in reply to: HART Compilence posted by Saurabh Prabhudesai on 17 April 2007 at 12:16:59.
Yes, to be properly HART-conformant, you must implement all Universal Commands.For commands 1, 2 and 3, you should allocate some of your measurements as the four Dynamic Variables. You could make a fixed allocation (perhaps corresponding to the first four input terminals), or better, use commands 50 and 51 to read and write allocations from the full set of device variables depending on the application.
You don't have to have all four dynamic variables, but this would be a design decision, not an application-specific configuration choice. Remember there is always the option of returning NaN (not-a-number) hex 7F A0 00 00 for a floating point variable which does not exist at this time, and hex FA (decimal 250) for not-used enumerated quantities such as units.
For your RS-485-only device, I suggest returning NaN as the Loop Current value in commands 2 and 3. The percent-of-range values should still be good, since you could use command 35 to configure lower and upper range values for the primary variable. Some Control Systems may like to use these to generate nice bar graphs. (Remember, the percent-of-range value is not limited to 0 to 100 percent.)