Is there a cadence meter available for the Shimano Ultegra Flight Deck 5601? Or is there a good alternative option for measuring cadence without changing the flight deck setup?
The Shimano Flight Deck computers use 'virtual' cadence. There are switches fitted to the shifters that tell the computer which gears you have selected. The wheel sensor tells the computer how fast you are travelling. The computer works what your cadence is from there.
I'm not aware of any Shimano product which is used for measuring cadence with these particular drive train components, but you can find many good products that will work for this application.
You can get good wireless cyclocomputers cheaply (that one has average cadence), so why go hardwired?