuilt from the machine itself — the wiring tells us every input and output, the operators know the sequence, and the HMI screens show what was being monitored.

Replacing an obsolete PLC

When spares are gone, waiting for the next failure is the expensive option. A planned migration maps every I/O point, rewrites the logic for a current controller, updates or replaces the HMI, and changes the panel wiring only where it has to. See PLC programming and HMI design for the platforms we work on.