The situation
The communications port on an Allen-Bradley CompactLogix L35E controller had failed, leaving the HMI with no control over the plant.
What was involved
A failed comms port is not a repair, so the controller had to be replaced. The replacement was a newer Series B unit, which requires a minimum firmware revision before it will accept a program at all. A controller on the wrong revision cannot be programmed, so the firmware work has to happen before anything else can.
Firmware alone was not the end of it. The existing project was still configured for the original controller, so both the controller type and the revision had to be changed in the program before it would recognise the new hardware and accept the download.
What we did
The work was carried out on night shift to keep the plant available for production. The replacement controller was brought up to the required firmware revision, and the existing program was then loaded onto it.
The outcome
Control from the HMI was restored and the plant returned to normal operation.
Worth noting
Firmware is where a controller swap either goes smoothly or stops dead. Getting a replacement onto the correct revision, reconfiguring the project for the new hardware and loading the existing program back into it is routine work if you have done it before, and a very long night if you have not.
