We have a TDC3000 utilizing Honeywell transmitters (instead of the before mentioned Rosemount). Loop check is a means to shorten the time for "cutover" (or actually commissioning the new instrument). During the loop check, the entire loop is connected from transmitter to DCS Gus. The instrument is not connected to the process at this time. Communication from DCS to instrument is verified. In some cases air pressure is applied to the instrument (for a PT, I/P, etc.) and a movement on the indication (or pressure) is verified. These steps are done prior to the cutover so that during the cutover phase, work can proceed in an accelerated manner (otherwise you would have to check all this each time for each loop). Many times wiring is not terminated right, etc. These problems are caught early and fixed. Our cutover was done with the unit running, so commissioning the loops in a accelerated fashion was very valuable to us. All we had to worry about were the process connections (not the electrical connections).