Your ship's PLC is discontinued. What happens next

A PLC does not fail often. That is the problem. It sits in a panel for fifteen years doing exactly what it was configured to do, and by the time one module finally dies, the manufacturer has moved through two generations of hardware and the part number means nothing to their current sales desk.

Meanwhile the ship is alongside and the ballast system, the fuel oil purifier or the engine room alarm loop is down.

First, identify precisely what you have

This is where most enquiries lose a day. A photograph of a whole panel is not enough. What decides whether a replacement will drop in is:

• The exact module number, including any suffix. `XGI-D22A` and `XGI-D24A` look identical in a photograph and are not interchangeable.
• The firmware or hardware revision, usually printed on a small label on the side of the module rather than the front.
• The rack and power supply it sits in. A CPU from the same family may not seat in an older backplane.
• Whether the program is backed up. This matters more than the hardware. A replacement CPU with no program is an expensive paperweight.

Photograph the rating plate on every module in the rack, not just the failed one. The neighbours tell you the generation of the system.

Route one: the same module, second-hand

Discontinued does not mean gone. Vessels of the same age are broken up continuously, and their control panels come out intact before the hull is cut. A module pulled from a working panel is the same part number from the same production era, and it will behave the way the rest of your rack expects.

For a system that is otherwise healthy, this is almost always the right answer. It is a component swap, not a project. No rewiring, no re-engineering, no class involvement.

The constraint is availability rather than price — it depends on whether that family passed through recently.

Route two: a compatible module from the same family

Manufacturers often maintain compatibility within a series even as individual modules are withdrawn. A digital input card may have a direct successor that seats in the same rack and presents the same address map.

Worth checking, but verify against the manual rather than the sales description. "Backwards compatible" in a brochure sometimes means "compatible with a firmware update we no longer supply".

Route three: migrate the system

Sometimes the honest answer. If the rack is failing in several places, the program has been lost, or the family has been out of production long enough that used stock is genuinely scarce, replacing the control system ends the problem instead of postponing it by two years.

This is a planned yard job, not an alongside repair. Treat it as one — budget engineering time for re-mapping I/O, not just hardware.

What actually shortens the outage

Send the part number and a photograph of the rating plate at the same time as the fault description. Most of the delay in sourcing obsolete automation is not the search; it is the three exchanges of email establishing which variant is on the vessel.

If you have a module you are trying to identify, send us the plate and we will tell you what it is and whether it can be matched. You can also look through what is listed.