There are 3 'bits' that control the mode of operation of the AST, called circulate, continuous and event which can be enabled or disabled by writing 1 (enable) or 0 (disable) to address locations CIRCULATE_MODE, CONTINUOUS_MODE and EVENT_MODE respectively.
Through the mode-bits the AST can be programmed to stop after outputting a certain number of tokens, to run continuously and to circulate the tokens in the FIFO.
When the circulate mode is enabled every token that is sent by the AST is reentered into the FIFO; in this mode therefore the number of data items in the FIFO is constant.
When the continuous mode is enabled the state of the counters is ignored.
When the event mode is enabled run is disabled when the AST sends out a control token (and the AST stops sending data), independent of the setting of the other mode-bits; to continue running run has to be explicitly enabled again (via VME or a NIM-signal); see chapter 2.4.5.
The stop-condition(s) as a function of the setting of the circulate and continuous mode-bits is as follows (also see table 6):
Table 6: The stop-condition as a function of the circulate and
continuous mode-bits.
Independent from the circulate and continuous mode the AST stops sending data when the FIFO becomes empty or when run becomes disabled (see chapter 2.4.5).
When the circulate mode is not enabled, the AST outputs data until the FIFO is empty (or TC=0); in this mode the AST's FIFO can be loaded with new data while the AST is outputting data; when the AST stopped sending data because the FIFO became empty, it will immediately resume sending when the FIFO is being filled with new data, until again the FIFO is empty or any other stop condition becomes valid.