Chapter 3 Hardware OverviewNational Instruments Corporation3-21AT-MIO/AI E Series User ManualYou can also individually enable each of the PFI pins to output aspecific internal timing signal. For example, if you need the UPDATE*signal as an output on the I/O connector, software can turn on the outputdriver for the PFI5/UPDATE* pin.Board and RTSI ClocksMany functions performed by the AT E Series boards require afrequency timebase to generate the necessary timing signals forcontrolling A/D conversions, DAC updates, or general-purpose signalsat the I/O connector.An AT E Series board can use either its internal 20 MHz timebase or atimebase received over the RTSI bus. In addition, if you configure theboard to use the internal timebase, you can also program the board todrive its internal timebase over the RTSI bus to another board that isprogrammed to receive this timebase signal. This clock source, whetherlocal or from the RTSI bus, is used directly by the board as the primaryfrequency source. The default configuration at startup is to use theinternal timebase without driving the RTSI bus timebase signal. Youselect this timebase through software.RTSI TriggersThe seven RTSI trigger lines on the RTSI bus provide a very flexibleinterconnection scheme for any AT E Series board sharing the RTSIbus. These bidirectional lines can drive any of eight timing signals ontothe RTSI bus and can receive any of these timing signals. This signalconnection scheme is shown in Figure 3-15.
