Microcode sequencer changing states in response to an external gating input level change upon the occurrence of a wait instruction

A sequencer controller for nuclear magnetic resonance imaging includes a level-sensitive external gating arrangement. When a sequencer microcode WAIT instruction is executed, the gating arrangement operates differently depending on the level of the signal existing at the external gating input. If the external gating signal level is at one level, the gating arrangement causes the sequencer to wait until the external gating input changes level--thus permitting an external gating event (e.g., closure of a breath switch or the like) to interact with and control the timing of the NMR sequence. If the external gating signal is at a different level when the WAIT instruction is first executed, however, the sequencer does not "wait" at all but instead ignores the WAIT instruction and goes to the next sequencer state. Such level sensitive external gating permits microcode sequences including WAIT instructions to be executed when external gating is desired and also when external gating is not desired, thus reducing the amount of code that needs to be maintained.