I decided to buy the FCU and EFIS ready built to save time as this is a complex unit and would have taken a lot of valuable hours. This was not without problems however.
The main difficulty was that the unit could not be used; whilst it is sold as a plug and play unit I could plug but not play as there was no interface software for AST. It was some months before this was developed by AST and even then the baro height adjustments did not work correctly and some of the buttons were scrambled. This was partly corrected 18 months later. There remains an issue with the scrambling of baro digits on the pilot side which I gather is suffered by all other users and the vertical speed settings below 1000 ft/min have never worked.
The development of the AST software has stalled and I decided I had to move if the simulator was ever going to work properly, alternatives at this time are Jeehell and in the future Prosim. I had trialled Jeehell for some time but the FDS FCU has an outdated USB interface and is not compatible. I decided to ditch the FDS FCU and go for the Skalarki product which is compatible with AST, Jeehell and Prosim.
I am very pleased with the result.