In my first job as a Software Developer about 5 or 6 years ago I worked at a company which connected to forecourt controllers (which managed ATGs, fuel pumps, etc) on petrol forecourts in the UK, retrieved the data and presented it to a team of analysts for reporting back to the forecourt owners.

To this day, the most difficult software challenge I have had to tackle was to retrieve the data from the serial port of a controller called the DOMS 2000. It had been replaced a number of years earlier by a new model but was still in wide service.

We had very limited documentation and I spent weeks observing the data being transmitted from the serial port while performing actions using a simulator. Eventually I managed to reliably intercept and correctly interpret almost every command.

I believe, to this day, I may well be one of the foremost experts on the DOMS 2000 on the planet.