Not to worry. You have company in this regard!

Despite all the benefits of a good documentation that outlines the whole project requirement, there are many instances where I have started a project without much of up front design inputs. In my field, there are instances where we develop some test facilities for the first time and as such no input is available from the client. These are tough ones but we manage very well based on experience.

If you want to list out the bare essentials these would be the bare minimum :

1. User interface - displays , indications, alarms, switches and user configuration parameters. And reports if any.

2. Input / Output summary - both digital and analog ( with required resolution ).

3. Time critical events. The minimum acceptable resolution of timing. While on this ALSO focus if there are any SLOOOOW process. Been once tripped by the requirement to generate a ramp of 0.5V / Hour at a very advanced stage of the project.

4. Power requirements. Particularly for mobile applications.

5. Certifications for sensor accuracy, safety regulation compliances and the like. If not properly planned, getting them at a later stage can cause a big hole in the pocket.

These would be the first level details to be agreed upon and SIGNED by both parties before proceeding further.

Raghu